Two solidly clear nights in a row – and tomorrow is supposed to be more of the same! Best stretch of weather since at least May and likely longer. 70 degrees for low tonight, almost no wind. Humidity 40% now, rising to 60% as temp drops. Seeing about typical, perhaps slightly better.
Rig #2: V1101 Aql early, ASAS-AN 13ck late
Same two targets as last night. V1101 looks a little fainter tonight – as does ASASAN 13ck; 25-second exposures for V1101 and 45-second exposures for ASAS-AN 13ck.
From last night – thoughts on variable PSFs and scattered light.
Reduced data from last night was actually pretty ratty, not really sure why. All three stars (variable, comp, and check) had S/N well above 100. But the biggest problem is what I’m guessing is variable PSF shape as a function of position on the chip. Enough that when the telescope is flipped when crossing the meridian the “curve” for the two comparison stars is different by about 0.04 magnitudes. The effect is easy to see in the plot from last night, below. The “drop” in comparative brightness is around 0.04 magnitudes. Outside of that drop the standard deviation is around 0.008 magnitudes.
It could also be due to scattered light getting into the flat fields, so not totally convinced that changing PSF shape is the problem. I’ve been thinking of a couple of different tests to try. One would be to pick a small region of the image plane and arbitrarily designate the flux of stars seen at that location as being “unity” and then seeing what the difference is for the same stars at their new location after the meridian flip. I have a number of nights now so could probably create a very crude map of improper flux levels due to scattered light in the flats.