[Aavso-photometry] sky flats

[Tom Krajci]

 >From: "Papini Riccardo" <paprika at inwind.it>
> At dawn,
> when the sun is -12 degrees below the horizon, I wake up and take dark
> frames remotely being cozy in front of my desktop inside home.
>   
If your CCD is temperature regulated, you don't need to take new dark 
frames every night.  I have found that I can use my master darks for 
months.  It saves time and allows you to do other things instead.  (Same 
for bias frames...they don't change over time unless you modify camera 
electronics.)

> At this point I switch the tracking mode from sidereal to terrestrial to
> avoid round stars in the frame and start a sequence of flats for about
> twenty-thirty minutes. The exposure of each flat is 10 seconds. Since the
> sky brightness changes quickly I take flats with different signal intensity;
> signal varies even from one picture to the next. 
>   
Question:  what do you do if there are clouds in the sky during this 
crucial time window?  (This is why I use a flat light box, and why you 
may want to consider using dome flats or other 'artificial' method of 
making flats.)

> Later (may be in the afternoon or the day after) I choose the flats with the
> maximum signal in the pixels greater than 10.000 ADU and less than 60.000
> ADU and average them creating a master flat.
>   
Have you analyzed your CCD to see how linear it is?...and at what high 
ADU level it deviates from linear?  See:
http://overton2.tamu.edu/aset/krajci/st-7/st-7.htm
and
http://overton2.tamu.edu/aset/krajci/st-7-new/st-7-new.htm  for some 
ideas on what to analyze and how to portray the results.

Because my CCD (at the binning mode I always use) is linear to a high 
ADU level, I expose my flats to get an average ADU of 45,000 to 50,000.  
After you analyze your CCD's linearity, you can make a good decision as 
to maximum permissible ADU level.

Also, does your software normalize the sky flats?  This can be important 
when average values change significantly between flats, which is the 
case with sky flats.

I hope this helps.

-- 
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Tom Krajci
Cloudcroft, New Mexico
http://picasaweb.google.com/tom.krajci

http://overton2.tamu.edu/aset/krajci/

Center for Backyard Astrophysics (CBA)
http://cbastro.org/ CBA New Mexico

American Association of Variable Star
Observers (AAVSO): KTC http://www.aavso.org/
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