MDM Trouble Report for 2017 May 01

Telescope: 2.4m
Observer(s): Hodges-Kluck, Lee (Michigan)
Instrument: OSMOS B4K

Problem(s) Encountered:
We observed NGC 4395 in a series of V-3x Halpha segments with
fixed exposure time. Partway through the night, the V-band counts
declined substantially, but the H-alpha was unaffected. The
number of counts in the background went down, as did the peak 
counts in point sources (by considerably more than the background),
and the integrated counts from these sources. The decline appeared
uniform across the field.

We do not know what caused this or if it is indeed a problem with
the filter or other instrumentation. The moon set around the time
that this happened, but the change was very abrupt and the filter
did not recover to the previous levels. It occurred around an
airmass of 1.0. I will send an e-mail to Eric with a plot of the
count rates measured to demonstrate the effect, but in histogram
form it essentially looks like (time increases down):

-----------------------> counts in 120s
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Submitted on 2017 May 2 [5:46:26]