Both Northern and Southern Survey Cameras have optical distortions which cause the psf to vary across the focal plane. This psf variation across the focal plane is also a function of temperature and camera focus. Although the resulting optics are quite good, enough non-isoplanicity exists to cause errors of several percent in the relative psf photometry across the focal plane. A software fix has been developed which corrects most of the cross-scan photmetric bias most of the time.
Problems still exist, however, for calibration scan blocks in low density areas.
The cross-scan profiles have been seen to change abruptly between blocks, and possibly within blocks, so interpolation from surrounding blocks can be risky. Interpolation between adjacent blocks may still be the best option for blocks with excessive errors in the offset determination. We are in the process of applying the preliminary cross-scan offset determination software to a number of nights of southern survey data to better evaluate the best way to diagnose correction problems and the best course of action when the correction errors are excessive. See Sherry Wheelock's analysis of the cross-scan offset corrections and the correction consistency. The psf shape and cross-scan psf magnitude errors are a strong function of the telescope focus.