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Creating Master Flats/Repair Maps

Alan Karty
in CCDStack
Hi Adam.  Let me start by saying that I am a great fan of your CCDStack tutorial and it has helped me immensely over and over again.  Still I have two questions that I hope you can help me with.

1.  I create my flats using an electroluminescent panel of fixed intensity.  My exposure times for each filter in seconds are as follows:
L  1x1  3.2
R  2x2  17.0
G  2x2  4.7
B  2x2  5.4

I have been applying a master bias to each flat as I create the master flat.  Is that still good enough for the 17 second red flats or do I need to apply a master dark of the same duration?  I did a comparison and the two flats did not appear the same - the one with the master dark was brighter.

2.  Should I create a repair map from the master bias or from the master dark?

Thank you.

Alan

Comments

  • BlockHeadBlockHead
    Wow... that is a really odd result for the RED filter. Can you please tell me the filters and camera you are using? I cannot think of a filter/camera combination that would yield such insensitivity in Red!  

    To answer your question, but I can't wrap might head around that time!, I suspect that even in 17 seconds you will not accumulate any significant number of electrons from dark current...and the bias is enough when creating your master flats. I think if you are going over 60 seconds... than maybe a dark for the flat is necessary. But then just make the panel brighter.

    With respect to #2 (I would need to review my section) - but I would use the dark frame as the reference. The dark frame has the bias in it right? So it will show you all issues (column problems and other things). Ultimately, as I recall, you can create the map using the Pixel Math code thing. That is the easiest way if indeed the problem are columns. I would not work about creating a map of hot pixels or something. The best way to handle them is with the hot pixel filter, dithered data, and proper rejection.

    Please let me know if this is helpful.

    -the Blockhead
  • BlockHeadBlockHead
    By the way Alan... the repair of columns and things is actually easier in PI. 
    Something to consider.
    I have a section on Defect Map that you will probably like.
    -the Blockhead

  • Alan Karty
    Hello Adam.  Thank you for your assistance.  In reply,

    My camera/filter combination is SBIG-STXL 11002 and Astrodon Tru-Balance LRGB I-Series CCD Filters Generation 2.

    Alan
  • BlockHeadBlockHead
    OK! 
    Wow... I found this useful chart:


    Your chip is significantly less sensitive in the red... but I am still struggling to see an RGB ratio like you describe. If you get a chance..it would be great for you to create an RGB image in PI and use Photometric Color Calibration. This would produce an independent measure of your RGB ratio (maybe you have already done this?).

    Anyway, thanks for the followup.

    -the Blockhead
  • Alan Karty
    Adam,

    I believe I have found the problem.  It is the ELP that I am using.  It emits highly in the green-blue region and very poorly in the red..  I am switching to sky flats.

    Alan
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