[Aavso-photometry] Flat Stuff

[Chuck Pullen]

>
>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
>
>


Richard - thanks for this.  I do take multiple exposures, trying to keep 
the median ADU about the same before combining, so I don't have to scale 
too much and introduce artificial noise.  Your exposure times are pretty 
short, you have to worry about shutter effects with anything under 4-5 
second exposure.  You might want to think about that.

Chuck