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Artifacts like "ghost " Diffraction spikes

Benoit Saintot
in PixInsight
Hello Adam.

After stacking, an artificat looking like a diffraction spike appears, but it's not linked to any star.
I went thorougly through my subs, Flats, Darks, Dark Flats, LRGB masters, actually it only appears on the LRGB ones.
At first I suspected a satellite and a too large rejection sigma, but I realized it was suspiciously exactly parrallel to the diffraction spikes.
It's really really hardly visible on the subs, but comes out after stacking.

Do you have any idea about where does this comes from ( reflexion on the double carbon spider vane, screw, ...), and how I can mange this and have it rejected ?

Thanks a lot for your help, and congarts for the wonderfull content you provide.

Comments

  • BlockHeadBlockHead
    HI Benoit,

    This is likely due to a very bright star outside of your field of view. It is linked to a star... even though it is not in the field..the diffraction from the star will be. There isn't anything in particular you can do about this... it is "real" photons in your image (even thought they are undesired). 

    The nearby 6 magnitude star is the likely culprit (the one closer to M81 in the attached picture). 
    I can't say for certain... perhaps you can see if it make sense. 

    Really the best you can do is substitute those pixels. There are a gazillion ways to do... of course no one actually does... most people just clone the thing out and get on with life. lol

    -the Blockhead

  • Benoit Saintot
    Thanks for your reply Adam.

    If I do understand, you mean that this spike could be due to another star, without  crossing it ?
    If thought spikes could only run through the center of a star.

    Many thanks

    Benoit
  • BlockHeadBlockHead
    The spike would definitely lead back to the originating star..but that star could be several fields of view away if it is bright enough. It looks like I forgot to attach the image. But there are a handful of 6th and 7th magnitude stars near by... and one 5th that could be the culprits. You need to look at a star chart/sky atlas and see if any fit the bill.
    -the Blockhead
  • Benoit Saintot
    OK, I do understand now.
    I wondered also if there was no image missing in your post.
    I looked in stellarium, there might be a culprit (hip 49230 maybe, it's in the axis of this spike)

    Thanks Adam
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