Problem:

This camera uses Peltier cooling to achieve the necessary temperatures (of around 15-20 K colder than ambient temperature), which is a technique based on electrical heatsinks (rather than liquid nitrogen cooling which is widely used at a number of observatories). As a result of this technique, there is a fan attached to the camera to blow out the residual heat from the system.

When the cooling starts, residual humidity inside the camera may condense and freeze on the inside of the camera. The whole CCD field starts to look "dirty" as a result of ice crystals forming rapidly. After a while, an ellipsoidal ring forms in the center of the field, the inside of which is free of ice (see left image which is a flat field image). As the humidity is decreasing, one can see this ring starting to grow on timescales of about 1 minute (see larger ring on right image). On the edge of the frame more dust-like icecrystals can form (left image), but the center clears out. It takes about 10-30 minutes for the whole CCD to become clean (depending on the temperature difference between the inside of the camera and the ambient temperature).

N.B.: the groove-like features (arc-shaped) across the whole field are most likely caused by the corrector lens of this Schmidt telescope.

Fix:

Several fixes are possible, though they are somewhat particular to this setup. One may either (a) wait until the entire CCD is ice-free, (b) start cooling the CCD earlier so that it is ice-free when observing starts, or (c) cool more slowly to avoid condensation altogether (often a temperature 5 C above the recommended value is good enough for observing).