BUILDING A ``COMPLETE'' QSO SURVEY BASED ON APS O MAGNITUDES

Because of their variability, it is an impossible task to compare two QSO surveys of the same region of the sky made at different epochs. However we now have, for a large fraction of the sky, the possibility to extract from the APS database, for any object, the O magnitude as measured on the Palomar Sky Survey plates [48] with an accuracy of about 0.2 mag [38]. By doing this for all known QSOs found in the same area of the sky during a number of different surveys, we may hope to get as near as possible from an ideal survey complete to a well defined limiting magnitude.

We have extracted the O magnitudes of 105 PG UV-excess stars (excluding CVs). We have compared these magnitudes with the photoelectric B magnitudes [44] and found a color equation: O-B = $0.23\,(U-B)+0.02$ (Fig. 4a); the rms error on the O magnitudes is 0.26 mag, slightly larger than the published value. For U-B = -0.8, the mean value for QSOs, the O magnitudes are systematically too bright by 0.16 mag.

We have extracted the APS O magnitudes, when available, for all objects in the QSO catalogue [55] brighter than B = 17, with M_B < -24.0 and z < 2.15, located in the 2400 deg² of the FBS at $\vert b \vert \gt$ 30$^\circ$. Whenever this O magnitude exists, we give it the preference. Table 4 contains 15 such QSOs with O < 16.2 (and 11 with O < 16.0, corresponding to B = 16.2) and three with B < 16.2. We have found ten additional QSOs with O < 16.2 (seven with O < 16.0) and four with B < 16.2 (listed in Table 5).

Thus our ``complete'' sample contains between 18 and 25 QSOs brighter than B = 16.2, or 0.0075 to 0.010 deg^-2; this is 1.2 to 1.6 times larger than the PG surface density. If we correct these surface densities for the Eddington effect (1.16 for our survey and 1.32 for the PG survey), our surface densities are 1.4 to 1.8 times larger than the PG values.

It should be possible, when the APS database will be completed, to check the O magnitudes of the seven objects for which they are not yet available.

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