[Aavso-photometry] Flat Stuff

[Chuck Pullen]

Michael-  Glad to see you are still a member of the Flat Earth Society.

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


At 21:02 1/16/05 -0600, Michael Koppelman wrote:

>Arne will tell all in his new book I'm sure, but he once told me that he 
>thought a good flat should have about a million electrons. I'm at 2.5 
>e/ADU so I need 400,000 ADU to get a million electrons. If I get 32,000 
>ADU per exposure I need about 13 of them. I like to do a min/max clipped 
>mean which throws out the highest and lowest value for each pixel so I 
>usually take 16 or more in each color.
>
>Another thing about flats is that one should not take too short of 
>exposures or there might be an effect from the shutter. I try to stay > 5 
>seconds on my flats to avoid any chance of this.
>
>Finally, redo your flats often! When you change focus the dust donuts 
>change size and your flat is invalidated. I take new flats every time I 
>focus if not more often than that.
>
>Just some random flats thoughts that have occurred to me on the discussion 
>lately.
>
>Cheers,
>Michael Koppelman
>http://www.lolife.com/astronomy/
>
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