[Aavso-photometry] Flat Stuff

[Steve Robinson]

 I have never had the option of using a light box.  The 18" mirror that I use
makes getting a wide steady light source awful hard.  Twilight flats are just about
all I ever take.

My procedure is to turn off the equatorial drive and point the scope a bit to the
east of the meridian when the sky is light enough to give me 60% full well using
the V filter using a shutter speed of up to 0.5 seconds.  I take several of these
until the illumination falls below 50 percent of full well.  Then I up the exposure
and continue until I get  9 or so exposures.  I then repeat using the R filter, 
and finally the I.

I use the Eastern direction because I believe the light gradient will be less in
that direction.   (what Arne said confirms what I have believed through experience.)  It is hard to say if east is better than zenith, but I do believe it is better than west.

Finally, whatever the longest exposure was that was needed to take the longest flat
frame, I take 10 flat darks.  I then take bias frames, and subtract scaled darks.

I still get gradients, but I suspect that has more to do with local light pollution
than any problem with the flats.

Steve Robinson