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Screened vs Unscreened

Darryl Madison
in PixInsight
I am a Horizons member. I was hoping to find a video explaining Screened vs Unscreened stars. I am confused when to use one or the other and what to use to put the stars back in the starless image i.e Combine with op_screen, or just stars + starless, or ~((~starless)*(~stars))? I'm just having a hard time grasping what and when to use ...

Comments

  • BlockHeadBlockHead
    I have the basic answer in a few different videos.
    The screening process is explained in the video on Blil Blanshan's star reduction process. 
    (However, I have been explaining the concept of screening images for at least 20 years in terms of an application in astrophotography.) The addition of images demonstrated most recently as an example in the video that explains how to remove super bad greadients from images by removing the stars first from the (linear) image.

    Here is the bottom line and why:

    1. When using linear images you will want to NOT use the unscreen stars. In this way what happens is that the stars are removed (SXT) and this image is subtracted from the original to give you the stars only. Note that when you subtract... you subtract the guessed background value that the star removal process (SXT) guessed shoudl be there. Then when you ADD (pixel math img1 + img2) your stars and starless you get almost exactly back what you originally had. 

    2. When using linear images its SEEMS like you can unscreen the stars and ADD them back with (img1 + img2) ... but this is mathematically wrong. The errors are likely small because the values are often small in linear images... but does not give you the correct answer. 

    3. When using linear images it SEEMS like you should be able to unscreen stars and screen them back with pixel math combine(img1,img2,op_screen()) ... but star removal programs can add a tiny pedestal value that makes not work. 

    4. When using a stretched image, you only USE UNSCREEN stars and then screen them back with combine(img1,img2,op_screen()) . Because everything is non-linear... the addition method simply does not work as well (correctly). This is the reason the unscreening became available. 

    I hope this helps clarify things?

    -the Blockhead 
  • Darryl Madison
    Thanks Adam for the info. It does help clarify things for me. Keep up the great work!.  Love your videos ...
  • BlockHeadBlockHead
    I did add the video on this topic...
    -the Blockhead
  • Darryl Madison
    Thanks Adam. Just watched it. Explains it well.
  • BlockHeadBlockHead
    Excellent!
    -the Blockhead
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