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I/289                  UCAC2  Catalogue                 (Zacharias+ 2004)

The Second U.S. Naval Observatory CCD Astrograph Catalog (UCAC2) Zacharias N., Urban S.E., Zacharias M.I., Wycoff G.L., Hall D.M., Germain M.E., Holdenried E.R., Winter L. <Astron. J. 127, 3043 (2004)> =2004AJ....127.3043Z
ADC_Keywords: Surveys ; Positional data ; Proper motions Description: The UCAC2 is the second release of the ongoing UCAC project, designed to observe the entire sky for R magnitudes of about 7.5 to 16. The observed positional errors are about 20 mas for the stars in the 10-14 magnitude range, and about 70 mas at the limiting magnitude of R∼16. For up-to-date information on the project, see the UCAC web page at . This web page will also serve as the location that the UCAC team will post addenda to the UCAC2 catalog. The UCAC2 is a high density, highly accurate, astrometric catalog of 48,330,571 stars covering the sky from -90 to +40 degrees in declination and going up to +52 degrees in some areas. The northern limit is a function of right ascension. Proper motions and photometry are provided for all stars. Positions and proper motions are on the ICRS (International Celestial Reference System) and given at the epoch J2000.0. The UCAC2 has a number of major differences with respect to UCAC1. These differences include: - much larger sky coverage - reduced systematic errors of CCD observations - positions given at a standard epoch (J2000.0) - the addition of several new catalogs for improved proper motions - photometry in the J, H, and K_s bands from the 2MASS project Additional details of the data are found in Sections 3, 4, and 5 of the "readme.txt" file. UCAC2 is the last intermediate data release before the final, all sky catalog will be constructed. Observations will end around mid 2004; the final catalog is expected out in 2005. The UCAC project, observations, and first data release are described in detail in the paper Zacharias et al. AJ 120, 2131 (2000). Versions of that and other related papers are placed on the UCAC Web page ( The paper describing UCAC2 is in preparation (AJ, 2003); some further details can be found in the "readme.txt" file. Notice: The data are freely available; however, any publication utilizing these data should make proper reference to the project and papers. File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file out.sam 157 100 Sample output readme.txt 77 1148 Original Description info/* 0 14 Various details about UCAC project and coverage
See also: : UCAC Home Page I/268 : UCAC1 Catalogue (Zacharias+ 2000) II/246 : 2MASS All-Sky Catalog of Point Sources (Cutri+ 2003) Byte-by-byte Description of file: out.sam
Bytes Format Units Label Explanations
1- 8 I08 --- 2UCAC [00000001,48330571] UCAC designation (1) 10- 20 F11.7 deg RAdeg Right ascension (degrees), ICRS, Ep=J2000 (2) 21- 31 F11.7 deg DEdeg Declination (degrees), ICRS, Ep=J2000 (2) 33- 35 I3 mas e_RAdeg Minimal mean error on RAdeg (3) 37- 39 I3 mas e_DEdeg Minimal mean error on DEdeg (3) 41- 43 I3 mas ePos Error of original UCAC observation (4) 45- 49 F5.2 mag UCmag Magnitude in UCAC system (579-642nm) (5) 51- 52 I2 --- No Number of UCAC observations for the star 54- 55 I2 --- Nc Number of catalog positions used for pmRA, pmDE 58- 59 I02 --- cfl Major catalogs used in pmRA, pmDE (6) 61- 68 F8.3 yr EpRA Central epoch for mean RA (3) 70- 77 F8.3 yr EpDE Central epoch for mean Declination (3) 79- 86 F8.1 mas/yr pmRA Proper motion in RA(*cos(Dec)) (7) 88- 95 F8.1 mas/yr pmDE Proper motion in Dec 97-100 F4.1 mas/yr e_pmRA Mean error on pmRA (8) 102-105 F4.1 mas/yr e_pmDE Mean error on pmDE (8) 107-111 F5.2 --- qpmRA Goodness of fit for pmRA (8) 113-117 F5.2 --- qpmDE Goodness of fit for pmDE (8) 119-128 I10 --- 2Mkey 2MASS (II/246) Unique source identifier 130-135 F6.3 mag Jmag ? J magnitude (1.2um) from 2MASS 137-142 F6.3 mag Hmag ? H magnitude (1.6um) from 2MASS 144-149 F6.3 mag Kmag ? K magnitude (2.2um) from 2MASS 151-153 I03 --- phf 2MASS photometric quality flag (9) 155-157 I03 --- ccf 2MASS confusion flag (10)
Note (1): this star number is not provided in the original files. It represents the running line number over all data, and uses an 8-digit after the string "2UCAC" Note (2): Positions are on the International Celestial Reference System Note (2): Positions are on the International Celestial Reference System (ICRS) as represented by the Hipparcos / Tycho-2 catalogs. The epoch for the positions of all stars is J2000.0; the weighted mean catalog position was updated using the provided proper motions. The observational UCAC position is but one of several going into these values and is not given in the UCAC2, thus the original UCAC observation cannot be recovered from these data. Note (3): Values for e_RAm and e_DEm are internal standard errors in position at the central epoch, NOT at the epoch J2000; for any epoch T (including "J2000.0" which corresponds to RAdeg and DEdeg columns) the error in position can be computed as: e_RAdeg(T) = sqrt(e_RAdeg2 + (e_pmRA * (T-EpRA))2) and similarly for e_DEdeg. Thus for all epochs other than EpRAm or EpDEm the positional errors for RA and Dec, respectively, increase for both going forward or back in time. Values for EpRAm and EpDEm are usually not equal due to different weights assigned to coordinates in the individual catalogs used in determining the mean positions and proper motions. Note (4): ePos is the internal, formal, 1-sigma error in position of the UCAC astrograph observations, expressed as the mean of the error in RA*cos(DE) and DE. This error is derived from the scatter of individual positions which form the mean observed position from the CCD observations of each star. These values are subject to small number statistics; many of the stars have only 2 to 4 observations. On average, for stars in the 7.5 to 10 mag range, 30 mas is expected, for 10 to 14.5 about 15 to 20 mas and increasing up to 70 mas toward 16th magnitude. Note (5): values for UCmag should be considered approximate. The UCAC bandpass (579-642nm) is between V and R. Observations were often made under non-photometric conditions and UCmag is provided here for identification purposes only. Estimated errors are 0.3 magnitude absolute, 0.1 magnitude relative. Note (6): identifier of some major catalogs used in the computation of proper motions, which also provides the user with information regarding known, high proper motion stars. However, this flag does not contain all catalogs going into the proper motions, about 150 were used in this project. The cfl value is actually a sum of numbers, each with the following meaning: 1 = modified Yellow Sky (NOFS scans of NPM, SPM; not published) 2 = AC2000.2 Catalog (I/275) 4 = Tycho-2 Catalogue observed position (I/259) 8 = USNO measurement of AGK2 plates (not published) 16 = Hipparcos Catalogue (I/239) 32 = Known high proper motion star from the revised New Luyten's Two-Tenths catalog (NLTT, Cat. I/98, match provided by A.Gould, see J/ApJ/582/1011) 64 = USNO A2 Catalog (I/252) Note (7): The original proper motion did not contain the cos(DE) factor, which was applied here to enable an easy computation of the total proper motion. Note (8): e_pmRA and e_pmDE are the formal standard errors of pmRA, pmDE respectively. In addition to these "model" errors, "scatter" errors have been calculated for each star having 3 or more catalog positions used in the proper motion determination. This "scatter" error depends on how well the individual catalog positions fit the computed proper motion. The "scatter" errors in position and proper motion component for each star are not explicitly given here, however the scatter error proper motion values can be recovered using the values qpmRA and qpmDE. qpmRA and qpmDE are termed "goodness of fit" for the pmRA and pmDE. These values give information on how well the computed proper motions actually fit the catalog positions used to compute them, termed "scatter error". The values qpmRA and qpmDE are actually ratios, "scatter error pm"/e_pmRA (or e_pmDE). Thus, when the actual fit of the proper motion to the catalog positions is better than the formal, model errors (e_pmRA, e_pmDE), the ratio is smaller than 1. In the opposite case, the ratio is larger than 1. Stars with qpmRA or qpmDE values exceeding 5 should be used with caution. If the ratio is larger than 12.5 it is set to 12.5. The ratio of "scatter error" to "model error" of the mean positions is highly correlated to those of the proper motions. Note that the "scatter error" only makes sense for stars with 3 or more catalog positions; if only 2 catalogs were used for the determination of the proper motion, the "fit" is perfect (going directly through each of the 2 positions, regardless of weights). In these cases, the ratios are set to 1.0. Note (9): photometric quality flag from the 2MASS catalog (II/246). For each color (J, H, K) one digit is given and has the following meaning: 0 = 2MASS quality is A or B, meaning good detections (snr > 7) 1 = 2MASS quality is C or D, meaning weak detections 2 = 2MASS quality is something else and the 2MASS documentation should be consulted. Note (10): artifact contamination and confusion from the 2MASS catalog (II/246). For each color (J, H, K) one digit is given and has the following meaning: 0 = 2MASS confusion is 0, no artifacts or contamination seen 1 = 2MASS confusion is "c", confusion with nearby brighter source 2 = 2MASS confusion is something else and the 2MASS documentation should be consulted.
Acknowledgements: It is a pleasure to thank the UCAC team who supplied a copy of the UCAC CD-ROMs in July 2003. History: A binary version of the UCAC2 catalog is available on CD-ROMs from the UCAC project (see the UCAC home page). This ReadMe file was essentially prepared from the contents of the "readme.txt" file contained on the UCAC CD-ROMs.
(End) Francois Ochsenbein [CDS] 17-Oct-2003
The document above follows the rules of the Standard Description for Astronomical Catalogues.From this documentation it is possible to generate f77 program to load files into arrays or line by line

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