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Thread: Calibration Frames: How many and why?

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    Default Re: Calibration Frames: How many and why?



    Quote Originally Posted by Clete View Post
    So, if I understand you correctly, you maintain a collection of dark frames taken at various temperature and you use whichever set is appropriate for the session of lights you're calibrating.

    How often do you update them?
    I try to refresh them once a year, so that my images are never more than 6 months from the time the darks were taken. I have seen that using dark frames that are more than 6 months earlier or later than my lights leads to more noise in the calibrated image. I suspect this time-frame is different for different cameras. I have only used one camera for AP, so can't say for sure.

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    Default Re: Calibration Frames: How many and why?

    Doing darks for a DSLR is problematic, even for different temperature settings, because DSLRs have a habit of heating up in the middle of imaging sessions. I've seen people write that they don't even bother with darks because of this and just do bias and flat frames.

    In terms of the number, doing 100 bias frames isn't a big deal. Bias frames are zero exposure and you're just waiting the download time. I have a cooled CCD and what I do is after an imaging session just leave it on in the garage overnight and take my calibration frames while I'm in bed. I get up and I have 100 bias and 30 or more dark frames ready to go.

    Processing them doesn't take that much extra time.
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    Default Re: Calibration Frames: How many and why?

    Quote Originally Posted by s24man View Post
    Doing darks for a DSLR is problematic, even for different temperature settings, because DSLRs have a habit of heating up in the middle of imaging sessions. I've seen people write that they don't even bother with darks because of this and just do bias and flat frames.

    In terms of the number, doing 100 bias frames isn't a big deal. Bias frames are zero exposure and you're just waiting the download time. I have a cooled CCD and what I do is after an imaging session just leave it on in the garage overnight and take my calibration frames while I'm in bed. I get up and I have 100 bias and 30 or more dark frames ready to go.

    Processing them doesn't take that much extra time.
    I've seen some say that too. I have, however, noticed a significant improvement in my images with dark frames used. The difference isn't as dramatic as the difference that flat frames makes but still worth doing.

    That may be because my exposure times are quite short. When I get an autoguider and start taking exposures that are 3, 4 or 5 minutes each, then that may create the sensor heating problem.


    Speaking of temperature. Does anyone have a definitely answer as to whether bias frames are significantly effected by temperature? The DSS user manual says no but some other's have told me otherwise.

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    Default Re: Calibration Frames: How many and why?

    No, bias frames are not significantly affected by temperature. The effect of temperature is cumulative over time. Since bias frames are taken at the shortest exposure that your camera is capable of, they have minimal thermal signal.

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    Default Re: Calibration Frames: How many and why?

    Quote Originally Posted by KathyNS View Post
    No, bias frames are not significantly affected by temperature. The effect of temperature is cumulative over time. Since bias frames are taken at the shortest exposure that your camera is capable of, they have minimal thermal signal.
    Cool!

    (see what I did there)
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    Default Re: Calibration Frames: How many and why?

    Correct. Thermal signal is a linear effect. So a sensor at -20 will have exactly 50% as much dark signal at 5 minutes than it would at 10 minutes. This is predictable and is why we can scale flats.

    Unfortunately, the same is not true for different temperatures. For example if you're going to have dark frames at 10, 15, and 20 degrees, the amount of signal will not be linear.
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