I/267               The APM-North Catalogue                 (McMahon+, 2000)
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Beginning of ReadMe : I/267 The APM-North Catalogue (McMahon+, 2000) ================================================================================ The APM-North Catalogue McMahon R.G., Irwin, M.J., Maddox, S.J. <Institute of Astronomy, Cambridge, CB3 OHA, UK (2000)> ================================================================================ ADC_Keywords: Surveys ; Photographic catalog ; Positional data ; Photometry, photographic Keywords: Optical; Positional data; Photometry; Stars; Galaxies Description: ****************************************************************** This version is a preliminary adaptation of the APM, covering the Northern sky at high galactic latitudes only. ****************************************************************** The catalogue APMCAT-POSS1-1.0 is derived from the first epoch (1949-1958) Palomar Observatory-National Geographic Sky Survey (POSS). The catalog is based on digitised scans with the laser based Cambridge Automated Plate Measurement(APM) machine of both the blue O plates and red E plates. The plates are scanned with a pixel sampling 8microns which corresponds 0.49 arcsecs at the nominal plate scale of 61arcsec/mm (16.4 micron/arcsec). Further details about the survey material can be found in Minkowski and Abell 1963 and Lund and Dixon 1973. Astrometry of the APM Catalogue: The main properties of the APM catalogue that differentiate it from other public catalogues of digitisation programs of the POSS1 are: o scanned at high spatial resolution 0.49arcsec pixels (cf DSS which used n.n arcsec pixels o image are classified into stars and galaxies(unlike USNO) The APM catalog currently includes all plates with J2000 plate centres between +90 declination and 0 declination inclusive and |b|>25. The catalogued area covers over 10000 square degrees and contains over 100million objects. At the plate limit we have attempted to detect all images that are detectable on the plates with the inevitable price that some spurious image are detected. The APM POSS1 catalogue contains measurements from both the blue(O) and red(E) plates. The POSS is based on plates taken in two wavebands during the period 1949 to 1955. The blue plates were taken using Eastman 103a-O emulsion, and the red plate were taken with Eastman 103a-E emulsion and hence are commonly known as O and E plates respectively. Between 1991 and 1995, the SERC Automated Plate Measuring Machine (APM) at the Institute for Astronomy, Cambridge (Kibblewhite et al. 1984) was used to digitize these plates at a resolution of 8.0 microns (0.54 arcsec), the highest spatial resolution yet applied to these images (McMahon and Irwin 1992). An object catalog has been constructed from these data which includes all objects down to the plate limits ---  20.0 in E and  21.5 in O --- and contains approximately 2000 stars and 2000 galaxies deg^-2 at high Galactic latitudes. The catalog contains positions, magnitudes, morphological classification parameters, major and minor axes, and isophotal areas for each source; a merged catalog which matches objects between plates also contains a color (or an upper limit thereto) for each entry. An automated classification algorithm interprets the morphological parameters to classify each object. Here, we present the basic procedures which establish the astrometric and photometric calibration of the APM catalog, and discuss the limits of the image classification system. The APM machine measures the x and y positions of all objects detected. The conversion relationship between these measured APM positions and celestial coordinates is derived by matching stars in the Tycho catalog (Hog et al. 1997, see Cat. I/250) with stars detected on each plate using a `standard' six plate-constant model that allows for shift, rotation, scale, and shear. The algorithm uses 2-sigma clipping to give a robust fit; the typical rms on the fitted positions of the Tycho stars are 0.4"-0.8" Irwin (1994) has studied the two-dimensional systematic errors in an earlier version of the APM catalog positions by investigating the intraplate residuals between the measured positions for bright stars in the Positions and Proper Motions Catalog (Roser and Bastian 1991, Cats I/146 and I/193)) and the astrometric fit. He found significant, systematic residuals ranging up to 0.5". A residual map generated from this analysis is applied to positions in the standard APM catalog. APM Photometry: The APM measures photographic density rather than flux; moreover, the central regions of all objects more than a factor of 10 brighter than the sky produce a nonlinear response and/or are saturated. The algorithms used to overcome these inherent difficulties are discussed in detail by Irwin (1985). Briefly, a local background is determined for each of 10^6^ locations on each plate by producing a histogram of the pixel values in 64 x 64-pixel regions and finding the mode of each distribution using a maximum likelihood estimator; a two-dimensional smoothing is applied to these million background estimates to derive a background model for the plate. The image detection algorithm then finds connected regions of pixels above a threshold level (typically 2-sigma above the estimated background level for the given plate position). This background-following technique has the advantage that faint objects lying in the halos of bright objects can be detected. However, large objects such as bright stars and galaxies with angular extents >30' have their raw fluxes underestimated. An additional problem for large images is that the limited memory available to the software means that bright objects sometimes overflow the pixel buffers and are lost. This occurs for images with sizes greater than roughly 1--2mm (i.e. 1-2'), corresponding to stellar magnitudes brighter than around V=9. Another inherent problem arises in attempting to derive magnitude estimates for extended objects from saturated images. Saturation effects can be corrected for in stars by assuming that stellar images have an intrinsic density profile independent of magnitude, and that this profile can be derived from the unsaturated parts of stellar profiles. A high signal-to-noise intrinsic profile is constructed by taking the core from faint stars and the wings from brighter stars (see Bunclark & Irwin (1983) for further details). This profile can then be integrated and used to derive a calibration curve to convert saturated stellar magnitudes to a linear system. In the APM catalog, this calibration is applied to all images. This has the unfortunate consequence that galaxies, which have shallower surface brightness profiles and lower central surface brightnesses than stars of the same total magnitude, will have their magnitudes over-corrected. This is a fundamental problem for galaxy photometry determined from photographic sky survey plates (see Metcalfe, Fong, and Shanks (1995) for a discussion). The basic APM catalog is to have a red-band (E) plate limit of m(r)=20.0. This limit was established during the early stages (1991) of the creation of the APM catalog via comparison with  10 photometric sequences (Evans 1989; Humphreys et al. 1991). Similarly, a single slope of 1.10, was assumed in converting between the linearized APM magnitudes (Bunclark & Irwin 1983) and the alpha-Lyrae based Johnston magnitude system. It was noted at the time that there were significant deviations(1mag) from a simple linear relation at magnitudes brighter than V=15. This is not surprising bearing in mind that the POSS-I glass plates measured by the APM are copies that may have different degrees of saturation and have had their contrast stretched to enhance faint features. The assumption of a constant flux limit seemed reasonable, since the plates were all taken in similar dark sky observing conditions with exposure times that were adjusted to ensure uniform sensitivity. A similar assumption is made in all modern photographic cameras where it is assumed that all photographic film has the specified speed. The blue band (O) limit was defined with respect to the red limit; for the 428 fields available in March 1999, this has a range of m(o)=20.6--21.3(+/-1sigma range). Eventually, a full photometric recalibration of the APM using the Guide Star Photometric Catalog (GSPC -- Postman et al. 1998a) CCD sequences is planned. Preliminary comparisons with CCD photometry for 5% of the POSS-I plates show that the APM magnitudes for stellar objects have a global rms uncertainty of 0.5 magnitudes over the range 16 to 20. As discussed above, the uncertainties in the magnitudes of galaxies are more complex, since galaxies have a range of surface brightness distributions, and hence may have complex, partially saturated surface brightness profiles on the POSS-I plates. This is compounded by the range in calibration slopes observed. At faint magnitudes (18--20) where the image profiles are unsaturated, the APM magnitudes may be more reliable, but it is left to the reader to verify this where precise magnitudes are required. For many programs, a uniform set of magnitudes or uniform selection criteria are more critical. The APM scans result in a parameterization of each detected image which includes an x,y position, a peak intensity, a total isophotal intensity, second moments of the intensity distribution, and areal profiles (defined as the number of pixels above preset levels which increase by powers of two above the threshold level). In addition, a parameter, psf, is calculated which reports by how many sigma the object differs from the stellar point-spread function of its plate. These parameters are then used to classify all images into one of four categories: stellar (consistent with the magnitude- and position-dependent point spread function, cl=-1), non-stellar (a measurably extended source, cl=1), merged objects (sources with two local maxima within a single set of connected above-threshold pixels, cl=2), and noise (objects with nonphysical morphologies, cl=0). For further details of the principles involved, see Maddox et al. (1990a,b). Very bright images can often be misclassified, since the limited set of parameters does not provide an adequate description and the background-following algorithm attempts to track over them in order to detect the faint images in the source halos. The merged/non-stellar boundary is not as reliable as the stellar/non-stellar boundary, so merged stars are often found in the non-stellar list (with a smaller number of galaxies in the merged list). Some objects classified as noise are real; objects found on both plates are the obvious examples. Bright objects (e.g. O, E < 13) cover a large number of pixels in the APM scans and, as a consequence, magnitude and source-size estimates are very sensitive to small uncertainties in the plate sky level and details of the background-following algorithm; as a result, large uncertainties in the parameter estimations can result, and very bright sources can even be completely missing from the catalog. In addition, bright galaxies with complex surface brightness distributions can be broken up into a swarm of discrete sources. At fainter magnitudes, the limitations of the plate material make reliable separation of stellar and non-stellar sources problematic.