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darks spots in center of tiny stars after Cosmetic Correction

Benoit Saintot
in PixInsight
Hi Everyone

First, let me introduce myself. I'm an intermediate astroimager, living in France, and I discovered Adam's Block website and videos recently.
I purchased the Pix Fundamentals course, and I'm REAAAALLy happy with that. I have read reference books on pix, seen tons of videos on subject, but non of them has been more accurate and sharper than this course.
Actually, unfortunately, my scope is under maintenance for almost 1 year now ( carbon out of stock, and somme other stuff), so I'm currently using a compact system, with a Canon 1300D defiltered and a Fornax LIghtrack II. I must say I'm pleased with it for the easy and nomad aspect.

Yesterday, I gave it a try on NGC7000
After prepocessing the files today, I had a strange artefact: I have a dark spot appearing in the center of some Tiny stars.
I suspect this coming from the cosmetic correction. I don't know if I'm right, but it looks like something's going wrong among hot pixel identification / cosmetic correction / or debayering.
I watched Adam's video about this subject (Star alignement, debayering and hot pixel), but despite applying the steps thoroughly , it leads me to that result. I wonder also if this might come frome the debayering process with a undersample rate (this is shot at 135mm, pixel size is 4,75um). One can see that there's a strange " color logic". Or is ot due to too close background / noise level ... Well' I'm lost  !!

Here is a picture of left, a non cosmetic corrected debayered light frame, and right the cosmetic corrected and debayered one

image

Thanks a lot for your help.
happy to join community here !

Ben

Comments

  • BlockHeadBlockHead
    What you are seeing is that with undersampled data (and noisy images) the auto detect method of CC will think the centers of some stars are hot pixels. They really are in a way- they are bright compared their neighbors so the algorithm does its job. 

    This is why it is so important to take many exposures and *dither* all of them as much as possible. (Shift the telescope many pixels). Then you might be able to avoid using CC. 

    However, this is difficult to do. A more robust method in this case that is specific for noisy and undersampled data- is to use the dark frame method of CC instead of Auto Detect. Please review my section on CC and how the dark frame method works. This will remove hot pixels at a threshold you choose as they appear in your dark. This removal will not be correlated with stars like the auto detect method and you should get a much better result.

    Finally, even if you leave *everything as it is* and combine your data- the averaging that occurs will likely give you a final image that does not show dark spots in stars- even though you see it in individual frames.

    I hope this information helps.
    -the Blockhead
  • Benoit Saintot
    Thanks for your reply Adam.

    I'm pretty happy about it.
    First, this is close to what I thought. So your course has proved it's efficenciy :-)
    Secondly, that was what I did. I preprocessed the files a couple of different ways, and came to the conclusion that using a master Dark to remove hotpixels was a better way than CC with low sampling frames, ( and noisy too)
    Actually I also noticed that something made the CC process fail even more in this particular situation : hot pixels in my case were barely higher values than mean signal.

    Thanks for your help, and congrats for the fantastic job.
  • BlockHeadBlockHead
    Yes... for DSLR cameras "warm" pixels are indeed an issue.
    -the Blockhead
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