MDM Trouble Report for 2007 Apr 21

Telescope: 2.4m
Observer(s): Mangala Sharma & Tom Statler (Ohio Univ)
Instrument: 8k CCD Camera

Problem(s) Encountered:
First night using the instrument after instrument change
(following a cloudy night). Once on primary targets, we found
that we were getting huge dark currents (thousands of counts
per minute). We suspected that the CCDs were getting warm but
had no way to verify this since DETCOM doesn't report actual
temperature. We phoned Bob around 9:30 PM and confirmed that the
dewar pressure was reading around 3.0 mTorr. We discussed our
experiences over the last day with a strangely loud dewar
filling. Bob told us that undoubtedly the dewar was warming up.
We re-filled the dewar, this time making sure it was truly full.
The pressure went down to nominal values and stayed there the
rest of the night. In hindsight we saw that we were being
fooled by the way the vapor was venting, and we thought the
dewar was full when it wasn't. We think it's odd, though, that
we were getting the loud warm-dewar symptoms even the previous
(cloudy) evening, when we filled it around midnight.

After getting the chips back to normal temperatures, we continued
to see very strong signatures of grounding problems (patterns
in the bias images that don't repeat from frame to frame). These
play havoc with the bias frame statistics; standard deviations
over small regions are around 40 counts, when they ought to be
just a few. This problem is now much worse than it was
on our previous runs (Feb 2006 and Sep 2006).

We also encountered trouble trying to focus the south guider
camera. We could loosen the lock ring but could not budge the
long stem to change focus. We gave up, and didn't even try to
use the north camera, which was even more out of focus.

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Submitted on 2007 Apr 22 [6:09:40]