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

    Joined:
    Jul 13, 2016
    Messages:
    125
    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

    upload_2019-4-18_23-7-24.png
    upload_2019-4-18_23-9-19.png
     
  2. Doug

    Doug Staff Member

    Joined:
    Sep 25, 2014
    Messages:
    6,088
    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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