[Aavso-photometry] Flat frame quality and its effect on photometric accuracy

[Lionel Catalan]

I've noticed that the main source of systematic error in my photometric
analysis is now due to the quality of calibration flat frames. I take sky
flats at dusk or dawn with an exposure time adjusted to achieve a pixel
value approximately 50% of the linearity range of my camera (SBIG ST8XMEI).
I usually combine 10 to 25 individual flat frames to make master flat
frames. I make master flats for each filter that I use. Because I use a
German equatorial mount, the stars change position in the CCD image after a
meridian flip. This change sometimes causes jumps or drops of up to 0.015
magnitude in the target or check stars. I attribute these jumps to
less-than-perfect flats. I reason that an error of just 1.5% in the master
flat pixel values would cause a systematic error of 0.015 magnitude, and
this error only shows up during meridian flips or when the stars slowly
drift in the image (I try to avoid that drift by using an autoguider to keep
my target star centered in the image). I've also noticed that if I try to
calibrate a flat done on one night with a master flat done a previous night
(without changing the camera orientation in between), then I don't get a
perfectly flat image having just random noise. Instead, I can see the edges
of dust donuts, and these patterns in the calibrated flat represent about 1%
variations above or below the image average pixel value. So, in summary, I
don't know how to further improve the quality of my flats to avoid (or
reduce) these errors.  I'd be grateful for any suggestions.

 
Lionel Catalan