[Aavso-photometry] Flat frame quality and its effect onphotometric accuracy

[Michael Newberry]

Correction: In item 4, I meant to ask what is the average signal of dark and
bias frames? This is most important if you are not applying them to the raw
flats.

Michael

----- Original Message ----- 
From: "Michael Newberry" <mnewberry at mirametrics.com>
To: <aavso-photometry at mira.aavso.org>
Sent: Tuesday, August 12, 2008 9:46 AM
Subject: Re: [Aavso-photometry] Flat frame quality and its effect 
onphotometric accuracy


> There are many variables involved in getting a "good" flat field. In 
> addition to what Arne and George have said, and in addition to the problem 
> of filter wheel repeatability, I would wonder about your processing of the 
> raw flats. Questions:
>
> 1. How much vignetting do you have from center to corner? (as in signal at 
> center vs signal at corner)
> 2. What is the signal level at the center of the flat?
> 3. So you apply dark and/ir bias corrections to your raw flats?
> 4. What is the average signal of a flat field frame?
> 5. Are the flats filtered, and do you use twilight, dark sky, or a screen?
> 6. What is your degree of artificial light pollution?
> 7. Are your master flats normalized to common signal level during or 
> before median combining?
>
> These can conspire to give you lousy flat field corrections. They wouldn't 
> be so important if, for example, your typical flat has a central signal 
> of, say, 40,000 and your dark is 200, and there's only a 5--10% vignetting 
> to the corner, the flats are filtered, and there's not much light 
> pollution.
>
> Michael
>
> ----- Original Message ----- 
> From: "George Sjoberg" <george_sjoberg at yahoo.com>
> To: <aavso-photometry at mira.aavso.org>
> Sent: Tuesday, August 12, 2008 9:27 AM
> Subject: Re: [Aavso-photometry] Flat frame quality and its effect 
> onphotometric accuracy
>
>
>> Hi Lionel,
>>
>> Couple of questions about your Flat frame acquisition with the GEM. When 
>> you say you take 10 to 25 flats for each filter is that 10 to 25 flats 
>> for each side of the meridian? (A total  of 20 to 50 for each filter?) 
>> For your masters do you average or median combine (or maybe some other)? 
>> What length exposures do you limit yourself to? I've been doing 24 (12 in 
>> each orientation) and i'm lucky if I can get two filters done at dusk and 
>> two more at dawn. I've become very frustrated at the lack of repeatable 
>> positioning of the sbig filter wheel (I'm using the same exact camera as 
>> you) Don't have any idea about what to do about the filter wheel.
>>
>> ..george
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>