[Aavso-photometry] sky flats

[Tom Krajci]

From: "Papini Riccardo" <paprika at inwind.it>

> >Also, does your software normalize the sky flats?  This can be important
>   
>>when average values change significantly between flats, which is the case
>>with sky flats.

>Can you explain what you mean here? I use MaxImDL.

Some software (I know AIP4Win does this) will normalize the average value of sky/twilight flat frames...typically to a value of 1.0000.  Then all flats have the the same average value...instead of a changing series of average values from twilight.  This makes it easier/better for certain combining algorithms (median, k-sigma combine, etc.)...but I don't think it really has too much impact on the averaging algorithm.  However, one point to consider...in your changing series of twilight flats the fainter flats have less signal and more noise.  I don't know if any/all/some software does this, but perhaps a weighted average is a good idea here so that the noisier/fainter flats don't contribute as much noise in the stacking process.

This is why I like 'artificial' flats...essentially constant/high brightness.  You can get a high quality/high SNR flat this way if you solve other problems.


> >I hope this helps.
>   

>A lot! Thank you again Tom. The key point of my question was: is it correct
>to average flats with different signal level? Flats are a topic of this list
>but it seeems to me that each person take flats his/her way.

See my above comments on averaging and weighting of various brightness flats.

-- 
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Tom Krajci
Cloudcroft, New Mexico
http://picasaweb.google.com/tom.krajci

http://overton2.tamu.edu/aset/krajci/

Center for Backyard Astrophysics (CBA)
http://cbastro.org/ CBA New Mexico

American Association of Variable Star
Observers (AAVSO): KTC http://www.aavso.org/
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