# Fast Translation from tex for: I/275/./intro.tex

Disclaimer: The following document (from catalogue I/275) results from an on-line translation from latex to html (the cgiprint tool developed at CDS). The correct presentation requires an execution of latex on the original file

6.5 in 9.0 in -1.0 in [missing file: psfig.sty] AC 2000.2: The Astrographic Catalogue on <BR> The Hipparcos System 2017-08-21

AC 2000.2: The Astrographic Catalogue on
The Hipparcos System

Sean E. Urban, Thomas E. Corbin and Gary L. Wycoff
United States Naval Observatory, Washington, DC ∧ Erik Høg, Claus Fabricius, and Valeri Makarov
Copenhagen University Observatory, Denmark

Catalogue of Positions Derived from the Astrographic Catalogue Measures. Positions are on the Hipparcos System (HCRS, J2000.0) at the Mean Epochs of Observation.

\listoftables

## 1  Introduction

The AC 2000.2 is a revised version of the 1997 release of the AC 2000 (Urban et al. 1998). It was decided that the availability of an improved reference catalogue and the inclusion of the Tycho-2 photometry would be sufficient to warrant a complete re-reduction of the data and a new distribution of the catalogue.

The AC 2000.2 is a catalogue of positions and magnitudes of 4,621,751 stars covering the entire sky around the epoch of 1900. The data are derived from the images measured and published as part of the Astrographic Catalogue (AC). The positions are on the Hipparcos reference frame (ESA 1997), having originally been reduced plate-by-plate using an updated version of the Astrographic Catalog Reference Stars. Each of the 22 zones of the AC has been reduced independently, since telescopes, observing techniques and measurement methods varied. This document describes the history behind the Astrographic Catalogue, the reference catalogue used to transform the measurements to equatorial coordinates, the methodology employed in this transformation, the resulting catalogue and information about each participating observatory.

## 2  The Carte du Ciel

The Carte du Ciel was an international effort begun more than a century ago to determine positions better than 0.5 arcsec for all stars 11th magnitude and brighter using photographic plates and, using another set of plates, to publish charts representing the relative positions of all stars of 14th magnitude and brighter. The charts – generally called CdC – proved to be very expensive to photograph and reproduce, so many institutions did not complete this part of the work. However, the astrographic program designed to measure all stars to 11th magnitude was completed. Actually, the original goal of 11th magnitude was generally surpassed. In fact, some observatories routinely measured stars as faint as 13th magnitude. These plate measures, as well as the formulae used to transform them to equatorial coordinates, have been published in what is known as the Astrographic Catalogue (AC).

In total, 20 observatories from around the world participated in exposing and measuring the AC plates. Each was assigned a specific zone, between two parallels of declination, to photograph. In order to compensate for any plate defects, each area of the sky was to be photographed twice, using a two-fold, corner-to-center overlap pattern. This pattern was continued even at the zone boundaries; each observatory's plates would overlap with those of the observatories responsible for the adjacent zones. The participating observatories agreed to standardize the type of telescope, so each plate photographed had a similar scale of approximately 60 arcsec/mm. The measurable areas of the plates were 2 × 2 degrees, so the overlap pattern consisted of plates that were centered on every degree band in declination, but offset in right ascension by two degrees. The first plates in the even degree bands were centered at right ascension 0 hours 0 minutes; the first plates in the odd degree bands were centered with right ascension several minutes higher (corresponding to approximately one degree).

In addition to the overlap pattern and type of telescope, the observatories also agreed to expose a grid, called a réseau, on each plate. It was originally used to monitor emulsion shifts. After the shifts were demonstrated to be quite small, the practice of exposing a réseau on each plate was continued because it aided in the measuring of the star positions by letting the measurer refer each image position to the grid lines. Each réseau unit was approximately 5 mm. The réseau orientation defined the plate's x,y coordinate system.

All participating observatories (with the exception of Vatican) used one of two measuring methods, short-screw or eyepiece scale. In both methods, a set of spider wires was centered over an image, then the distance traveled by the slides carrying the spider wires was read. With the short-screw method, this distance was read off of the screws used to move the slides. Whereas for the eyepiece scale method, this distance was inferred by a scale in the focal plane of the microscope. In general, the eyepiece scale method was faster, but less precise.

Although telescope type, plate size, and use of a réseau were standardized, many other factors, such as reference catalogue used, reduction technique and printing formats were left up to the individual institutions.

## 3  Published Data

Most observatories published their results in several volumes, each of which consists of measures from plates centered on the same degree of declination. Generally, each line in the printed volumes contains data for one star, including the measured x,y value, a measure of brightness (magnitude or diameter), the plate number and a running number on the plate. Other data concerning epoch of exposure, hour angle at mid-exposure, air temperature, barometric pressure, réseau used, observer, measurer and measuring machine are usually provided by plate in separate tables. Additionally, provisional plate constants used to transform the x,y measures to standard coordinates are typically supplied.

The published data have been transferred to machine-readable form via double-entry (that is, typing each record twice to remove most keying mistakes) including errata found in the published volumes. An additional literature search has been conducted on all zones to increase the probability that all published errata not found with the original volumes have been corrected. Each page of each volume was searched for pertinent notes. If important notes were found, they were entered and the measures to which they refer were flagged. A summary of the data entry information can be found in Table tab:keypunch.

## 4  Compiling a New Reference Star Catalogue

The reference catalogue used throughout the individual plate reductions of the 1997 version of AC 2000 (hereafter referred to as AC 2000.1) was the Astrographic Catalog Reference Stars (ACRS; Corbin & Urban 1988, Corbin & Urban 1990, Corbin & Urban 1991). This was compiled on the system of the FK5 as realized by the International Reference Stars (IRS, Corbin 1991), and utilized the best data and reduction techniques available in the early 1990s.

Since its compilation, the FK5 has been superseded by the Hipparcos Catalogue. It was recognized that with the increased number of stars in the Hipparcos Catalogue over the FK5/IRS (~ 118,000 vs. ~ 40,000), the conversion of the catalogues making up the ACRS to the system of Hipparcos could be done more rigorously than the earlier conversions to the FK5 system. Since the systematic errors could be determined and removed more thoroughly, the final combined catalogue would be better. Additionally, the FK5/IRS data become very sparse fainter than magnitude 9.0, whereas Hipparcos contains over 35,000 stars listed as V=9.0 or fainter. This magnitude extension allows the removal of systematic errors in the fainter stars – something that was problematic for the original ACRS.

In addition to Hipparcos, several new catalogues unavailable at the time of compiling the original ACRS have been released. The most notable are the Tycho-1 Catalogue (ESA 1997), the Twin Astrograph Catalogue (TAC, Zacharias & Zacharias 1999), FOKAT (Polozhentsev et al. 1989), and a recompiled Second Cape Photographic Catalogue (CPC2, Zacharias et al. 1999).

Due to these improvements in astrometry, it was decided to compile a new reference catalogue. In this document, this new catalogue will be referred to as ACRS_1999. The ACRS_1999 is not being released as a separate catalogue because the data used in computing the astrometry were subsequently used in computing the proper motions of the Tycho-2 Catalogue (Høg et al. 2000). The ACRS_1999 can be thought of as a specialized subset of the data going into Tycho-2, which completely supersedes it.

### 4.1  Conversion of input catalogues to HCRS

The Hipparcos Catalogue (ESA 1997) plays a special part in the ACRS_1999. It is recognized as the optical realization of the International Celestial Reference Frame, ICRF (IAU 1999). The frame defined by the Hipparcos data is termed Hipparcos Celestial Reference Frame, HCRF (IAU 2001); the HCRF contains all Hipparcos stars with the exception of those believed to be multiple. These data – taken from the Hipparcos Catalogue – were used to convert all input catalogues to the HCRF by the method described below.

In total, 145 different observational catalogues (termed input catalogues in this document) were included specifically to strengthen the positions of the ACRS_1999 stars when brought to the epochs fo the AC plates. A list of these catalogues is found in Table tab:acrs. Each of the catalogues was converted to a standard format. Next, zero point corrections and elliptical aberration terms were applied, when necessary, to convert the positions to roughly coincide with the FK5 J2000.0 system. Following this, the Hipparcos stars contained in each catalogue were identified. Differences in right ascension and declination were computed for these stars following the application of the Hipparcos proper motions to bring the Hipparcos positions to the epochs of the input catalogue positions.

To remove the systematic differences between these input catalogs and Hipparcos, local'' mean differences between each catalogue and Hipparcos were made, and corrections to the input catalog positions based on those differences were applied. To compute these differences, for each star in the catalogue, nearby stars in common with Hipparcos were identified. In order to compute local systematic errors and not follow the random errors, a minimum number of stars in common to the input catalogue and Hipparcos had to be enforced. This minimum number was based on each input catalogue's random errors. For catalogues with large random errors, more Hipparcos stars were needed. The range in required Hipparcos stars was from 3 for the most accurate catalogues to 30 for the least accurate.

Additionally, since determining local systematics was the goal, only those stars in common with Hipparcos and close on the celestial sphere were used. For astrographs, typically the Hipparcos stars within two degrees of the star to be corrected were utilized. For transit circles, typically the Hipparcos stars within 30 minutes of time in right ascension and 5 degrees in declination were used. If the minimum number of stars discussed in the previous paragraph could not be met, the area could be expanded. How far this expansion was carried out was dependent on how quickly, spatially speaking, the systematic deviations from Hipparcos were changing. Catalogues whose density of Hipparcos stars was too low to adequately perform a local reduction were dropped from the input catalogue list.

Once nearby catalogue entries of Hipparcos stars were found for each input catalogue star, their positional differences with the Hipparcos data – that is, their residuals – were used to compute the local reduction to the Hipparcos system. Weights were assigned to each residual based on the distance to the star being corrected. To more heavily favor nearby Hipparcos stars, an elliptical, parabolic weighting method was used, as seen in the following equation.
 wi = 1 – \left[ \left( Dαi/ maxDα \right) 2+ \left( Dδi/ maxDδ \right) 2\right] (1)
The value of wi is the weight of the ith residual, Dαi and Dδi are the distances in right ascension and declination between the catalogue star whose correction is being computed and the ith Hipparcos star, and maxDα and maxDδ are the maximum distances whose weight is non-zero in right ascension and declination. Once weights to each nearby Hipparcos residual are computed, the weighted mean residual vector can be computed in the standard way. That is, for right ascension,
 Δ(α) = Σwi Δi(α)/Σwi (2)
where Δ(α) is the weighted mean vector in the right ascension direction, Δi(α) is the residual of the ith Hipparcos star. Similar formulation is used for the declination vector. A summation is used to get the mean residual vector, which is the systematic difference between the Hipparcos Catalogue and the input catalogue at that specific location. Hence, subtracting this vector from a star position will put it close to the HCRF.

The above technique describes correcting a catalogue for systematic differences with Hipparcos based on right ascension and declination. However, it is well known that other systematic deviations exist, most notably those based on magnitude and color. These, in turn, may have a positional dependency associated with them; so, corrections based on right ascension, declination, magnitude, color and all combinations of the four were computed and applied. This was done similarly to the technique just describe, however the definition of local'' was expanded to include those stars nearby not only in right ascension or declination, but also similar in magnitude or color. In order for a magnitude-dependent correction to be computed, typically only Hipparcos data from stars that were within 0.1 magnitudes of the input catalogue star were used. The same 0.1 magnitude difference in B-V was usually utilized in performing color corrections.

### 4.2  Computing catalogue weights

Once a catalogue was believed to be on the HCRS, individual star differences from Hipparcos were computed. (Note that the conversion to the HCRS utilized weighted mean differences using several stars, not individual differences.) These were used to compute the catalog standard error and weight utilizing the well-known formulae
 σα,δ = sqrt( Σ(xi)2– (Σxi)2/N/N) (3)
and
 Wα,δ = 1/σα,δ2 (4)
where σα,δ is the standard error per catalogue entry in right ascension and declination, xi are the individual differences between the stars in common between the input catalogue and Hipparcos in right ascension and declination, and N is the total number of stars in common between the two. The value Wα,δ is simply the computed weight of the catalogue in right ascension and declination. For some input catalogues, the number of observations per catalog entry given. In these cases, an error (hence weight) per observation could be computed. These catalogues have a Yes'' under the σ/sqrt(N) column of Table tab:acrs. For many catalogues, however, only a standard error (hence weight) per catalogue entry could be computed. These catalogues have a No'' under the σ/sqrt(N) column of Table tab:acrs.

This procedure was used for most catalogs; a few, however, had error estimates available for each star that closely coincided with the newly computed estimates. In these cases, the additional information in the form of published error estimates (hence weights) was used.

### 4.3  Computation of ACRS_1999 positions and motions

The reduced positions were grouped together by indivisual star. Mean positions, proper motions, and error estimates were computed following the method outlined by Corbin (1977), briefly described here.

#### 4.3.1  ACRS_1999 mean positions

Computations are made using position unit vectors, defined by
 xi = cosδi ×cosαi (5)
 yi = cosδi ×sinαi (6)
 zi = sinδi (7)
Weighted mean (x,y,z) values, denoted as ({bar}x,{bar}y,{bar}z) and their associated epochs can be easily computed using the computed weights. Note that the weights wx, wy, and wz are equal to wα, wα, and wδ, respectively. From ({bar}x,{bar}y,{bar}z), a conversion back to mean right ascension and declination, ({bar}α, {bar}δ), is made using the formulae
 {bar}α = \left( {bar}y/{bar}x \right) (8)
and
 {bar}δ = \left( {bar}z/sqrt( 1 – {bar}z2) \right) (9)

#### 4.3.2  ACRS_1999 proper motions

To compute proper motions, first the time derivatives of (x,y,z), denoted ({dot}x,{dot}y,{dot}z), are computed using
 {dot}x = Σxi ⋅wi ⋅τi/Στi2⋅wi (10)
with similar equations for {dot}y and {dot}z. The variable τi is the epoch difference between the ith catalogue position and the weighted mean epoch. Finally, proper motions in right ascension and declination, µα, µδ, are computed using
 µα= {dot}y {bar}x – {dot}x {bar}y /{bar}x2+ {bar}y2 (11)
and
 µδ= {dot}z/sqrt( (1–{bar}z2)) (12)

#### 4.3.3  ACRS_1999 error estimates

The U.S. Naval Observatory typically computes standard error estimates based on the residuals of catalogue positions making up the mean position and proper motion. This has been termed the scatter method, because it is based on the scatter of the data around the computed value. The formulae used are detailed in Corbin (1977). For standard errors of the mean right ascension and declination, σ{bar}α, σ{bar}δ, we have
 σ{bar}α = \left[ Σ(Δyi {bar}x – Δxi {bar}y)2wi/(1–{bar}z2)(n–2)Σwi \right] 1/2 (13)
 σ{bar}δ = \left[ Σ(Δzi wi)/(1–{bar}z2)(n–2)Σwi \right] 1/2 (14)
where the values of Δxi, Δyi, Δzi are the residuals of the ith catalogue position from the computed (x,y,z) values at ith catalogue epoch. For the computation of the standard errors of µα and µδ we have
 σµα = \left[ Σ(Δyi {bar}x – Δxi {bar}y)2wi/(1–{bar}z2)(n–2)Σwi ⋅τi2 \right] 1/2 (15)
 σµδ = \left[ Σ(Δzi wi)/(1–{bar}z2)(n–2)Σwi ⋅τi2 \right] 1/2 (16)
where n is the number of catalogue positions used to compute {bar}α, {bar}δ,µα, and µδ. Even a cursory look at the above formulae will show that computation of the standard errors using this method cannot be made if there are only two catalogue positions used. In this case, the formulae
 σ{bar}α = \left[ 1/Σwi2 ∑wi2⋅σαi2 \right]1/2 (17)
and
 σµα = \left[ \left( 1/Στi2 \right)2 1/Σwi2 ∑wi2⋅σαi2⋅τi2 \right]1/2 (18)
were used. Similar formulae were used to compute position and proper motion errors in declination.

#### 4.3.4  Refining the catalog

Since the purpose of compiling the ACRS_1999 was to provide a source of reference stars used to reduce the AC data on to the system defined by Hipparcos, it was decided to utilize the astrometry from the Hipparcos Catalogue for the ~30% of the stars in common. Hence the Hipparcos data were substituted for the compiled data. Hipparcos stars flagged with a G, V, or X in the Double and Multiple Flag field (H59) were removed from the ACRS_1999, as their astrometry is suspect. Additionally, stars marked in that same field with an O (orbit stars) were removed if the semi-major axis exceeded 100 mas. All stars marked as orbit stars in the Washington Double Star Catalog (WDS; Worley and Douglass 1996) were removed. Since measurements of blended images on the AC plates are often suspect, when an ACRS_1999 stars was found within 5 arcsec of another ACRS_1999 star, both were removed. Finally, all ACRS_1999 stars whose positional errors in either coordinate at epoch 1900 were computed to be at 450 mas or higher were removed.

#### 4.3.5  ACRS_1999 characteristics

The ACRS_1999 contains 391,838 stars distributed over the entire sky, resulting in an average density of just under 10 stars per square degree. The magnitude distribution, using the Tycho-1 visual magnitude, can be seen in Fig fig:magdist.

The astrometric errors are magnitude dependent. Figures fig:poserr and fig:muerr – indicating positional errors at the ACRS mean epoch and proper motion at the same epoch, respectively – show this dependency. The wide range in positional errors, from near 1 mas for the brighter stars to over 30 mas for stars between magnitude 10 and 11, is due to the Hipparcos star distribution in the ACRS_1999. Positional errors in the Hipparcos Catalogue are generally 1 to 3 mas; for the best non-Hipparcos catalog, Tycho-1, the errors run from 10 to 50 mas. This is reflected in Fig. fig:poserr. For stars brighter than 8th magnitude, the primary source of astrometry is Hipparcos. Between 8 and 11, the ratio of Hipparcos to non-Hipparcos stars drops, and hence the errors increase. Beyond about magnitude 11, this ratio begins to increase, so the positional errors once again drop.

This same trend is reflected in the proper motion errors, seen in Fig. fig:muerr. The span in errors is less, however, since the Hipparcos values are generally between 1 and 3 mas/year whereas without Hipparcos, values of 2 to 5 mas/year are normal. No downturn of errors in the faintest stars is seen; this is where the Hipparcos proper motion errors are their highest.

Having the ACRS_1999, with a density providing an adequate number of reference stars on each AC plate (35 on average) and being on the Hipparcos system, allows one to continue the plate reduction process.

## 5  Preparing the Data for the Plate Reduction Software

It was necessary to prepare the data for the plate adjustment software. The preparation process consisted of three discrete tasks: matching the images with reference stars; matching images with those of the same star on another plate; and converting the data to a standard format. The last two steps were performed during the AC 2000.1 work. However, since they are an integral part of the reduction process, the descriptions are repeated here.

### 5.1  Matching images with the reference stars

Equatorial coordinates for all AC images were computed from the rectangular coordinates via the published plate constants. The ACRS_1999 data were then brought to the average epoch of each zone and transformed to the AC equinox, B1900.0. A positional match was then made between the AC and ACRS_1999. Checks were made to ensure that illegitimate matches had not taken place. An example of an illegitimate match is an ACRS_1999 star that matches with two separate AC images that are on the same plate. Also at this stage, all plates were checked to ensure that each contains an adequate number of identified reference stars. Fewer than expected matched reference stars on a plate may indicate problems with the published constants or plate centers. These plates were investigated, and the problems were corrected.

### 5.2  Matching images with those on other plates

The same equatorial coordinates computed in the previous step were used to identify images of the same stars that lie on different plates. Images within 2.0 arcsec of each other were generally identified as the same star at this stage. Illegitimate matches were investigated and treated appropriately. Each image was then assigned an internal star number that is unique for each star in the zone, regardless of the number of plates on which it appears. The data were verified to ensure that all images marked with an ACRS_1999 number have the same internal star number, and vice versa. No two images on one plate were allowed to have the same internal star number.

### 5.3  Converting data to a standard format

The data were combined in one, standard format file. This file contained all pertinent information about each plate and image. Plate information such as plate centers, sidereal time of exposure, meteorological data, measuring machine and measurer, réseau used, emulsion type, and epoch of exposure were included, if they were published. Data pertaining to each star, that is the x,y values, image diameter or magnitude, internal star number, and ACRS_1999 identifier, were included. A conversion from the published x,y units to units of millimeters, along with a translation of the coordinates to ensure that the origin is in the approximate plate center, was performed when necessary. The persons responsible for the zone preparations can be found in Table tab:people.

#### 5.3.1  Conversion of the AC Magnitudes

The Astrographic Catalogue as originally published contains magnitude measures – usually in the form of image diameters and formula to convert them to stellar magnitudes. However these are non-uniform between zones, in part because of different techniques used by participating observatories. Many of the published magnitude measures are unreliable, especially for the faintest and brightest stars. Thus, it is desirable to transform them to a well-known – or often used – system, preferably to the same system as the reference catalogue, thus facilitating the removal of systematic errors that are a function of magnitude. The plates used were most sensitive in the blue spectral region, so a logical choice of systems was the Tycho-2 blue (BT). Tycho-2 contains about 2.5 million stars covering much of the AC magnitude range. The Tycho-2 photometry was made available for this in advance of publication.

Each zone was treated independently. Only stars identified in Tycho-2 as being single and with negligible variability were used for the calibration. Differences between AC and Tycho-2 BT magnitudes as a function of AC magnitude were computed. Polynomial expressions describing the results were computed via least-squares fitting. Some extrapolation of these polynomials was required because for most zones the magnitude limit is fainter in the AC than in Tycho-2. (Usually an extrapolation of less than 0.5 magnitude was used. Beyond that the corrections were held constant). For stars not used to compute the calibration – primarily non-Tycho-2 stars – the magnitudes found in the AC 2000.2 are based on the published diameters and the derived polynomial expressions. For stars in common with Tycho-2, the Tycho-2 BT magnitudes are given.

## 6  Preliminary Reductions and Investigation of Plate Models

The core of the plate reduction software was the same as that used in the reductions of the Cape Photographic Catalogue 2 data (Zacharias et al. 1992). For all zones, an eight-constant plate model, consisting of four orthogonal terms (a,b,c, and d), two non-orthogonal terms (e and f) and two tilt terms (p and q) was initially used, as shown in Eqs. eq:xmodel1 and eq:ymodel1 .
 ξ= ax + by + c + ex + fy + px2+ qxy (19)
 η= ay – bx + d – ey + fx + pxy + qy2 (20)
At this step, no corrections to the published x,y values were applied. Investigations of radial distortions, tangential distortions, magnitude equation, coma, periodic measuring errors and réseau-dependent systematic errors were investigated for each zone following the procedures outlined previously (Urban & Corbin 1996; Urban et al. 1996). Since an uncompensated systematic error may cause a star to be an outlier, only reference stars with residuals over five times the standard deviation of unit weight of the solution were removed while various plate models were investigated.

### 6.1  Corrections to the x,y values

Results of investigations of systematic errors may lead to applying a correction to the published star measures prior to final plate constant determinations. These are typically done on a zone-by-zone basis.

To investigate if radial distortion exists in the data, the reference star radial residuals were plotted against distance from the plate center. If a radial distortion was present, the corrections were described by a non-zero function. Additionally, the radial residuals were examined for magnitude, measurer and measuring machine dependence. A similar investigation was conducted looking for the existence of a tangential distortion, however none was found in any of the zones.

The presence of a magnitude equation, which is a systematic offset of stars based solely on their brightness, was investigated by plotting the reference stars' residuals with respect to the Tycho-2 BT magnitude. This was performed separately for both the x and y coordinates. The existence of a magnitude equation that is dependent on the x,y measures was also investigated. Additionally, magnitude equation varying with measurer and plate epoch was considered. Note that a magnitude equation outside the range of the reference stars is extremely difficult to find (Eichhorn 1974). It is possible to compensate for a magnitude equation in the magnitude range of the reference stars, but it is unwise to extrapolate those corrections to field stars that may be two or three magnitudes fainter.

The presence of coma, a change in scale based on magnitude, is uncovered in two ways. First, investigating the x,y-dependent magnitude equation will reveal coma if it exists within the range of the reference stars. Second, using fainter stars, a plot of the difference between the mean position (computed from overlapping plates) and an individual image position vs. plate coordinates of the individual position will, using data from hundreds of stars and different magnitude ranges, reveal the presence of coma. If coma was found, it was examined for variations with respect to plate epoch.

Errors caused by the measuring mechanism were investigated by analyzing the residuals of reference stars as a function of location on the x and y measuring apparatus (usually a screw or an eyepiece scale, see Section 2). The presence of an error with a period corresponding to the length of the measuring screw or eyepiece scale is quite likely caused by the machining of that part, but may also be a function of the measurer. In either event, these errors can be corrected.

Additionally, the presence of a remaining field distortion pattern can be examined by using plots similar to those produced while investigating coma. These patterns were investigated for dependence on magnitude, réseau, measuring machine and measurer.

Using the information from these investigations, a plate model was developed for each of the AC zones. Equations (eq:xprime) and (eq:yprime) show the corrected x,y values used in the final plate adjustments.
 x′= x+RD + TD + ME x + MC x + S x mx + MA x + FDP x (21)
 y′= y+RD + TD + ME y + MC y + S y my + MA y + FDP y (22)
In Eqs. (eq:xprime) and (eq:yprime), RD is the correction applied to compensate for radial distortion; TD is the correction to compensate for tangential distortion; ME and MC are corrections to compensate for magnitude equation and x,y-dependent magnitude equation; S is the correction for coma; MA corrects for measuring apparatus errors, and FDP compensates for any remaining field distortion pattern. The variable m is the computed magnitude. By substituting x, y for x,y in Eqs. eq:xmodel1 and eq:ymodel1, the 8 constants for each plate were computed. The significant systematic errors found and corrected in each zone are listed in Table tab:const.

## 7  Investigation of Discordant Data

Once a suitable plate model was determined, the computed positions were used to investigate problems such as mismatched images, blended images of multiple stars and typographical errors. Much of this work was performed during the AC 2000.1 reductions; the description is repeated here for completeness.

### 7.1  Incorrectly matched images from overlapping plates

Images may be incorrectly matched in a variety of ways. Images are originally matched in the way described in the subsection Matching images with those on other plates.'' Positions computed from the provisional plate constants can change significantly due to the new reductions. So, images earlier believed to be from two stars because their positions were dissimilar may now be recognized as coming only from one if their new positions are closer. To investigate this possibility, images closer together than a certain distance were investigated. The distance chosen depended on the range in plate epochs (since proper motion will then be a factor) as well as accuracy of the x,y measures. Images that fall outside the expected precision may be a result of a double star, poor measurements of the same star or good measurements of a star that has moved due to proper motion. Deciding which was the case was often difficult because no comparable catalogue with the epoch of the AC plates exists with which to check the stars in question. The Preliminary Version of the Third Catalogue of Nearby Stars (Gliese & Jahreiss 1991) aided in detecting high proper motion stars, but this catalogue, as its name states, contains only nearby stars. The Luyten's Two-Tenths Catalogue and its supplement (Luyten 1979, 1980; Luyten & Hughes 1980) has also been used in aiding in the identification of high proper motion stars, as was the Hipparcos Catalogue. The Washington Double Star catalogue (WDS; Worley & Douglass 1996), the internationally recognized source of visual double star data, was also used but is limited because of its incompleteness over the AC magnitude range and its occasional use of AC data. However, certain criteria were followed in deciding if two images were from the same or different stars. These criteria were zone specific, but for most zones images were identified as coming from the same star if an image was within 3.5 arcsec of a mean position and there was no indication of duplicity. Images from different stars, but incorrectly identified as coming from the same star, will result in a high standard deviation in right ascension or declination, σα or σδ. Stars with the largest σα and σδ were investigated for this possibility.

### 7.2  Duplicate entries

Virtually every zone has mistakenly printed some of its measures more than once. These were easily found since the data in question was either exactly the same as another record (in cases of true duplication) or the resulting star positions of two records were ridiculously close for the telescope scale and typical seeing. In general, images closer than 1.0 arcsec that appear on the same plate were suspected of being duplicate entries. In cases of this type, generally only one of the measures was kept.

### 7.3  Blended images

Images of double stars that are blended on one plate but discrete on another require special treatment. Two possible problems exist under these circumstances. First, if the blend is identified as one of the separate images, then the computed separation of the double star will be smaller than it is in actuality (since the blend will fall near the photocenter). Second, if the blend is not identified with either discrete image, then three sets of coordinates will be computed where only two stars exist.

Under the first scenario (a blend is matched with a discrete image), the data will consist of a multiple star system with at least one star having a large standard deviation of position, σα or σδ. To investigate this possibility, the area around each star was searched for the presence of another star. If a nearby star was found, then the data were examined for a blend if σα or σδ of either star exceeded a certain amount (usually 1.0 or 0.9 arcsec). Additionally, any two stars closer than about 3.0 arcsec, or any triple or quadruple systems, were examined.

Under the second scenario, (a blend is not identified with either of the discrete images), then a star system will appear to have one more member than it really has. To minimize this occurrence, an area with a radius of about 10 arcsec around each star was searched for the presence of two other stars. Additionally, an area with a radius of about 15.0 arcsec was searched for three other stars. If multiples were found, they were examined to ensure that images were identified correctly. In the situation described above, blends are discarded.

### 7.4  Large proper motion candidates

Due to the span of plate epochs, images from large proper motion stars may be displaced from one another by several arcsec and hence not be matched as coming from the same star. To minimize this occurrence, a search in the Luyten's Two-Tenths Catalogue and its supplement was made to identify high proper motion stars. An additional search for large proper stars in the Third Catalogue of Nearby Stars was also made. Stars in the final catalogue having large standard deviations of position are known high proper motion stars.

### 7.5  Typographical errors

A bright star may only appear to have one image if one of its records contains a typographical error. To find and correct this, all bright, single image stars were investigated. (The exact magnitude limit depends on the zone. Typically all stars down to 11.5 are investigated.)

The process involved generating positions for these images if one altered one of the digits of the printed x or y value, then performing a search within 2 arcsecs around these pseudo-positions. If another image was found at one of the locations, a typographical error may be present. For zones with x,y values published in millimeters, investigations down to the tenths position were made (0.1 mm ~ 6 arcsec). For those zones with x,y values published in réseau units, investigations down to the hudreths position were made (0.01 réseau unit ~ 3 arcsec). The Twin Astrograph Catalogue (TAC; Zacharias et al. 1996), was used to determine which of the two records, if either, should be changed for all zones north of, and including, Tacubaya. For the more southerly zones which are not covered by TAC, the Tycho Input Catalogue (Halbwachs et al. 1994) has been used. If a corresponding star was not found in the TAC or TIC, then a search in the the Digitized Sky Survey using the Skyview interface was made to determine which, if either, contains a typographical error.

To minimize the possibility of a printing error in the magnitude data, a standard deviation of the magnitude, σ mag, was generated from the computed magnitudes for every star appearing on more than one plate. The stars with the largest σ mag were investigated. Additionally, range checks were made on the original data as well as the final computed magnitudes to ensure that all values were reasonable.

### 7.6  Investigation of other potential problems

To investigate the possibility of a few plates being adjusted incorrectly due to such oddities as several poor reference stars on a plate or incorrectly removing accurate reference stars for poorer ones, the positions of the 500 stars with the largest σα and σδ were plotted for each zone. Additionally, the percentage of stars on each plate whose σα and σδ are in the highest 1% of that zone were computed. Plates that have more of these than expected were investigated.

Positions of all stars were plotted to ensure that the data contain no missing areas, such as a block of data having not been typed or accidentally discarded. Positions of stars having only one image, and positions of stars having multiple images, were plotted separately to investigate the possibility of missing plates.

To avoid the presence of non-stellar objects in the final catalogue, a search in the New General Catalogue of Nebulae and Clusters of Stars (NGC), the Index Catalogue (IC), and the Second Index Catalogue (Sinnot 1988) was made. The Digitized Sky Survey, using the Skyview interface, was used to graphically show potential NGC and IC objects present in the data. Those records found to be non-stellar NGC or IC objects have been discarded.

## 8  Final Plate Model and Weights

It was necessary to ensure that the plate models developed in each zone and the corrections applied to the data were still valid because they were originally developed on the data that included erroneous identifications and typographical errors. Once any needed revisions were made, plate weights were computed. For most of the AC zones, a plate's weight is the inverse of the variance of the positions of the stars which it contains, after removing the stars with the highest 1% σα and σδ. The removal of these stars prior to computing the plate weights reduced the possibility that a few, high proper motion stars adversely affected the weighting of an entire plate.

No grating was used in the AC program, so bright stars are over-exposed on the plates and should not be used in the final adjustment nor be included in the final catalog. All stars with mean computed magnitudes brighter than 4.0 have been removed prior to final plate adjustments. Also removed from determining the final plate parameters were reference stars with residuals larger than 3.0 times the standard deviation of unit weight of a plate solution. Once removed, the plate adjustment is performed again. These stars are included in the final catalog, but are treated as field stars.

## 9  Linking of Individual Zones

The Astrographic Catalogue was observed in discrete zones in the sky, and the reductions, by necessity, were made on individual zones. However it was desirable to link these together to make one, cohesive catalog. To do this, stars in common to adjacent zones were identified. Blended images, high proper motion stars, and typographical errors were investigated as described above. Each star was assigned a new internal star number. All images were combined to yield weighted mean right ascensions and declinations on the system defined by Hipparcos (HCRS) at the weighted mean epochs of observation.

### 9.1  AC 2000.2 plots

Figures fig:errra1 through fig:epoch1 clearly demonstrate the zonal dependence of the AC 2000.2. Figures fig:errra1 and fig:errdec1 show the average positional error for a single image with respect to location on the sky. Of course individual errors will vary, and many of the stars have more than one image. Figure fig:epoch1 provides information on the mean epochs of observation as function of location of the sky. The combination of the positional errors and epochs being a function of declination has consequences for any proper motions derived using the AC 2000.2 data. That is, the proper motion accuracies will also contain this dependence. This was true for the now superseded ACT Reference and Tycho Reference Catalogues, both of which utilized earlier reductions of the Astrographic Catalogue data. It is also true for the Tycho-2 Catalogue, but some of this zonal dependence was eliminated by the inclusion of additional astrometric catalogues. Figure fig:zones1 shows, on the same scale as Figs. fig:errra1 through fig:epoch1, the observatories responsible for photographing and measuring each area of the sky.

The average density of the AC 2000.2 is 112 stars per square degree; however, the sky is hardly uniform. The mean density as a function of sky position is shown in Fig. fig:dens1. The galactic plane is clearly visible, where some areas exceed 500 stars per square degree. On the other extreme, for large areas surrounding the galactic poles, the density drops by more than half of the average.

As a courtesy to the reader, Figs. fig:errra1 through fig:dens1 are provided on this CD-ROM in a larger format. They can be found on this directory in the files errra1.ps, errdc1.ps, epoch1.ps, zones1.ps and dens1.ps; each is in postscript format.

## 10  Cross-Referencing Information

The numbering between the AC 2000.1 and AC 2000.2 has been maintained. It should be noted, however, that there is not a strict one-to-one correspondence between the two versions; the main reason is that some images now known to be from high proper motions stars were not identified as such in AC 2000.1. Note that AC 2000.1 contains 4,621,836 whereas AC 2000.2 contains 4,621,751 stars. However, the numbering between the two remains consistent. In other words, the star numbered 1'' in both versions refers to the same star.

In order to aid users and to facilitate other work utilizing the Astrographic Catalogue, most stars from the Hipparcos and Tycho-2 Catalogues have been identified. This cross-reference is not intended to be 100% complete; however, the vast majority of stars from these catalogues are identified.

## 11  Description of AC 2000.2

#### 11..1  Right ascension

The mean right ascension for each star as computed from its weighted images, in units of hours, minutes and seconds of time, referred to the Hipparcos system (HCRS, J2000.0) at the weighted mean epoch of observation.

#### 11..2  Declination

The mean declination for each star as computed from its weighted images, in units of degrees, minutes and seconds of arc, referred to the Hipparcos system (HCRS, J2000.0) at the weighted mean epoch of observation.

#### 11..3  B-Magnitude

The magnitude is either taken directly from the Tycho-2 Catalogue or is an average of the computed magnitude based on the measured image diameters from the AC plates. To determine which is the case, one will need to check the V-Magnitude field. All stars with the V-magnitude field non-blank contain Tycho-2 photometry in both the B-Magnitude and V-magnitude fields. Stars with the V-magnitude field blank contain magnitudes based on the measured image diameters on the AC plates, which should roughly correspond to the Tycho-2 BT system. See the section Conversion of the AC Magnitudes'' for details.

#### 11..4  Epoch

The mean epoch for each star as computed by its weighted images, in years.

#### 11..5  Number of images used

The number of individual images used to compute position, magnitude (if from image diameters), epoch and standard deviation of position.

#### 11..6  Standard deviation of weighted mean

The standard deviations of weighted means, σ\ and σ\ are computed for every star with more than one image. The formula used is:
 σ2x = x – x2/N′– 1 (23)
where
 x = αi, δi (24)
and
 N′≡N \left[ 2/ \right] (25)
where w is the weight of an individual observation (generally the same for all stars on a plate).

#### 11..7  AC 2000 number

This number is used in the reduction process to identify all images of the same star that may appear on different plates. This is generated at the U.S. Naval Observatory and added to the original x,y data. When the x,y data are released, these numbers can be used to link the data to the final catalog.

#### 11..8  Hipparcos number

If a star has been identified as being in the Hipparcos Catalogue then the Hipparcos number is provided. This cross-referencing information is not 100% complete.

#### 11..9  Tycho-2 number

If a star has been identified as being in the Tycho-2 Catalogue, then the Tycho-2 identifier is provided. Zeros have been inserted in blank fields. This cross-referencing information is not 100% complete.

#### 11..10  V-Magnitude

The magnitude listed here is taken directly from the Tycho-2 Catalogue, otherwise it is blank. There are 20 cases where the Tycho-2 BT is given in the B-Magnitude column, but their is no corresponding Tycho-2 VT magnitude. For these stars, the V-Magnitude is set to .000 and a 3' is set in the Magnitude Flag field.

#### 11..11  Magnitude Flag

This flag will give users caution regarding the photometry. A 1' is given if the star has a B magnitude fainter than 13.5 or a V magnitude from Tycho-2 VT fainter than 12.5. A 2' is given if the star was identified as a Tycho-2 star but the AC magnitude computed from the published diameters is given. This is done when the Tycho-2 BT magnitude is either not given or is listed as fainter than 14.00. No Tycho-2 VT magnitudes are given for these stars. In cases where both 1' and 2' are set, only 2' is given. A 3' is given if the star was identified as a Tycho-2 star, but there is no Tycho-2 VT magnitude. In these cases, the Tycho-2 VT magnitude is set to .000. There are 20 such instances. Only 1 star (AC 408635, Tycho-2 107200084801) should have a '3' flag in combination with another. For this star, the Tycho-2 BT is 13.51 and no Tycho-2 VT magnitude exists; the flag is set to 3'.

#### 11..12  Verification Flag

A 1' in this field indicates the star has a single image and is not found in Hipparcos, Tycho-2, ACRS_1999 or the Hubble Guide Star Catalogue 1.2. These stars'' may not exist but instead may be the result of typographical errors, plate defects or other such blunders.

## 12  Participating Observatories

Work from the observatories that participated in the photographing and measuring of the Astrographic Catalogue data are summarized below. Some important characteristics of each zone can be found in Table tab:obschar1 and in Appendix A. The telescope characteristics agreed upon were a normal astrograph with an aperture of roughly 33 cm and a this instrument having a focal length of 3.43 m. This created a scale close to 60 arcsec/mm on the plates. All observatories, with the exception of Nizamiah, used this type of instrument.

### 12.1  The Royal Observatory at Greenwich

The Royal Observatory at Greenwich was an original participating institution in the Astrographic Catalogue project, sending representatives to the International Congress on Astronomical Photography held in Paris in 1887. This meeting outlined the plans for the Carte du Ciel project. Funding was provided shortly following this meeting. A telescope built by Sir Howard Grubb following the design agreed upon in Paris was delivered in May 1890. Greenwich centered its plates between +65 and +90 degrees, with five plates taken on the pole in different orientations. In total, 1153 plates were exposed and measured in this zone. Three exposures were made on each plate lasting six minutes, three minutes and 20 seconds. The telescope was moved 20 arcsec between the exposures, so the six and three minute exposures are offset from each other in declination, the six minute and 20 second exposure are offset in right ascension. The plate epochs span from 1892 to 1905.

After initially measuring some plates using micrometer screws, Professor H.H. Turner realized that a different measuring technique was required if the job were to be completed in a timely fashion. At his request, an eyepiece scale type measuring machine was built and greatly reduced the time to measure a plate. Many other observatories adopted this measuring procedure. Systematic measuring of the plates began in October 1894. A duplex micrometer, which is a measuring machine capable of measuring two plate simultaneously, was put into use in February 1895. This aided the identification of images of the same star but on different plates since it was possible to arrange the plates in the machine so that the same field of sky was coincident under the measuring apparatus. (Remember that each plate overlapped surrounding plates so that each area of sky appears on at least two plates.) All plates were measured in two orientations, with the plates being rotated 180 degrees between measurements. In all degree bands except the +65, +66, and +67 degree bands, the same measurer was used for both orientations. Both the six minute and three minute exposures were measured for all stars that appeared on the 20 second exposure. Details can be found in the introductions to the published volumes (Christy & Dyson 1904-1932).

### 12.2  The Vatican Observatory

The Vatican Observatory, located in Vatican City, was founded in 1888 while the Carte du Ciel program was in its infancy. Vatican staff members realized that participation in this program would immediately give their young observatory international recognition. Pope Leo XIII commissioned Father Francesco Denza and Father Giuseppe Lais to attend the Astrographic Congress and enroll the Vatican as one of the participating institutions.

After being accepted as a participant, the Vatican commissioned the Henry brothers of France to build the telescope and P. Gautier to build a machine to measure the stars on the plates. Father Denza describes the finished telescope: The instrument consists of two parallel telescopes: The photographic telescope with an aperture of 33 cm and a focal length of 3.43 m; and the finding telescope or collimator with 20 cm aperture and a focal length of 3.6 m. Both are housed in a metal tube with a rectangular cross section of 37 by 68 cm. Both objectives are fixed on the same block of bronze at one end of the tube and at the other end is the photographic plate holder and the eyepiece of the collimator. A thin metallic diaphragm separates the two telescopes. ...The photographic objective is a doublet of flint and crown and it is both achromatic and aplanic for the most intense chemical rays of the spectrum.'' (Denza 1891).

The telescope was installed in the Leonine Tower in 1891. This tower, located on the highest point of Vatican Hill, was originally constructed in 840 AD under Pope Leo IV as a defense against the Saracen invasions. It is about 20 meters above ground (about 100 meters above sea level) with walls about 4.5 meters in thickness.

The Vatican was assigned the strip of sky between +55 and +64 degrees on which to center the plates. In order to achieve the two-fold sky coverage 1040 plates would be needed. (Actually, 1046 plates were exposed and measured.) The job of photographing and developing the plates was carried out primarily by one person, Father Lais, who worked on this for over 25 years until the time of his death in 1921. Lais's aid, Carlo Diadori, completed the photographing in 1922. For the first several years that Father Lais was working at the telescope, no plates were being measured.

Father Johann Hagen, appointed director of the Observatory in 1906, was committed to seeing the project completed. He soon realized that the machine built by Gautier to measure the stars was too slow. After investigating different measuring techniques employed by other institutions, he decided on the use of an eyepiece grid. In the method used by Vatican, a 10 × 10 mm area of a plate, corresponding to 2 × 2 réseau intervals, was magnified in a microscope along with a grid which is segmented into 0.05 mm steps. The edge of the grid was aligned with the edge of the 2 × 2 réseau interval area. The location of a star within the magnified area was read from its apparent position on the grid. This method was indeed efficient, allowing the Vatican to be one of the first observatories to complete its assigned zone despite utilizing minimal manpower. However, the accuracy was not as high as can be obtained with the measuring techniques used at other participating institutions. Vatican was the only observatory in the Astrographic Catalogue program to use the eyepiece grid technique. Also during his trips to other observatories, Father Hagen saw extensive use of women in measuring the plates, freeing the full-time, male astronomers from this time-consuming, repetitive task. He brought in three nuns from the Instituto di Maria Bambina to measure the plates. These nuns worked from 1910 to 1921 and measured the vast majority of the Vatican data. The Oxford University Observatory agreed to compute the plate constants used to convert the rectangular measures to equatorial coordinates.

For additional information concerning the history of the Vatican Observatory and its personnel, see the book In the Service of Nine Popes (Maffeo 1991). Additional details regarding the Vatican's participation in the Astrographic Catalogue project can be found in the introductions to the data (Vatican 1914-1928), written in Italian.

### 12.3  The Catania Observatory

The Catania University Observatory, located in Catania, Sicily, centered its plates between +47 and +54 degrees. In total, 1010 plates were exposed and measured in this zone. The epochs span from 1894 to 1932, but over 95% were exposed prior to 1906. All the plates were measured using the short-screw method. Additional details can be found in the introductions to the published data (Catania 1907-1963).

### 12.4  The Helsingfors Observatory

The Helsingfors Observatory, located in Helsinki, Finland, centered its plates between +40 and +46 degrees. In total, 1008 plates were exposed and measured in this zone. The epochs span from 1892 to 1909, but over 94% were exposed prior to 1897. All the plates were measured using the short-screw method. One screw was used; the plates were rotated 90 degrees to measure both x and y coordinates. The plates measured early in the work had images from both the longest and middle exposures measured in one orientation. Later (after 1896), only the images from the longest exposure were measured, but the plates were rotated 180 degrees between measurements so these images were measured twice (this also helped remove any bias a measurer may have). Various aspects of the work are detailed in the introduction to Volume 1 (Helsingfors 1903-1937).

### 12.5  The Potsdam Observatory

In 1887, at the meeting of the International Congress which established the Astrographic Catalogue, the zone with plate centers from +39 to +32 degrees was assigned to the Potsdam Observatory, Germany. Potsdam began photographing the plates by 1893 and 1226 plates were exposed by the end of 1900. Each plate had two exposures of 5 minutes duration. Unfortunately, the measurements of the plates lagged behind the exposures. By the start of World War I only 406 plates were measured. Following the war, Potsdam announced it could no longer continue with the project, and a re-photographing of its zone was made by Oxford, Uccle, and Hyderabad. The remaining plates were never measured. An allied bomb during World War II destroyed virtually the entire set of plates (Dick 1988, 1990).

In total, 406 plates were measured and published as the Potsdam zone. These plate are scattered throughout the zone, so many are not overlapped by others. All were measured using one of two short-screw type measuring machines. The plates were measured in one direction only; the plates were not rotated. Various aspects of the work are detailed in the introductions of the printed volumes (Potsdam 1889-1915).

### 12.6  The Nizamiah Observatory, Hyderabad

In 1887, at the meeting of the International Congress which established the Astrographic Catalogue, the zone from –17 to –23 degrees was assigned to the Observatory of Santiago, Chile. By 1900, the work was still not progressing, so a proposal to establish an observatory in Montevideo, Uraguay was made. This, too, did not progress so Santiago asked to re-undertake the project. At the same time, the Nizamiah Observatory, located in Hyderabad, India, offered to work on this zone as well. So, in 1909 there were two observatories offering to work on the –17 to –23 zone. A resolution passed by the Congress in 1909 assigned the –17 to –20 zone to Hyderabad. After completing the photographing and measuring of these four bands in 1920, the International Astronomical Union recommended that Hyderabad continue photographing down to –23 degrees declination. This work was completed in 1928. This zone between –17 and –23 degrees is known as the Hyderabad South zone. Hyderabad also observed a section of the sky in the Northern hemisphere that Potsdam was originally assigned. This zone, between +36 and +39 degrees, is known as the Hyderabad North zone.

In total, 1260 plates were exposed and measured in the Hyderabad South zone. The epochs span from 1914 to 1928, with only a handful taken after 1923. For the Hyderabad North zone, 592 plates were exposed and measured. The epochs span from 1928 to 1937, with just a very few taken after 1934. The telescope used was not one of the Henrys' design, as all the other AC participation observatories used. Instead of a 33 cm, the Hyderabad instrument, built by Cooke and Sons of York, had an aperture of only 20 cm. Its objective was described as a patent photo-visual lens''. The smaller aperture meant longer exposures were required to achieve the desired magnitude limit set for the AC. The telescope's focal length was 133 inches. All the plates were measured using one of four eyepiece scale type measuring machines, all built by Cooke and Sons. Various aspects of the work are detailed in the introductions of the printed volumes (Edinburgh 1918-1930, London 1934-1946).

### 12.7  The Uccle Observatory

The Royal Observatory of Belgium, located in Uccle, was assigned the zone with plate centers running from +34 to +35 degrees. Although Uccle was not an original participating observatory in the AC project, it became one because the Potsdam Observatory, originally assigned to cover this area, was unable to fulfill its commitment. The telescope used was of similar design as the Henry brothers, but built by Gautier. In total, 320 plates were exposed between 1939 and 1950. The epochs of the plates are spread fairly uniformly, except for a lack of plates exposed between mid-1943 and mid-1945. The measurements took place at the Paris Observatory with the use of three short-screw measuring machines. The réseau used was one from the Toulouse Observatory. An introduction can be found in Volume 1 of the printed catalog (Paris 1960,1962).

### 12.8  The Oxford Observatory

The University Observatory at Oxford was originally assigned the zone +25 to +31 degrees on which to center the plates. This is known as the Oxford I zone. An additional zone with plates centered on +32 and +33 degrees declination was photographed at Oxford after Potsdam announced they would not be able to complete their assigned area (+32 to +39). This two degree band is referred to as the Oxford II zone.

The telescope used was the same design as the Henry brothers' instrument located in Paris. The Oxford lens was made by Sir Howard Grubb and attached to an existing Grubb 12 1/4 inch, which was utilized as a guiding instrument. All plates of the Oxford 1 zone were taken between mid-1892 and 1910, with over 80% exposed by the end of 1903. These plates were measured mostly by boys from the New College Choir School, and Mr. T. J. Moore, a gardener who was interested in astronomy. The measuring apparatus was designed with an eyepiece scale, similar to that employed by the Greenwich Observatory in their AC work, with the exception that only one plate was measured at a time whereas Greenwich measured two.

Mr. F.A. Bellamy of University Observatory at Oxford supervised much of the work in photographing the 320 plates required to complete the Oxford 2 zone. He employed the same techniques used in the Oxford 1 zone and used the same Grubb refractor operated 30 years earlier. Mr. Bellamy would have photographed the entire zone himself, however he died with 32 fields left unobserved. These 32 fields were exposed at the Royal Observatory at Greenwich by Mr. H.G.S. Barrett with the telescope used for the Greenwich Zone (+65 to +90). These last 32 plates were the only plates in the Astrographic Catalogue that did not have a réseau exposed on them, however they were measured with one clamped on the glass. All plates of the Oxford 2 zone were exposed between 1930 and 1936. (Actually there are two plates that appear to have typographical errors in their epochs. One is dated in 1918; the other in 1930, but 8 months prior to any other plate.) Mr. Barrett also supervised the measurement and reduction of these plates at Oxford, using the eyepiece scale technique. However World War II intervened preventing the determination of the plate constants for 92 of the fields. After the war, the constants for these remaining plates were determined under Dr. H. Kox at the Hamburg Observatory at Bergedorf. A detailed introduction covering the participation of the University Observatory at Oxford in the Astrographic Catalogue project can be found in Volume 1 of the Oxford I zone catalogue (Turner 1906-1911), as well as in the book The Great Star Map also by Turner. An introduction to the Oxford II zones can be found in Volume 1 of the Oxford II zone catalog (Paris 1953-1954).

### 12.9  The Paris Observatory

The Paris Observatory agreed to photograph the zone with plate centers between declinations +18 and +24 degrees. The telescope used was the original Henry brothers' instrument, after which all other AC telescopes were supposed to be patterned. In total, 1261 plates were photographed and measured. The measuring technique employed at Paris was the short-screw method, which was the most accurate utilized with the AC plates. The epoch range of the plates are from October 1891 through November 1927, however only 7 plates were exposed after 1907 and all but 100 were exposed prior to 1900. An introduction to the Paris Observatory's participation in the Astrographic Catalogue can be found in the introductions to the individual volumes (Paris 1902-1932).

### 12.10  The Bordeaux Observatory

The Bordeaux University Observatory, located in Floirac, France, was assigned the zone between +11 and +17 degrees declination on which to center its plates. The telescope used was a similar design to the Paris instrument and was built by the Henry brothers. In total, 1260 plates were exposed between 1893 and 1925, all but five being taken before 1913. The plates were measured at Bordeaux using the short-screw method. An introduction to the Bordeaux Observatory's participation in the Astrographic Catalogue project can be found in Volume 1 of the published data (Paris 1905-1934).

### 12.11  The Toulouse Observatory

The Toulouse University Observatory (France) agreed to participate in the Astrographic Catalogue project by taking plates centered between +5 and +11 degrees declination. In total, 1260 were exposed and measured. The epochs vary from 1893 to 1935, and were taken in three fairly distinct groupings. Most were exposed before the end of 1910. Another set is taken between 1918 and 1922. The last few plates are scattered between 1930 and 1935. All the plates were measured using a short-screw type measuring machine; however, not all plates were measured at Toulouse. Ninety plates were measured at the Bordeaux University Observatory and 36 were measured at the Paris Observatory. Various aspects of the work are detailed in the introductions of the printed volumes (Paris 1903-1948).

### 12.12  The Algiers Observatory

The Algiers Observatory was assigned the zone between –2 and +4 degrees on which to center its plates. All 1260 plates were exposed between 1891 and 1911. The plates were measured using the short-screw method. Details about the Algiers Observatory's participation in the Astrographic Catalogue project can be found in the introduction to the catalogues (Trépied 1903,Paris 1903-1924).

### 12.13  The San Fernando Observatory

The Naval Observatory of San Fernando (Spain) was assigned the area between –3 and –9 degrees declination on which to center its plates. The telescope used was built by Gautier, with the objective made by the Henry brothers. All of the 1260 plates were exposed between 1891 and 1917. Over 1000 were taken before 1899, then only a few per year until 1917. The plates were measured using a short screw micrometer. An introduction to San Fernando's participation in the Astrographic Catalogue project can be found in Volume 1 of the published data (San Fernando 1921-1929).

### 12.14  The Tacubaya Observatory

At the meeting of the International Congress which established the Astrographic Catalogue, the zone from –10 to –16 degrees was assigned to the National Astronomical Observatory of Tacubaya, located near Mexico City, Mexico. In total, 1260 plates were exposed and measured in the Tacubaya zone. (Actually, one of the plates had an incorrect and unknown plate center and has been discarded from the reductions, leaving a total of 1259 plates). All but five plates were exposed between 1900 and 1912. The five later plates were exposed between 1926 and 1938. All the plates were measured using the eyepiece scale method. An introduction to the history of the work can be found in Volume 1 part 1 of the –15 degree zone (Tacubaya, 1913-1962).

### 12.15  The Cordoba Observatory

At the 1887 Paris meeting, the zone from –24 to –31 degrees was assigned to the La Plata Astronomical Observatory. In 1900, the zone was re-assigned to Cordoba. The telescope was installed at the end of 1901. In 1908, Dr. Thome, the Director of Cordoba, died and Dr. Perrine was named new director. After some investigations, Perrine decided to re-observe all areas. He discovered that the telescope was out of focus and many of the plates were impaired, and that the plates were centered on apparent place coordinates, not at those defined at equinox 1900. Plates of this series were exposed starting in 1909 and finished by the end of 1913. These plates were measured between 1909 and 1920. In total, 1360 plates were exposed as part of the Astrographic Catalog.

The telescope used was one of the Henrys' design and build, following the standard with a 33 cm aperture and 3.47 m focal length. In the introduction written by Perrine, he states that the guiding is difficult because the guide scope has only a 19 cm aperture, as opposed to the more conventional 25 cm in use by most of the participating observatories. The mount was built by Gautier. Some plates showed a triangular distortion'' that was traced to a warp in the ring which held the lens in place. This ring was replaced in 1911. The lens, from August 9, 1910 until the end of the program, was stopped down to 11  inches. This, according to Perrine, greatly improved the image quality.

Virtually all plates were taken and developed by R. Winter or F.P. \linebreak Symonds. Four exposures on each plate were made; two long exposures of the same duration (both of 5 or 6 minutes) one medium exposure (of 60 to 90 seconds) and one short exposure (of 5 to 8 seconds). The telescope was moved in declination between exposures. In order to expedite the work, two measuring machines of the short-screw type were retro-fitted with eyepiece scales. As a result, 140 plates were measured using the short-screw method, the remaining 1220 plates were measured using the eyepiece scale method. In total, five different measuring machines were used, allowing each measurer to have his or her own machine. The réseaux used were supplied by Gautier and Prin; four were used throughout the work. Investigations of two of the réseaux were made in Paris and the deviations were found to be negligible; no corrections for the réseaux were applied to the measures. Only stars within one degree in right ascension and declination of the plate center were measured. All stars having three images were measured unless images ran together, which was the case of the brightest stars. Four measures were made on each star; a measure was made on both of the long exposures in both orientations of the plate. (Following an initial measurement of all stars, the plate was reversed 180 degrees and all stars re-measured.) In general, measures in the direct and reverse orientations of the plate were made on the same day by the same measurer. In total, 37 man-years went into measuring the plates. Various aspects of the work are detailed in the introduction of Volume 26 of the Observatory Results (Cordoba 1925-1934).

### 12.16  The Perth Observatory

At the International Congress which established the Astrographic Catalogue, the Observatory of Rio de Janeiro was assigned to photograph the area between –32 and –40 degrees declination. In 1900, the work at Rio had not progressed and so the Perth Observatory undertook the task. The telescope used was from Sir Howard Grubb, and was of similar design to other telescopes used for the AC work. Although observing was progressing, no resources were available to measure the plates. At this time, the Perth Observatory was primarily a meteorological station, and the meteorological work took precedence. This changed in 1908 when the Australian Federal Government established the Australian Weather Bureau. About this time, four women were hired as plate measurers. Professor Dyson of the Edinburgh Observatory offered assistance in the measuring. Perth accepted this offer and started sending those plates of the –40 degree zone. By 1915, the Edinburgh Observatory completed its commitment by measuring all of the plates centered on the –40, –39 and –38 degree band. This area is known as the Perth-Edinburgh AC zone. Observing continued at Perth until 1919, at which time all areas had been photographed. Personnel at Perth measured all plates between –37 and –32 degrees; this is known as the Perth zone. In total, 432 and 944 plates make up the Perth-Edinburgh and Perth zones, respectively.

All plates had three exposures taken, one of 4 minutes, 2 minutes and 13 seconds, or that of 6 minutes, 3 minutes or 20 seconds. The change in exposure times took place following a 1909 meeting of key personnel from different observatories participating in the AC. At that meeting, many people expressed concern about the uniformity of limiting magnitudes on different plates. Many of the Perth plates were re-examined and found to be unsatisfactory. The areas affected were re-observed and a method of ensuring more uniformity was developed.

In general, no guiding of the instrument other than the sidereal drive was made, as it was found unnecessary. All plates were the brand Ilford special rapid''. All were measured using an eyepiece scale machine, similar to the Oxford Observatory's. For all but 5 plates, the measurements done at Perth were made in two orientations of the plate by the same person; the plate being rotated 180 degrees between measurements. In total, 10 measurers were used at Perth. For the plates measured at Edinburgh, images were measured in two orientations of the plate, with the plate being rotated 180 degrees between measurements. For the direct orientation, the second exposure (either 2 or 3 minutes) was measured; the longest exposure was measured with the plate in reverse orientation. Quite often more than one measurer was used on each plate. Various aspects of the work can be found in the introduction to Volume 23 of the Perth-Edinburgh data (Perth 1922, Paris 1949-1952) and in Volumes 1 and 17 of the Perth data (Perth 1911-1921).

### 12.17  The Royal Observatory at the Cape of Good Hope

The Royal Observatory at the Cape of Good Hope, South Africa, took the zone between –41 and –51 on which to center its plates. In all, 1512 plates were exposed and measured. The epochs range from 1897 to 1912, with 97% of them being exposed prior to 1906. The telescope used was built by Sir Howard Grubb. Two machines were used to measure the plates, both were designed by David Gill and built by Repsold of Hamburg (Gill 1898). Both machines employed the short-screw measuring method and were of similar design. An introduction to the participation of the Royal Observatory at the Cape of Good Hope in the Astrographic Catalogue project can be found in Volume 1 of the published data (London 1913-1926) .

### 12.18  The Sydney Observatory

H.C. Russell, the Government Astronomer at Sydney, was in attendance at the 1887 meeting of the International Congress. He committed the Sydney Observatory to photograph the sky between declinations –52 and –64 degrees. The lens used for the project was built by Howard Grubb of Dublin, and it followed the general design established by the International Congress. The lens was delivered in December of 1890. Most of the telescope was built in Sydney. The observing program began in earnest in 1892. The telescope was moved twice in the course of the AC work; first from inside Sydney to Redhill (located about 12 miles from Sydney) in 1899, and then back to its original location in 1931. The observing was left unchanged until 1912, when W.E. Cooke took over the project. He was unsatisfied with the quality of many plates and eventually rejected and rephotographed many of the areas. Until this time, plates were being sent to Melbourne for measurement, but under Cooke the Sydney Observatory began to measure their own photographs. Prior to Cooke's arrival, plates were positioned so the center of the plate was in the sharpest focus (The astrograph used in photographing the plates did not have a flat field of focus, so some areas of the plate are in focus while others are not. This is true for all the telescopes used in the Astrographic Catalogue work). Cooke altered this and made the ring about 50 arcmin from the center the place with the sharpest focus. In 1926, Cooke retired and James Nagle took over. He altered the place of best focus in a ring about 40 arcmin from plate center. Nagle died in 1941, and H.W. Woods took over. Some plates were found unsatisfactory or missing. The remaining areas were photographed between 1944 and 1948. In total, 1400 plates were taken as part of the Astrographic Catalogue. All plates exposed between 1890 and 1930 were taken by James Short.

Plate measuring did not start for about six years after the first plates were taken. Evidently there was a suggestion about having all plates from all participating observatories sent to Paris for measurement. This plan was not ever put in place, but the Australians liked this idea so they decided that Melbourne would be used to measure both the Sydney and Melbourne plates. As mentioned above, this changed with Cooke's arrival and Sydney started measuring their own plates. There were four short-screw measuring machines used in Melbourne, and two eyepiece scale machines used at Sydney. From this point on, all stars were measured twice with the plates being rotated 180 degrees between measurements. Various aspects of the work are detailed in the introduction of Volume 53 of the data (Sydney 1925-1971).

### 12.19  The Melbourne Observatory

In 1887, following the meeting which established the Astrographic Catalogue, the British government agreed to have the Melbourne Observatory participate in the project. Melbourne was assigned the zone from –65 to –90 degrees. The telescope used was built by Howard Grubb of Dublin, and it followed the general design established by the International Congress. The telescope was delivered in December of 1890. The observing program started about one year later in January 1892 and continued until 1927 (Actually, one plate was exposed in 1940). Over 80% of the plates were exposed prior to 1898. In total, 1149 plates were taken as part of the Astrographic Catalogue. All plates had three exposures of 5 minutes, 2.5 minutes and 20 seconds duration, with the exception of the plates taken prior to February 26, 1892, whose exposures were slightly longer. The réseau was exposed on the plates shortly after the plates were removed from the telescope. In total, six different réseaux were used during the program; three were supplied by Gautier and three were made at Melbourne.

Plate measuring did not commence in earnest until November 1898, when six women were hired at Melbourne. Two measurers were used for each plate, one taking the northern half and one the southern. The plates were rotated 180 degrees and each half was remeasured by the same person. All reference stars on each plate were measured by both measurers. Four measuring machines were used throughout most of the work; all used short-screws for the star measurements. The Melbourne staff tried using an eyepiece scale measuring machine for the plates but found the measuring error too high.

Periodic and progressive screw errors were investigated. These were not applied, as they were found to be negligible by the Melbourne astronomers. (Investiations of the screw errors performed at the U.S. Naval Observatory as part of the AC 2000 work show this not to be true.) Investigations into errors of the réseaux were made and these corrections were applied to the data prior to publishing. Various aspects of the work are detailed in the introduction of Volume 1 of the data (Melbourne 1926-1929; Paris 1955-1958; Sydney 1963).

## 13  References

• Catania, 1907-1963, Catalogo Astrofotografico Internazionale 1900 Vol 1 through 8
• Christy, W.H.M., and Dyson, F.W., 1904-1932, Astrographic Catalogue 1900.0, Greenwich Section Vol 1 through 6
• Corbin, T.E. and Urban, S.E., 1988, IAU Symposium 133 Mapping the Sky, ed. S. Debarbat, J.A. Eddy, H. K. Eichhorn and A. R. Upgren, Kluwer, Dortrecht, p. 287
• Corbin, T.E., and Urban, S.E., 1990, IAU Symposium 141 Inertial Coordinate System on the Sky, ed. J. Lieske and V.K. Abalakin Kluwer, Dortrecht p. 433.
• Corbin,T.E., and Urban, S.E., 1991, Astrographic Catalog Reference Stars, NASA, NSSDC 91-10
• Corbin,T.E., 1977, The Proper Motion System of the AGK3r. Ph.D. dissertation, Leander McCormick Observatory, Univ. of Virgina
• Corbin,T.E., 1991, International Reference Stars, NASA, NSSDC 91-11
• Cordoba, 1925-1934, Resultados del Observatorio Nacional Argentino, Vol 26 through 34
• Denza, F., 1891, Cenni Strorici sulla Specola Vaticana, Pubblicazioni della Specola Vaticana (1891) Fascicolo II, p. 88
• Dick, R.W., 1988, Info Bull of CDS, 34 155
• Dick, R.W., 1990, Info Bull of CDS, 38 19
• Edinburgh, 1918-1930, Astrographic Catalogue 1900.0 Hyderabad Section, Vol 1 through 7
• Eichhorn, H., 1974, Astronomy of Star Positions, Fredrick Ungar, p. 286
• ESA, 1997, The Hipparcos and Tycho Catalogues, SP 1200
• Germain, M.E., 1997, Variance of a Weighted Mean with Astrometric Applications, in preparation
• Gill, D., 1898, MNRAS 59 61
• Gliese, W., and Jahreiss, H., 1991, Preliminary Version of the Third Catalog of Nearby Stars
• Halbwachs J.L., Baessgen G., Bastian U., Egret D., Hoeg E., van Leeuwen F., Petersen C., Schwekendiek P., Wicenec A., 1994, A&A 281 25
• Helsingfors, 1903-1937, Catalogue Photographique du Ciel zone de Helsingfors Vol 1 through 8
• Høg, E., Fabricius, C., Makarov, V.V., Urban, S., Corbin T., Wycoff, G., Bastian, U., Schwekendiek, P., and Wicenec A., 2000, A&A, 355, L27
• IAU. 1999, Trans. IAU, 23B 39
• IAU. 2001, Information Bull. 88, Resolution B1.2, p 29
• London, 1913-1926, Cape Astrographic Zones, Vol 1 through 11
• London, 1934-1946, Astrographic Catalogue 1900.0 Hyderabad Section, Vol 9 through 12
• Luyten, W.J., 1979, 1980, New Luyten Catalogue of Stars with Proper Motions Larger than Two Tenths of an Arcsecond (Minneapolis: University of Minnesota)
• Luyten, W.J. and Hughes, H.S., 1980, Proper Motion Survey with the Forty-Eight Inch Schmidt Telescope. LV. First Supplement to the NLTT Catalogue (Minneapolis: University of Minnesota)
• Maffeo, S. 1991, In the Service of Nine Popes: 100 years of the Vatican Observatory, Vatican Observatory Foundation
• Melbourne, 1926-1929, Melbourne Astrographic Catalogue 1900.0, Vol 1 through 3
• Paris, 1902-1932, Catalogue Photographic du Ciel, Vol 1 through 7
• Paris, 1903-1924, Observatoire D'Alger Catalogue Photographique du Ciel, Vol 1 through 7
• Paris, 1903-1948, Observatoire du Toulouse, Catalogue Photographique du Ciel, Vol 1 through 7
• Paris, 1905-1934, Observatoire de Bordeaux, Catalogue Photographique du Ciel, Vol 1-7
• Paris, 1949-1952, Astrographic Catalogue 1900.0, Perth Section, Vol 1 through 3
• Paris, 1953-1954, Astrographic Catalogue 1900.0, Potsdam-Oxford Section, Vol 1-2
• Paris, 1955-1958, Melbourne Astrographic Catalogue 1900.0, Vol 4 through 7
• Paris, 1960 and 1962, Catalogue Photographique du Ciel, Zone Uccle-Paris, Vol 1 and 2
• Perth, 1911-1921, Astrographic Catalogue 1900.0, Perth Section, Vol 1 through 24
• Perth, 1922 , Astrographic Catalogue 1900.0, Perth Section, Vol 33
• Polozhentsev, D., Potter, K., Sal'Es, R., Selaiia, K., & Bystrov, N., 1989, Astronomicheskii Zhurnal 66 425
• Potsdam, 1889-1915, Publicationen des Astrophysikalishen Observatoriums zu Potsdam, Photographische Himmelskarte Catalog, Vol 1 through 7
• San Fernando, 1921-1929, Catálogo Astrográfico para 1900.0, sección del Observatorio de Marina de San Fernando, Vol 1 through 8
• Sinnot, R.W., 1988, NGC 2000.0, The Complete New General Catalogue and Index Catalogue of Nebulae and Star Clusters by J. L. E. Dreyer, ed. R. W. Sinnott, Sky Publishing Corporation and Cambridge University Press
• Sydney, 1925-1971, Astrographic Catalogue 1900.0, Sydney Section, Vol 1 through 53
• Sydney, 1963, Melbourne Astrographic Catalogue 1900.0, Vol 8
• Tacubaya, 1913-1962, Catalogo Astrofotografico 1900, Vol. 1 through 7
• Trépied, C. 1903, Observatoire D'Alger Catalogue Photographique du Ciel, Introduction
• Turner, H.H., 1906-1911, Astrographic Catalogue 1900.0, Oxford Section, vol 1 through 7
• Turner, H.H., 1912, The Great Star Map, John Murray
• Urban, S.E. and Corbin T.E., 1996, A&A 305 989
• Urban, S.E., Martin, J.C., Jackson E.S. and Corbin, T.E., 1996, A&AS 118 163
• Urban, S.E., Corbin, T.E., Wycoff, G.W., Martin, J.C., Jackson, E.S, Zacharias, M.I., and Hall, D.M., 1998, AJ 115 1212
• Vatican, 1914-1928, Catalogo Astrografico 1900.0, Sezione Vaticana, Vol 1-10
• Worley, C.E. and Douglass, G.G., 1996, The Washington Double Star Catalog
• Zacharias, N., de Vegt, C. Nicholson, W. and Penston, M.J., 1992,
A&A 254 397
• Zacharias, N., Zacharias, M.I., Douglass G.G., and Wycoff, G.L., 1996, AJ 112 (5), 2336
• Zacharias, N. & Zacharias M., 1999, AJ 118 2503
• Zacharias, N., Zacharias M., & de Vegt, C., 1999, AJ 117 2895

## A  Notes on Participating Observatories Data Characteristics

• Greenwich notes:
• Five plates are centered on the pole.
• A duplex micrometer was used; two plates were measured simultaneously. Formats of the published volumes reflect this.

• Vatican notes:
• One plate has a published epoch of 1891.
• A measuring grid was used instead of eyepiece scale or screw.
• Most plates were measured by one of three women. Measurer E'', who measured about 50% of the plates, was significantly worse than the other two.
• Plate weights, computed at USNO, are based on measurer.

• Catania notes:
• Over 95% of the plates were exposed prior to 1906.
• Errata are found throughout volumes, mostly at the end of each plate's x,y data.
• Measures of large images (bright stars) are often listed many times (with slightly different values).
• Multiple stars were often measured both as center of light AND individual images.
• One plate has been taken with the same plate center as another, but only faint stars measured on the second. Hence only reference stars near the edges were available on that plate.
• Many typographical errors in the printed volumes were found by USNO. These have been corrected.

• Helsinki notes:
• Over 95% of the plates were exposed before 1897.
• Helsinki was under Russian rule during the period of exposure and measurement of the plates.

• Potsdam notes:
• The number of plates exposed was 1226, of which only 406  were measured and published. No funds after World War I were available to continue the work.
• All but 34 plates were destroyed during World War II.
• In cases where stars were measured and published twice, USNO used the mean of the measures in the reductions.
• Personal equation tables (describing observer dependent magnitude equation) are published. USNO developed its own corrections by observer that supersedes the Potsdam tables.
• Generally, only stars brighter than 11.0 were measured
• Plate weights, computed at USNO, are based on measurer.

• Nizamiah Observatory had the only instrument not of standard design. Objective diameter was 20 cm (not 33 cm) so longer exposures were required to achieve the desired limiting magnitude. The plate scale was approximately 61 arcsec/mm.
• This zone was photographed in response to the Potsdam region not being completed.
• India was under British rule during the period of exposure and measurement of the plates.

• Uccle notes:
• This zone was photographed in response to the Potsdam region not being completed.
• One plate was broken prior to diameter measurements.
• Measurements were made at Paris.
• Hour angles of exposures are not given.

• Oxford II notes:
• This zone was photographed in response to the Potsdam region not being completed.
• Thirty two plates in this zone were exposed and measured at Greenwich. These are the only plates of the AC without a reseau printed on them.
• One plate has a published epoch of 1918, 12 years prior to the supposed start of this region.
• Kox (Hamburg Observatory) determined plate constants for 92 of the plates. All others were computed at Oxford.

• Oxford I notes:
• Some reseaux had rulings of 5.04 mm (5.00 mm was standard). Introductions indicate which ones, but the introduction appears to be wrong for 3 plates.

• Paris notes:
• Only 7 plates were exposed after 1907.
• Corrections indicated in the ARI version (1991) of the keypunched data have been applied.
• Additional typographic errors found in Bull. of CDS 12 (p32-40) by Dunham and Herget were applied, where needed.
• One plate was printed with the x,y measures of two plates (see end of Volume 2).
• Nine plates were measured using Baillaud method, not utilizing the reseau. Since the field distortion pattern applied during USNO's redudction is believed to arise from the reseaux, these corrections were not applied to these nine plates.
• Notes indicate that several stars were not measured to their full precision (0.1 microns). Images were not kept if the x,y values were not given to 1.0 micron or better.

• Bordeaux notes:
• Both Toulouse and Bordeaux have plates centered on +11 degrees.
• Only five plates were taken after 1913.
• Corrections indicated in the ARI version (1991) of the keypunched data have been applied.
• Many more typographical errors were found by USNO in this zone than most other zones.

• Toulouse notes:
• Both Toulouse and Bordeaux have plates centered on +11 degrees.
• Ninety plates were measured at Bordeaux and 36 plates were measured at Paris.
• Corrections indicated in the ARI version (1991) of the keypunched data have been applied.
• Notes indicate that several stars were not measured to their full precision (0.1 microns). Images were not kept if the x,y values were not given to 1.0 micron or better.
• The odd zones(+5,7,9 and 11) are split. The plates of the first part (usually 0 to about 6h) do not have reseaux corrections applied, but information was given as to which measuring machine (1 or 2) and which reseau was used. After the split, there is no information regarding micrometer or reseaux. For the even zones, there is information regarding where the plates were measured and often what types of plate emulsions were used. However, there are no data regarding which reseau was used.

• Algiers notes:
• Algeria was under French rule during the AC data acquisition.

• San Fernando notes:
• Y-coordinate increases to South
• 80% of the plates were taken before mid-1898.
• One plate was exposed with Saturn and 3 of its satellites.
• Four records have footnotes saying body moved during exposure of the plate''.

• Tacubaya notes:
• All but five plates were taken before 1913.
• One of the plates was not used by USNO because it has an unknown plate center.
• Many typographical errors in the printed volumes were found by USNO. These have been corrected.
• The type is very poor in some of the volumes.

• The Nizamiah Observatory had the only instrument not of standard design. The objective's diameter was 20 cm (not 33 cm) so longer exposures were required to achieve the desired limiting magnitude. The plate scale is approximately 61 arcsec/mm.
• India was under British rule during the program.
• Five reseaux were used, some with non-standard rulings. Rulings of 5.00, 5.04 and 4.985 mm used.
• Y increases to South.

• Cordoba notes:
• 1220 plates were measured using an eyepiece scale, 140 were measured with a short-screw.
• Errata were found in Information Bulletin of CDS 22 79-86 (1982).
• Lens was stopped down to 28 cm in August 1910 to give better images.
• Reseaux with rulings of 5.00 mm were used for all plates except those numbered 2001 through 2290. These have 5.05 mm between rulings. Note that this is slightly different than what is given on page xviii of Volume 26.
• Many typographical errors in the printed volumes were found at USNO. These have been corrected.

• Perth notes:
• This zone covers only those plates that were both photographed AND measured at Perth (see Perth-Edinburgh notes).
• A change of scale for plate 2063 through 2209 is present. This was the result of someone altering the telescope focus.
• A Scale value, usually given as a letter, is used as an estimate of magnitude for all but the brightest stars.
• Very few typographical errors were found at USNO.

• Perth-Edinburgh notes:
• This zone covers plates taken at Perth, but measured at Edinburgh, Scotland.
• Very few typographical errors were found at USNO.
• Y increases to South.

• Cape notes:
• Images measured with micrometer one have significantly lower mean errors than those measured with micrometer two.
• Actual diameters were measured for many stars, but the faintest stars were given a density'' measurement, dependent on how dark each image appears.
• South Africa was under British control during the AC work.

• Sydney notes:
• The telescope was relocated twice during the AC work.
• The plate location of sharpest focus altered during the work, depending on observatory director (see Section 12).
• Plates measured prior to 1912 were sent to Melbourne for measurement, using screw method. Later, the plates were measured at Sydney with an eyepiece scale.
• The plates were exposed in three discrete epoch spans, 1892 to 1901, 1918 to 1929, and 1944 to 1948.
• Many typographical errors in the printed volumes were found at USNO. These have been corrected.
• Authors suspect a magnitude equation by measurer, but no identification of the measurers of individual plates is given.
• Scale value as estimate of magnitude is given for the majority of images.
• X increases to the West

• Melbourne notes:
• Over 80% of the plates were taken before 1898.
• One plate has an epoch of 1940.
• Additional Melbourne errata were found in Sydney Volume 53, pg 64.


Bytes  Format  Units     Label   Explanations

1- 2 I2     h      RAh              Right Ascension (hours)
4- 5 I2     min    RAm              Right Ascension (minutes)
7-12 F6.3   s      RAs              Right Ascension (seconds)
14 A1     —    DE-              Declination (sign)
15-16 I2     deg    DEd              Declination (degrees)
18-19 I2     arcmin DEm              Declination (minutes)
21-25 F5.2   arcsec DEs              Declination (seconds)
27-31 F5.2   mag    B(mag)           Magnitude - Tycho-2 BT if col 86-91 are
non-blank; else from image diameters
33-40 F8.3   yr     Ep               Mean epoch of position
42-43 I2     —    Num              Number of images used
44-49 F6.3   arcsec e_RAs           ?Standard deviation of mean, RA
50-55 F6.3   arcsec e_DEs           ?Standard deviation of mean, Dec
57-64 I8     —    AC2000           AC 2000 Number
66-71 I6     —    HIPP             ?Hipparcos Number
73-84 A12    —    TYCHO-2          ?Tycho-2 ID
86-91 F6.3   mag    V(mag)           ?Magnitude - Tycho-2 VT else blank
92-92 I1     —    MGFL             ?Magnitude flag
93-93 I1     —    VER              ?Verification Flag


Byte-by-byte description of AC 2000.2 using format requested by the CDS.


Zone         Country    declination mearuring  num of      epoch
or region  of centers  technique  plates

Greenwich    England     +90  +65  scale   1153  1892  1905
Vatican      Italy       +64  +55  grid    1046  1895  1922
Catania      Sicily      +54  +47  screw   1010  1894  1932
Helsinki     Finland     +46  +40  screw   1008  1892  1910
Potsdam      Germany     +39  +32  screw   406   1893  1900
Hyderabad N  India       +39  +36  scale   592   1928  1938
Uccle        Belgium     +35  +34  screw   320   1939  1950
Oxford II    England     +33  +32  scale   320   1930  1936
Oxford I     England     +31  +25  scale  1188   1892  1910
Paris        France      +24  +18  screw  1261   1891  1927
Bordeaux     France      +17  +11  screw  1260   1893  1925
Toulouse     France      +11  +5   screw  1260   1893  1936
Algiers      Algeria     +4  –2   screw  1260   1891  1912
San Fernando Spain       –3 –9   screw  1260   1891  1918
Tacubaya     Mexico     –10 –16  scale  1259   1900  1938
Hyderabad S  India      –17 –23  scale  1260   1914  1929
Cordoba      Argentina  –24 –31  both   1360   1909  1913
Perth        Australia  –32 –37  scale   944   1902  1919
Perth-Edinb. Australia  –38 –40  scale   432   1903  1914
Cape         S. Africa  –41 –51  screw  1512   1897  1912
Sydney       Australia  –52 –64  both   1400   1892  1948
Melbourne    Australia  –65 –90  screw  1149   1892  1928


Characteristics of each observatory's plates


Zone          stars    images  keyed and verified
(thousand)(thousand)

Greenwich    179  322  USNO
Vatican      256  480  CIDA, Venezuela (verified USNO)
Catania      163  320  USNO
Helsinki     159  284  USNO
Potsdam      108  143  CIDA, Venezuela (verified USNO)
Uccle        117  159  USNO
Oxford II    118  161  USNO
Oxford I     277  471  Strasbourg
Paris        254  436  Strasbourg
Bordeaux     224  355  Strasbourg
Toulouse     270  433  Strasbourg
Algiers      200  330  Strasbourg
San Fernando 226  346  USNO
Tacubaya     312  518  USNO
Cordoba      309  467  Sternberg, Russia
Perth        228  402  USNO
Perth-Edinb. 139  202  USNO
Cape         545  901  USNO
Sydney       431  743  USNO
Melbourne    218  392  Univ of Fla., USNO


Numbers of images in each zone and institutions aiding in keypunching the data, by zone


Zone            prepared  reduced

Greenwich      Gary Wycoff, John Martin                Sean Urban
Vatican       John Martin                             Sean Urban
Catania       John Martin, Gary Wycoff                Sean Urban
Helsinki      Harry Crull, Edward Jackson, David Hall  Sean Urban
Potsdam       Marion Zacharias                        Marion Zacharias
Hyderabad N   Edward Jackson                          Sean Urban
Uccle         Edward Jackson                          Sean Urban
Oxford II     John Martin                             Sean Urban
Oxford I      John Martin                             Sean Urban
Paris         Edward Jackson                          Sean Urban
Bordeaux      Edward Jackson                          Sean Urban
Toulouse      David Hall                              Sean Urban
Algiers       Edward Jackson                          Sean Urban
San Fernando  Gary Wycoff                             Sean Urban
Tacubaya      John Martin, Gary Wycoff                Sean Urban
Hyderabad S   Edward Jackson, Sean Urban              Sean Urban
Cordoba       Gary Wycoff                             Sean Urban
Perth         David Hall, Gary Wycoff, John Martin    Marion Zacharias
Perth-Edinb.  David Hall, Gary Wycoff                 Sean Urban
Cape          Sean Urban                              Sean Urban
Sydney        David Hall, Gary Wycoff                 Sean Urban
Melbourne     Gary Wycoff                             Sean Urban


Personnel involved in USNO plate reductions

adjustment. Also includes single image precisions by zone.]Corrections applied to x,y data prior to final least squares adjustment. Also includes single image precisions by zone.

In the table, a Y'' indicates a correction was applied, an n'' indicates it was not. Values in parentheses indicates an additional dependence on those characteristics.

mg=magnitude, mr=measurer, mp=microscope, rs=reseau, ep=epoch,
x=x-coordinate, and y=y-coordinate.

Note that Ox II/Grn zone'' refers to the 32 plates observed at Greenwich for the Oxford II zone.


Zone        radial dist Mag Eq.       Coma   Screw  FDP      \ ra    \ dec

Greenwich     Y(mg)      Y             n      n      n         .29  .30
Vatican       Y(mg)      Y             n      Y(mr)  Y(mp)     .42  .44
Catania       Y(mg)      Y(x,y)        n      n      Y(mg)     .33  .30
Helsinki      Y(mg)      Y(x,y)        Y      Y(mr)  Y         .23  .22
Potsdam       n          Y(mr)         Y      n      Y         .26  .25
Hyder. N      Y          Y(x,y)        Y      n      Y(mg)     .33  .30
Uccle         Y(mg,x,y)  Y             Y      Y      Y(mg)     .33  .38
Oxford II     n          Y(x,y)        n      n      Y         .33  .32
Ox II/Grn     Y          Y             n      n      n
Oxford I      n          Y(x)          n      n      Y         .32  .31
Paris         Y(mg)      Y             Y      Y(mp)  Y(rs)     .21  .21
Bordeaux      n          Y(ep)         n      Y      Y(rs)     .22  .21
Toulouse      Y(mg,mp)   Y(y)          n      Y(mp)  Y(rs)     .29  .28
Algiers       Y(mg)      Y(y)          n      n      n         .19  .18
San Fer.      Y(mg)      Y             Y      n      Y(mg)     .32  .33
Tacubaya      n          Y(x)          n      n      Y(mg)     .25  .24
Hyder. S      Y(mg)      Y(x,y)        Y      n      Y(mg,rs)  .31  .32
Cordoba       Y(mg)      Y(x,y)        n      Y(mp)  Y(mg)     .32  .29
Perth         Y          Y             Y      n      Y         .29  .28
Per-Edin.     Y(mg)      Y(x,y)        Y      Y      Y(mg)     .29  .28
Cape          n          Y(x,y)        Y(ep)  Y(mp)  Y(mg,rs)  .28  .26
Sydney        Y(mg)      Y(mp,rs,x,y)  n      Y(mp)  Y(mg,rs)  .44  .40
Melbourne     Y          Y             n      Y(mp)  Y(mg)     .34  .32


Corrections applied to x,y data prior to final least squares

Catalogues used in the ACRS_1999 (units of mas)

Name  type  σα  σδ  σ/ sqrt(N)

1ST CAPE FUND 1900   Transit Circle    589   564  Yes
2ND CAPE FUND 1900   Transit Circle    529   462  Yes
2ND MELBRN GC 1880   Transit Circle   1380   859  Yes
3RD MELB GC 1890     Transit Circle   1507  1117  Yes
ABBADIA 00 –3/–9   Transit Circle    975   849  Yes
ABBADIA 00 +16/+24   Transit Circle    988   810  Yes
ABBADIA 00 –2/+4    Transit Circle    951   929  Yes
AGK2  (BERGEDORF)     Astrograph   205   198  No
AGK2  (BONN)          Astrograph   261   259  No
AGK2a                Transit Circle    165   217  No
AGK3                  Astrograph   204   191  No
AGK3R                Transit Circle     75   107  No
ALBANY –2/+1 1900   Transit Circle    840   844  Yes
ALBANY –20/–40  00 Transit Circle    836  1156  Yes
ALBANY 10            Transit Circle    294   282  No
ALGER AG   1900      Transit Circle   1156   958  Yes
BERGEDORF  I - 25    Transit Circle    432   540  Yes
BERLIN 10  79/90     Transit Circle    504   617  Yes
BERLIN 1920          Transit Circle    559   660  Yes
BONN 00  +40/+50     Transit Circle    506   461  Yes
BONN 20              Transit Circle    533   504  Yes
BORD 00-II RE-OBN    Transit Circle    397   377  Yes
BORDEAUX II  1900    Transit Circle   1038   734  Yes
BUCHAREST –11/+11   Transit Circle    381   493  Yes
CAMC Series          Transit Circle    176   237  Yes
CAPE  I - 50         Transit Circle    368   373  Yes
CAPE  II - 25        Transit Circle    468   453  Yes
CAPE  III - 25       Transit Circle    474   420  Yes
CAPE –40/–52  1900 Transit Circle    743   671  Yes


Catalogues used in the ACRS_1999 (units of mas)

Catalogues used in the ACRS_1999 (units of mas), continued

Name  type  σα  σδ  σ/ sqrt(N)

CAPE 17 –30/–35     Astrograph   391   407  No
CAPE 18 –35/–40     Astrograph   319   350  No
CAPE 19 –52/–56     Astrograph   298   319  No
CAPE 20 –56/–60     Astrograph   210   193  No
CAPE 20 –60/–64     Astrograph   196   200  No
CAPE 21 –64/–68     Astrograph   182   187  No
CAPE 21 –68/–72     Astrograph   190   195  No
CAPE 21 –72/–76     Astrograph   211   211  No
CAPE 21 –76/–80     Astrograph   186   202  No
CAPE 22 –80/–89     Astrograph   176   227  No
CAPE G. C. (1900)    Transit Circle    674   600  Yes
CAPE I - 25          Transit Circle    469   459  Yes
CAPE II - 50         Transit Circle    467   588  Yes
CAPE ST 50 –30/–35 Transit Circle    742   668  Yes
CAPE ST 50 –35/–40 Transit Circle    534   510  Yes
CAPE ST 50 –52/–56 Transit Circle    722   608  Yes
CAPE ST 50 –56/–60 Transit Circle    330   344  Yes
CAPE ST 50 –60/–64 Transit Circle    302   336  Yes
CAPE ST 50 –64/–68 Transit Circle    374   398  Yes
CAPE ST 50 –68/–72 Transit Circle    376   380  Yes
CAPE ST 50 –72/–76 Transit Circle    372   416  Yes
CAPE ST 50 –76/–82 Transit Circle    328   370  Yes
CAPE ST 50 –82/–90 Transit Circle    226   326  Yes
CORDOBA 6429 ST 00   Transit Circle    722   715  Yes
CORDOBA D  1950      Transit Circle    589   628  Yes
CORDOBA E  1950      Transit Circle    618   952  Yes
CPC2                  Astrograph      Ind.  Ind.  NA
FAYET +5 TO +15      Transit Circle    559   607  Yes
FAYET –5 TO +5      Transit Circle    165   240  No
FOKAT                 Astrograph   220   238  Yes


Catalogues used in the ACRS_1999 (units of mas), continued

Catalogues used in the ACRS_1999 (units of mas), continued

Name  type  σα  σδ  σ/ sqrt(N)

GREENWICH 1910       Transit Circle    426   324  No
GREENWICH 9Y2        Transit Circle    950   854  Yes
GREENWICH I-50       Transit Circle    504   487  Yes
GREENWICH II-25      Transit Circle    450   466  Yes
HEIDELBERG ZOD 50    Transit Circle    375   453  Yes
HIPPARCOS            Satellite        Ind.  Ind.  NA
KONIGSBERG 25        Transit Circle    183   199  Yes
KONIGSBERG 25-II     Transit Circle    482   426  Yes
KUSTNER - BONN 00    Transit Circle    368   332  No
LA PLATA 3710 S 50   Transit Circle    712   658  Yes
LA PLATA A  1925     Transit Circle    808   790  Yes
LA PLATA B  1925     Transit Circle    811   679  Yes
LA PLATA C  1925     Transit Circle    801   698  Yes
LA PLATA D  1925     Transit Circle   1000   867  Yes
LA PLATA E 1925      Transit Circle    911   857  Yes
LA PLATA EROS 1930   Transit Circle    500   387  No
LA PLATA F  1935     Transit Circle    485   518  Yes
LA PLATA GC  1950    Transit Circle    564   585  Yes
LEIDEN 25  XV/1      Transit Circle    460   576  Yes
LEIDEN 25  XV/2      Transit Circle    458   664  Yes
LEIDEN 25  XV/4      Transit Circle   2308  2069  Yes
LICK 17 HARVD AG     Transit Circle    246   345  No
LICK 28  20/30       Transit Circle    222   202  No
LICK ZODIACAL 1900   Transit Circle    424   335  Yes
LUND 50 FAINT AG     Transit Circle    710   749  Yes
LUND AG 25 35/40     Transit Circle    661   696  Yes
MADISON 1910         Transit Circle    444   468  Yes
MUNICH A  1900       Transit Circle    681   749  Yes
MUNICH B  1900       Transit Circle    730   680  Yes
NICOLAEV –5/–20 50 Transit Circle    306   367  Yes
NICE                 Transit Circle    111   148  No
PARIS 1900           Transit Circle   1299   848  Yes


Catalogues used in the ACRS_1999 (units of mas), continued

Catalogues used in the ACRS_1999 (units of mas), continued

Name  type  σα  σδ  σ/ sqrt(N)

PARIS ASTR +17/+25    Astrograph   290   323  No
PERTH 83             Transit Circle    211   405  Yes
PERTH VOL 2  1900    Transit Circle    717   713  Yes
PERTH VOL 3  1900    Transit Circle    899   974  Yes
PERTH VOL 4  1900    Transit Circle    844   803  Yes
PERTH VOL 5  1900    Transit Circle    909   798  Yes
PERTH VOL 6  1900    Transit Circle    763   779  Yes
PFKSZ 50             Transit Circle     59    62  No
PULKOVA   1900       Transit Circle   1272   914  Yes
PULKOVO 10  39/44    Transit Circle    300   288  No
SAN LUIS 1910        Transit Circle    716   661  Yes
SCHWACHER STERNE     Transit Circle    108   148  No
SRS                  Transit Circle    268   367  Yes
STRASBOURG AG 1900   Transit Circle   1005   908  Yes
SYDNEY 1499 IMD 25   Transit Circle    782   771  Yes
SYDNEY   –51/–63.5  Astrograph    84    80  No
SYDNEY  –48 TO –54  Astrograph   156   144  No
TOULOUSE 00 III-B    Transit Circle    462   380  No
TOULOUSE III         Transit Circle    684   572  Yes
TOKYO PMC 86-89      Transit Circle    463   543  Yes
TYCHO-1              Satellite        Ind.  Ind.  NA
USNO TAC              Astrograph      Ind.  Ind.  NA
W1J00                Transit Circle    204   236  Yes
W2J00                Transit Circle    274   319  Yes
WASH 25 ZOD  6-IN    Transit Circle    349   405  Yes
WASH 33 –10/–20 33 Transit Circle    564   438  Yes
WASHINGTON 00 9-IN   Transit Circle    590   535  Yes
WASHINGTON 20        Transit Circle    642   540  Yes
WASHINGTON 250       Transit Circle    262   381  Yes
WASHINGTON 350       Transit Circle    290   383  Yes
WASHINGTON 40 9-IN   Transit Circle    370   447  Yes
WASHINGTON AG 1900   Transit Circle   1217   976  Yes
WIEN AG  1900        Transit Circle    806   807  Yes


Catalogues used in the ACRS_1999 (units of mas), continued

Catalogues used in the ACRS_1999 (units of mas), continued

Name  type  σα  σδ  σ/ sqrt(N)

YALE v11  –10/–14   Astrograph   360   358  No
YALE v12/1 –14/–18  Astrograph   316   264  No
YALE v12/2 –18/–20  Astrograph   278   254  No
YALE v13/1 –20/–22  Astrograph   285   251  No
YALE v13/2 –27/–30  Astrograph   336   350  No
YALE v14  –22/–27   Astrograph   305   277  No
YALE v16   –6/–10   Astrograph   242   236  No
YALE v17   –2 / –6  Astrograph   247   233  No
YALE v18  +15 / +20   Astrograph   204   213  No
YALE v19,22/2 +9/15   Astrograph   205   190  No
YALE v20   +1 / +5    Astrograph   202   190  No
YALE v21   +1 / –2   Astrograph   226   230  No
YALE v22/1 +5 / +9    Astrograph   198   207  No
YALE v24,25 +20/+30   Astrograph   187   153  No
YALE v26/1  +85/+90   Astrograph   158   173  No
YALE v26/2,27 50/60   Astrograph   118   135  No
YALE v28  –30/–35   Astrograph   291   323  No
YALE v29  –35/–40   Astrograph   278   288  No
YALE v30  –40/–50   Astrograph   201   197  No
YALE v31  –70/–90   Astrograph   193   219  No
YALE v32  –60/–70   Astrograph   299   306  No

`
Catalogues used in the ACRS_1999 (units of mas), continued