Magnitude calibrations

Magnitudes, the prerequisite for later applications, are obtained in a lengthy process: each measured isophotal magnitude or aperture magnitude is subjected to transformations, corrections and calibrations, amounting to 8 steps for galaxies and 6 steps for stars. Plate overlaps are used for the calibrations. Fig. 1 shows the arrangement of the 216 plates and their larger or smaller overlap regions. The small ones are needed to equalize the magnitude zero points of all plates. CCD sequences from plates with not too many intermediaries, are then used to transform the plate magnitudes into an international system.

All hypersensitized Schmidt plates have a tendency to loose sensitivity towards the plate edges. This effect, together with that of vignetting, must be corrected. Desensitisation is more severe for J-plates, especially the older ones, taken before UK astronomers became aware of the effect and found partial remedies. Although the effect is smaller on R-plates, it cannot be neglected. The best remedy, short of having tens of calibration sequences over each plate - which would require a dedicated 1m-telescope on La Silla for several years - are magnitude corrections based on the comparison of object numbers per unit area in the central part of the plate and at the edges. Galaxy counts on several plates near the SGP as a function of local background densities yield a good average correction curve for the magnitudes. The quality of this curve is tested by comparing the results in the regions of large plate overlaps.

Magnitudes from the J- and R-plates are labeled b_J and r_F, the symbols indicating the blue and red passbands and the photographic emulsions Kodak IIIa-J and Kodak IIIa-F, used in combination with filters. About 70 % of all objects brighter than r_F = 20, i.e. 35000 stars on an average R-plate, and more than 11000 galaxies are sufficiently well matched with their J-counterparts to yield the colour index b_J - r_F.