The APM Sky Catalogues: Brief Details of the Catalogues

1. The Northern sky catalogue is based on degree scans of glass copies of the Palomar O and E sky survey taken in the 1950's. The field centres are on a 6 degree grid. The limiting magnitude is 21.5 for the O plates and 20 for the E plates.

2. The Southern sky catalogue is based on degree scans of glass copies of the UKST B sky survey and originals of the, current UKST SES R survey. The UKST field centres are on a 5 degree grid and unlike the old Palomar plates have density wedges present (in the SE and NW corners). The limiting magnitude is 22.5 for the B plates and 21 for the R plates.

3. Plates were scanned using a 1/2 arcsec sampling interval and a scanning resolution of 1 arcsec. The image detection thresholds on the O and E plates were around 24 mag arcsec and 23 mag arcsec respectively. For the B and R plates the equivalent numbers are 25 mag arcsec and 24 mag arcsec .

4. Detected images were parameterised to preserve coordinate, intensity and general shape information. The on-line catalogue uses a compressed subset of this, namely: x and y plate coordinate, magnitude, semi-major axis radius at detection isophote, ellipticity and ellipse position angle, image morphological classification and a stellarness index.

5. The magnitudes (and colours) are internally calibrated. Tests on the Northern catalogue show that the colours are consistent to +/-0.2 magnitudes and the magnitude faint end zero-points are consistent to +/- 0.25 magnitudes. Eventually external photometry will be folded in but we have found the internal calibration is accurate enough for most applications.

6. Both catalogues use the red plates as the reference list; the blue plates are referred to as the comparison list. Note that ALL detected images on both red and blue plates are stored in the catalogue.

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