This is the end of an otherwise fine night of observing. The weather goes seriously downhill shortly after this image is taken. This graphic also illustrates how the same noise across an image has different effects. The green trace, for Reference star 3, at instrumental magnitude 8.97, shows a moderately increasing deterioration as the evening progresses. The check star (orange trace), at instrumental magnitude 10.42, or about 4x fainter, goes completely haywire with the same noise. Notice, too, that Reference star 1 (black trace) appears to be dimming slightly. Is it also a variable star?

(Chart from MaxIm DL/CCD)