Image Quality With and Without Auto Dark Subtraction

Discussion in 'Image Capture' started by Mike Hambrick, Apr 19, 2019.

  1. Mike Hambrick

    Mike Hambrick Cyanogen Customer

    Jul 13, 2016
    While waiting for the next clear, moonless night I have been experimenting with the Auto-Grab function in CCDOps with my STXL16200 and I have noticed something that I wanted to get some input on. Compare the two images below. Both are 20 second "light" frames that were taken with the cover on the camera (Sorry - No M42). The Test5, Clear003 image was taken without any dark frame subtraction, and the Test6, Clear003 image had an auto dark frame subtraction. The cooler was running at -30 Deg C for both images.

    In my mind, the Test6 image with the dark frame subtracted is much grainier than the Test5 image. I would have expected just the opposite to be the case. Is this increase in graininess due to the fact that I am subtracting a dark frame from a dark image, or can I expect to see a similar increase in the graininess with real images when I do an auto dark subtraction ?

    I don't normally do auto darks because I process my images in Maxim DL and I will do an auto-calibration of the raw images with a stack of dark frames that I take separately. upload_2019-4-18_23-7-24.png upload_2019-4-18_23-9-19.png

  2. Doug

    Doug Staff Member

    Sep 25, 2014
    You can’t go by the appearance because of the screen stretch (brightness/ contrast) function. Having fewer hot pixels will make it stretch more, making it look grainier.

    You have to go by the numbers.

    Subtracting a single dark without averaging dark frames will actually increase the random noise by 41%, but it will immensely improve the hot pixels, resulting in a large overall improvement. Averaging a bunch of dark frames will improve the overall random noise.

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