[Aavso-photometry] Flat Stuff

[Richard Miles]

> I have pretty much taken to dawn twilight flats.  At best, I get 3 or 4 
> per filter of about 50% well depth.  I use a SiTE chip camera, so could 
> probably go higher, but I've never rigorously verified it's linearity. 
> So, it's impossible to get 15 or so flats!
>
> I wonder what other folks who use twilight flats do?

Chuck,

I'm a believer in twilight flats too.  One thing that will help is to point 
your scope at areas of the sky that are relatively devoid of stars and that 
also have the least twilight light gradient across the frame - the latter 
depends on where the setting/rising sun is below the horizon.

I also use 2 or 3 different exposures.  So taking a dawn set, the first set 
are with say 2-sec exposures, and I image in the range 20-50% full-scale. 
Then I move on to say 0.5 sec, finish that set before moving onto the last 
at 0.2 sec.  The shutter on the camera is electronic and is in effect 
<<0.001 sec so shutter speed is not an issue.  I also take darks at the 3 
exposure settings (when it is dark).  Generally manage about 50-100 images 
making up the final flat - the number you get depends on your download time.

I also have a German mount and it occurred to me that I could take one half 
before, and one half after telescope reversal across the meridian, so as to 
minimise the brightness gradient across the field.

Just some ideas.
Richard