[Aavso-photometry] Flat frame quality and its effect on photometricaccuracy

[Steven Orlando]

----- Original Message ----- 
From: "Steven Orlando" <sorlando at sorlando.com>
To: "Lionel Catalan" <lcatalan at lakeheadu.ca>
Sent: Monday, August 11, 2008 8:41 PM
Subject: Re: [Aavso-photometry] Flat frame quality and its effect on 
photometricaccuracy


> So would I!
>
> Steve
>
> ----- Original Message ----- 
> From: "Lionel Catalan" <lcatalan at lakeheadu.ca>
> To: <aavso-photometry at mira.aavso.org>
> Sent: Monday, August 11, 2008 4:25 PM
> Subject: [Aavso-photometry] Flat frame quality and its effect on 
> photometricaccuracy
>
>
>> I've noticed that the main source of systematic error in my photometric
>> analysis is now due to the quality of calibration flat frames. I take sky
>> flats at dusk or dawn with an exposure time adjusted to achieve a pixel
>> value approximately 50% of the linearity range of my camera (SBIG 
>> ST8XMEI).
>> I usually combine 10 to 25 individual flat frames to make master flat
>> frames. I make master flats for each filter that I use. Because I use a
>> German equatorial mount, the stars change position in the CCD image after 
>> a
>> meridian flip. This change sometimes causes jumps or drops of up to 0.015
>> magnitude in the target or check stars. I attribute these jumps to
>> less-than-perfect flats. I reason that an error of just 1.5% in the 
>> master
>> flat pixel values would cause a systematic error of 0.015 magnitude, and
>> this error only shows up during meridian flips or when the stars slowly
>> drift in the image (I try to avoid that drift by using an autoguider to 
>> keep
>> my target star centered in the image). I've also noticed that if I try to
>> calibrate a flat done on one night with a master flat done a previous 
>> night
>> (without changing the camera orientation in between), then I don't get a
>> perfectly flat image having just random noise. Instead, I can see the 
>> edges
>> of dust donuts, and these patterns in the calibrated flat represent about 
>> 1%
>> variations above or below the image average pixel value. So, in summary, 
>> I
>> don't know how to further improve the quality of my flats to avoid (or
>> reduce) these errors.  I'd be grateful for any suggestions.
>>
>>
>> Lionel Catalan
>>
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>