No Light on Detector (4K) Weirdness

Sunday, June 2, 2019 1:52 PM

Problem(s) Encountered:

I had a weird thing happen early in the evening when I read out a basically blank exposure (/data/hiltner/n1.1075.fits). There were cosmic rays present, so this is different from what was reported a few weeks ago.
This was a long (1800s) spectroscopic exposure and there was no light from the sky on the CCD. Note that I had already used this setup, so it is not like something was mislabeled. I double-checked the prospero display and the FITS header but the instrument status and configuration seemed normal. I did a shorter test exposure afterwards and it was also blank.
That seemed to me to indicate some sort of obstruction in the light path, either in the filter wheels and mechanisms or possibly the shutter not opening correctly. I rotated the filter, slit, and disperser wheels but didn't hear anything out of the ordinary, so I put the instrument in imaging mode and then I was able to successfully take an image of the field.
My next attempt at a spectrum had a faint trace from the target plus some sky lines, but even the sky lines were much fainter than normal, possibly indicating a partial obstruction.  I moved the mechanisms around a few more times and I went out into the dome and wiggled the cord for the shutter to make sure it had good contact.
After that, I didn't have any more problems for the rest of the night, but I'm not sure what resolved the issue.