[Aavso-photometry] (no subject)

[Tom Krajci]

>From: Yenal Ogmen <yenalogmen at yahoo.com>

>Today, for the first time, I tried flat fielding,
>using double t-shirt and twilight zenith. It was 20
>minutes before sunset and I got ~31000ADU at 0.0075
>sec. Is this exposure suitable for flats or it does
>not matter how long you expose as long as you get
>30000-35000ADU?

Your analysis of your CCD's linear behavior will answer that question.  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 how to test for linearity.

For my CCD I can go to 55,000 ADU in my flats and still have linear behavior.

If you need more help on CCD testing, feel free to contact me.

>One last question, after taking flats, if you fine
>tune focus, does this destroy flat fielding?

Small focus changes are not too bad...rotation of the CCD is worse (i.e. you remove and then replace the CCD).

If you analyze your focus position versus temperature on some dark nights...you can preset focus during twilight to anticipated night temperature and be very close in your focus setting.  That will help minimize this problem.

 
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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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