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.
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.
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 .
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.
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.
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.