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Thread: Calibration Frames: How many and why?

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    Default Calibration Frames: How many and why?



    Dark Frames, Flat Frames and Bias Frames - How Many of Each and Why?

    Sounds like the title of a good book and at least one chapter of one anyway.

    I've heard a lot of different answers. I've seen people use 10 of each while others insist that you should have at least 30 of each. I've heard one guy say that he likes to have 10-15% as many of each calibration frame as he has of light frames and I've recently seen one youtube video where they used 100 bias frames. I've even heard that using both dark frames and bias frames ends up subtracting the bias frame data twice.

    It's truly all over the place!

    So, what's the real answer? How many of these calibration frames are really necessary and why?

    Clete

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    Default Re: Calibration Frames: How many and why?

    Hi Clete,

    Here is your answer direct from the Deep Sky Stacker FAQ.
    DeepSkyStacker FAQ

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    Default Re: Calibration Frames: How many and why?

    The answer is ... it depends.

    I like about 20 calibration frames of each flavour. That is enough to eliminate most noise from them. More frames will eliminate more noise, but there is a diminishing return.

    The effect of bias frames depends on how the software uses them. If you are doing the calibration by hand, you could indeed subtract the bias signal twice if you are not careful. However, bias frames can be used to scale darks by exposure time. If you subtract bias from darks, what is left is pure thermal signal, and is proportional to exposure time. That allows 5-minute darks to be used with 2-minute lights, when properly scaled, a very useful thing to do. PixInsight's Batch Preprocessing does this.
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    Default Re: Calibration Frames: How many and why?

    I do about 100 bias frames and about 50 dark frames.
    I do 35 flat frames with a target of 35,000 ADU.
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    Default Re: Calibration Frames: How many and why?

    Well, that all makes perfect sense to me now.

    The number of each type of calibration frame used is only (well, primarily) relevant to the signal to noise ratio in the resulting master frame.

    So, while 100 frames is better than 10 frames, the difference is probably not worth the extra time it takes to collect and process them.

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    Default Re: Calibration Frames: How many and why?

    I normally collect 40-50 flat frames for each session. It takes a couple of minutes, so not a big time eater. My current master bias is made up of 200 frames. I figure I am going to be using these frames to calibrate, and more frames = better SNR, so I am adding less noise to the final image with the larger number of frames.

    I shoot darks outside of my AP sessions and the library has a range of 20 - 40 subs for each temperature range I am likely to encounter. Doing that takes quite a while, but I normally set up the camera on an unheated porch and let it slog away all night unattended. I don't like using valuable AP time to collect the darks. I usually use 3 minute subs, and while shooting darks I pause for a couple of minutes between frames to try to control the sensor temp, so to get 20 darks takes an hour and 40 minutes to two hours.

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    Default Re: Calibration Frames: How many and why?

    I generally use none, spend my time capturing the target not the back of the lens cap.

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    Default Re: Calibration Frames: How many and why?

    Quote Originally Posted by bobharmony View Post
    I normally collect 40-50 flat frames for each session. It takes a couple of minutes, so not a big time eater. My current master bias is made up of 200 frames. I figure I am going to be using these frames to calibrate, and more frames = better SNR, so I am adding less noise to the final image with the larger number of frames.

    I shoot darks outside of my AP sessions and the library has a range of 20 - 40 subs for each temperature range I am likely to encounter. Doing that takes quite a while, but I normally set up the camera on an unheated porch and let it slog away all night unattended. I don't like using valuable AP time to collect the darks. I usually use 3 minute subs, and while shooting darks I pause for a couple of minutes between frames to try to control the sensor temp, so to get 20 darks takes an hour and 40 minutes to two hours.

    Bob
    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?

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    Default Re: Calibration Frames: How many and why?

    If you use an AP camera, they have a TEC/Peltier cooler for temperature control, so you can take you dark frames at any time and make libraries of them to use later. I take them in the house without a scope.
    So I have libraries of dark's taken at different exposure lengths, gains and temperatures.
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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?
    Once a year is a good plan. That is enough to keep up with any deterioration of the sensor.

    Flats, of course, are a different story, and should normally be taken new every session.

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