[Aavso-photometry] Flat Stuff

[Robert Koff]

I've also gone to dawn or dusk twilight flats.  I use exposures of 10 and 20 
seconds as the light changes.  the download time of my camera is about 45 
seconds, so I don't get a lot of images.

I use a diffuser to prevent stars from interfering.  For the diffuser I use 
the same white translucent sign material I have used with a light box.  Any 
thoughts on the validity of using a diffuser?  How about using it with a 
back-illuminated chip and I-band filter?

Bob Koff
----- Original Message ----- 
From: "Chuck Pullen" <cpullen at pacsafe.com>
To: "Michael Koppelman" <lolife at bitstream.net>; 
<aavso-photometry at mira.aavso.org>
Sent: Monday, January 17, 2005 20:19
Subject: Re: [Aavso-photometry] Flat Stuff


> 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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>
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