[Aavso-photometry] Flat Stuff

[Arne Henden]

Michael Koppelman wrote:
> These seems to ignore the fact that the low S/N frames are going to be 
> weighted very, very low vs. the high S/N frames, doesn't it? The 
> fractional uncertainty of the high S/N frames is going to be an order of 
> magnitude less than the fractional uncertainty of the low S/N frames.
> 
Few programs weight scaled images any different than unscaled
images when performing the final summation, which is why a
poor signal/noise image impacts so much on the composite.  I agree
that you could scale *and* weight flats to remove much of this
effect.  My usual technique is much like Chuck's:  I modify
the exposure times to give reasonably constant DN levels as
the sky darkens.  With my slow readout system, I can get 4-5 flats
per filter before the exposures get too long, and that is
adequate for decent signal/noise in the flat.  This exposure
modification of course means that you either have to take a lot
of darks for amateur cameras, or use a scaled master dark.  Luckily,
dark current is a minor contributor to half-well flatfield images.
Arne