focus specific flats.

Forums Photometry focus specific flats.

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    Eric Watkins

    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

    Dr Andrew Smith

    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 

    Dr Paul Leyland

    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

    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.

    Eric Watkins

    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,


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