2. The quasars sample
2.1. Definition of the sample
2.2. Calibration of the photographic material and error estimation
8 plates taken with the ESO La Silla and UK Schmidt telescopes and a copy of a POSS E-plate (E1453) have been analysed. In Table 2 a detailed list is given. The combination IIIa-F + RG630 filter was used in all cases except for the plate E1453, that is a combination of a IIIa-E + a red Plexiglas 2444, providing a transmission similar to that of a Wratten filter (peak sensitivity near ). The natural photometric system defined by the IIIa-F + RG630 combination (in the following defined ) is close to the Johnson-Kron-Cousins R and the color transformation is given by Bessel (1986):
Table 2. List of the plates
The combination IIIa-E + red Plexiglas 2444 provides a system for our purposes indistinguishable from the previous one, with no significant color term in the range , as verified on our photometric material.
The plate material has been scanned with the COSMOS microdensitometer (MacGillivray & Stobie 1984) in IAM mode. The resulting tables, one per each plate, containing the instrumental magnitudes and other useful parameters for the objects detected, have been merged together in one table. Only objects with at least 3 detections in the 9 plates have been accepted in this final table containing 82 984 entries. The astrometric error box defining a common detection has been defined as a circle of 3.5 arcsec of radius. In this way spurious detections (plate defects) are minimized to an acceptable level, while real measurements are in practice never discarded.
The calibration of the magnitudes of each plate has been carried out according to the following procedure (see also Paper I):
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998