NOTE: this problem has been fixed by replacing the shutter!!! (2018-MAR)
tl;dr: Please avoid targets in the Azimuth range 0-90° (North to East). Due to shutter problems you will not get good images!
It seems that we're having some technical problems with the camera shutter at Monet/S: sometimes it opens only for a (too) short time and sometimes it doesn't open at all. Apparently that's connected to the position of the telescope, or more precisely, to that of the derotator.
For this test during the day with a closed roof we moved the telescope to different positions and took 5 pictures at each one. We expect some variations due to an inhomogeneous illumination of the roof, but for all mean count rates below 1000 we assuma that the shutter didn't open. In the following plot the mean count rates are indicated by the radii of the circles and those smaller than 1000 are marked red:
As one can see that is a vertical strip in the azimuth range 0-90° (North to East), where the problem occurs. The numbers in the circles are the absolute derotator positions. Plotting the mean counts over those positions, we get the following plot:
So it seems that there is some range in the derotator rotation that causes the problem, probably too much strain on some cable.
Therefore we recommend avoiding observations in that range for now. We will let you know as soon as the problem is fixed.