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Catalogue Parameters
Celestial Coordinates
Global astrometry is tied to the Tycho-ACT reference catalogue, and the mean errors in RA or DEC will be around 0.2" for bright stars. Very faint objects will have increased random noise and additional systematic error at a similar level (a few tenths of an arcsecond).
Proper motions
Proper motions in units of mas/yr are provided for every image paired between the UKJ and UKR (SES-R) plates. We followed the prescription given by Evans & Irwin, MNRAS, 277, 820 (1995) in the derivation of the pms using the UK Schmidt J plates as primary (first epoch) and the UK Schmidt R plates as secondary data. The mean proper motions of all galaxies are set to zero in this procedure, so they are on an extragalactic reference frame. Differential colour refraction corrections have been applied to minimise systematic errors in the pms of stars having extreme colour.Note that when an image is not paired between the J and second epoch R plates, no proper motion is available and the null value of 9.99E8 is returned (this can happen if a particular object is not detected for some reason on the J or R plate). This also applies to fields where no second epoch R data are currently available.
The pairing limit between any two plates is 3 arcsec; consequently for a median epoch difference between UKJ and UKR of 15 years, the upper proper motion limit is 0.2 arcsec/yr. In fields where the epoch difference is smaller, the upper proper motion limit will be higher of course.
Error on proper motion
Each proper motion value is accompanied by an error estimate since the plate qulaity can vary from field to field, and also because different epoch differences yield different proper motion errors. Moreover, galaxy centroiding is in general about a factor 2 less accurate than stellar image centroiding, so error estimates depend on image type; the errors are also a function of magnitude. In the case of stellar proper motion errors, these will be slightly overestimated from the data itself since error distributions are affected by real stellar motions. Note that brighter than 18th magnitude, there will be systematic errors in the stellar proper motions (increasing with magnitude) at the level of 10 mas/yr (for a median epoch difference of 15yr).Calibrated Magnitudes
Roughly calibrated magnitudes are provided in this parameter. These are currently tied to bright photometry from the Tycho and Guide Star Photometric Catalogues. Field-to-field zeropoints have been removed, and the calibrations are tied to faint CCD standards from Croom et al. (1999, MNRAS, 306, 592), Boyle et al. (1995, MNRAS, 276, 33), Stobie et al. (1985, A & AS, 60, 503), Cunow et al. (1997, A & AS, 125, 71 and references therein), Maddox et al. (1990, MNRAS, 246, 433) and Collins (private communication). Bright stars will have systematic errors as a function of position on the plate. Systematic errors of a few tenths of a magnitude will be likely; errors of 0.5 mag or more are less common but nonetheless present. Note that stellar magnitudes are based on calibrated profile magnitudes, while galaxian magnitudes are calibrated isophotal magnitudes, sky divided and corrected to total magnitudes.In October 2000 the R/I photometric calibration was changed to account for systematic errors in colour as a function of position and magnitude. The corrections have been done with respect to the J magnitudes, which have not changed (ie. J-R and J-I colours are now self-consistent and anchored at the same median value across the entire survey).
SuperCOSMOS Magnitudes
The full 32 IAM parameter sets for each plate have two instrumental magnitudes called COSMAG and PRFMAG. The COSMAG parameter is an isophotal magnitude (-2.5log{sum of intensities of thresholded pixels above sky}) while the PRFMAG parameter is a `linearised' profile magnitude computed following the prescription of Bunclark & Irwin (1983, Proc. Statistical methods in Astronomy Symp., ESA SP-201, 195)Classification Flag
Image classification (based on morphological parameters and areal profile shape) provides a classification flag: 1=galaxy, 2=star, 3=unclassifiable, 4=noise. In listings showing the full object parameters the profile classification stage also writes a classification statistic which is 1000x the number of sigmas an image profile set is away from the mean stellar areal profile set at that particular brightness and position on the plate. A "linearised" profile magnitude is also written and is used for stellar magnitude calibration.Quality Flag
The quality flag gives and indication of how "good" an image is in the sense that the higher the value, the more likely an image is to be spurious. Bits are set in this flag by the image analysis software such that more severe conditions are flagged using more significant bits. The nominal threshold for a severe defect is 65535 - images having a quality value less than this value but greater than zero are not necessarily spurious. non-zero values should be regarded as a warning of possible problems with the image.
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or +44 131 668 8100 (switchboard)
M.Read@roe.ac.uk 21-Jan-2002
