Creating & using calibration masters

Discussion in 'Image Processing' started by Adrian Jones, Jun 23, 2020.

  1. Adrian Jones

    Adrian Jones Cyanogen Customer

    Joined:
    Oct 23, 2014
    Messages:
    14
    I hope someone can answer this question that has bugged me for a while.

    I like to build a library of dark, flat and bias masters that I can use for a while and refresh from time to time. I'll usually do this as a completely separate operation from processing any Lights/ science images.
    My question is about what options I should be selecting in 'Set Calibration' when I'm processing only the calibration frames to build masters (as opposed to later when I'm calibrating the light frames using the masters so created.)

    For example:
    I have a set of bias, flat and flat-dark images and I want to produce a master flat. The flats have differing exposures so I need to scale the flat-darks. Now, at this stage when all I'm doing is creating a master flat, should I be selecting Auto-scale darks and should I tick all the boxes in the Advance menu (Calibrate bias, Calibrate dark, Calibrate flat, Bias subtract flat, Dark subtract flat)? Also, should I set a pedestal value?

    Or should I leave these options un-checked here .... and only check them at the next stage when I come to apply the master calibration frames to the lights? Or do I select all options in both cases?

    Thanks

    Adrian
     
  2. Doug

    Doug Staff Member

    Joined:
    Sep 25, 2014
    Messages:
    6,860
    The settings when making master frames should be identical to when you are calibrating images. There is no reason whatsoever to change them.

    You should always tick all the boxes in the Advanced menu (i.e. click the Defaults button). It is very rare that anyone should ever need to turn one off. That's why it's the Advanced menu.

    The only time you need to adjust Pedestal is when the standard deviation in your image background is so large that you get pixels clipped to zero after dark/bias subtraction. You will be able to see that immediately by looking at the histogram in the Screen Stretch window. If you are concerned about this then set the value to 1000 and you should never have to worry about it again.
     
  3. Adrian Jones

    Adrian Jones Cyanogen Customer

    Joined:
    Oct 23, 2014
    Messages:
    14
    Great - thank you!

    Adrian
     

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