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Photometric calibration

Although the illumination corrections for VST make heavy use of APASS, the overall photometric calibration of each pointing is still based on the limited number of standard fields observed each (mostly) night.  The calibration is currently in a VST Vega-like system, but as the u-band part of the Vega spectrum, and also the average standard star,  and the detector reponse, are plummetting rapidly here it would be surprising if  there were not offsets (and nonlinearities and duplicities) in at least the u-band
calibration.  The next step is to redo all the zero-points based on the illumination-correction fixed catalogues - they are still based
on the originals.  At the same time we will also compute an APASS ABmag zero-point and put that in the headers too.  When the recalibration is done we will also compute a VST -> Vega offset for all bands and post it on our web pages.

Global photometric calibration of the array from observations of SA's over the period 16th August - 5th October.  Top to bottom: g,r,i,z,u,NB_659