Although the illumination corrections for VST make heavy use of APASS, the original overall photometric calibration of each pointing was based on the limited number of standard fields observed each night (mostly). That calibration is 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 linear colour equations used to convert the standards to an internal VST system for that calibration are shown below:
u_vst = U + 0.035 * U-B
g_vst = B - 0.405 * B-V
r_vst = R + 0.255 * V-R
i_vst = I + 0.115 * R-I
z_vst = I - 0.390 * R-I
The current 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 are also computing an APASS ABmag zero-point and putting 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.
Examples of the global Vega-like photometric calibration of Omegacam from observations of SA's over the period 16th August - 5th October are shown below. Top to bottom: g,r,i,z,u,NB_659