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.

Solutions:

The drop-off was in counts but not count rate, and the exposure time for the high ones was 180 s and the low ones was 120 s. In our observing log, we''d just noted 180 s the whole time, since this was the intended exposure; I guess that at some point my co-observer decided that the S/N was good enough to reduce to 120 s and didn''t note it. Thus, I didn''t even consider this and glossed over the difference in the header until reading through the full ''diff''.