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focus specific flats.

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Americo Watkins's picture
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Joined: 11/03/2014 - 13:12
focus specific flats.

Given that the optical train remains the same and that there is no rotation of the fov is it necessary to create a set of master flats for a given filter due to a nightly change in focus due to ambient temperature change particularly in the case of photometry?

Could I take a set for each filter and use them for several nights or weeks?

Cheers, Eric

andrew.j.smith1905's picture
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Theory v experiment

I am sure in theory their could be some changes but they might well be of the same order as the changes between taking the flats and the observations on a given night.

I would do a set on two nights with a reasonable temperature difference. Then subtract them and see if the is a significant residual or not.

Regards Andrew 

Xilman's picture
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Yes, you should take frequent

Yes, you should take frequent flats if you wish to do photometry at the highest precision possible.  The out of focus dust ring-images change in size and intensity as the focus changes.

Most everyone, myself included, ignore this effect and use a set of flats for lengthy periods.

Peter Carson's picture
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Used for long periods

I take flats, made up of about 50 averaged images, each time something in the optical train changes, like moving the camera or a new blob of dust appears or I use the camera at a different set tempetature. I redo the flats if the season changes but nothing else has changed. However apart from that I keep using the same ones. I've experimented and can't find any decernable improvement if I take flats every time I slightly tweak the focus.

Americo Watkins's picture
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focus specific flats

Thanks,  Andy, Peter and Paul,

at the end of a nights's session I can barely keep my eyes open and I have always taken a new set of flats and corresponding darks. I actually have a medical problem of not getting enough sleep which is getting to be a problem.  If I was simply imaging I would't bother, but as I do a lot of photometry the flats may have been an issue.  I'll try experimenting and see if there is any difference.  Peter's experience seems promising.

Many thanks,

Eric