focus specific flats.

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  • #574322
    Eric Watkins
    Participant

    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

    #581032
    Andrew Smith
    Participant

    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 

    #581033
    Paul Leyland
    Participant

    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.

    #581038
    Peter Carson
    Participant

    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.

    #581040
    Eric Watkins
    Participant

    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

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