We inspect four flat fields taken of the Hohlraum lamp using AIR, ND1.0, ND1.3 and ND2.0 (ND0.9 was skipped as it is apparently AIR). All images were exposed so that count levels were between 10000 and 56000.
At upper left is AIR, upper right is ND1.0, then ND1.3 lower left and ND2.0 lower right.
Looking at these images there is evidently some light-leakage problem at the upper right corner, that has to do with which ND filter is used - or the lamp shifted slightly between exposures. Other experiences with the Hohlraum lamp underline how exactly positioned the telescope must be - this is difficult since small mount alignment problems apparently accumulate over time (the 'creep problem'). The mount should therefore be freshly calibrated before use of the Hohlraum, which is not always convenient. It must furthermore be calibrated on a part of the sky near the lamp or the 'offset problem' will be evident.
'offset' and 'creep' are relatives, I think - something to do with the axes of the mount and the polar alignment.
A wishlist for this system might include an item about 'proper polar axis alignment' or perhaps an item about 'automatic centering of telescope on flattest part of hohlraum field'.