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

[Michael Newberry]

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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