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Dark pixels around stars pixinsight


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I stacked over 1000 images in pixinsight today, most painful thing in my life. But when I started processing I noticed dark pixels around my stars. I did stack them in pixinsight so I don't know if thats the issue, but I really don't want to go through stacking again. Is there a way I can fix this? 

Horshead Nebula 1.jpg

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1000 images! Wow! That is some achievement and congratulations on your perseverance to capture 1000 images of this one target. I really like the composition and framing.

I am not at all sure what is causing that phenomena you mention but I am drawn to the lack of definition and clarity in the horsehead feature itself. Are you confident all 1000 images are good? Have you tried the SubframeSelector process in PI? I presume not all the 1000 images were collected on the same night so we are looking at multiple imaging sessions. I think it might be worth grading the images and selecting only the very best to include in the final stack.

Sometimes less is more - maybe - irksome though it may be to not use all 1000 subframes.

Can you provide details of the equipment used and subframe details, exposure times etc.

Good luck.

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I used the az-gti, nikon d3300 dslr. exposures were about a minute each. Very minor star trailing in half of them as I didn't have the best guiding until a few months ago. I did some noise reduction and no sharpening yet so it does look a bit softer. I didn't check the clarity and stacked them all. But I'm sure if this is the issue to the rings around my stars...

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I presume the artifacts around the stars aren’t on the calibrated frames. Not sure what the cause is but I wonder whether it’s something to do with the pixel rejection, what settings did you use in ImageIntegration?

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1 hour ago, TheMan said:

I used the az-gti, nikon d3300 dslr. exposures were about a minute each. Very minor star trailing in half of them as I didn't have the best guiding until a few months ago. I did some noise reduction and no sharpening yet so it does look a bit softer. I didn't check the clarity and stacked them all. But I'm sure if this is the issue to the rings around my stars...

Looks like strongly under-sampled maybe as all stars are square

Which lens have you used?

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To me this looks like an artifact that you can get when you under sample and use for example Lanczos3 or Bicubic interpolation for registering images.

It is sort of aliasing problem. Don't know how to fix it without restacking. Maybe gentle blur (I know, counter productive, but you can apply gentle blur to separate copy, do the same stretch, and then just blend in missing pixels).

If you restack your images - select linear interpolation for registration. This will make the problem go away.

Here is graph of "explanation" for effect:

image.png.ecc711a582df2008ffa6f924866fb1f7.png

When you under sample you get sort of "sharp" edges, and registering images applies certain interpolation that assume your sampling is close to optimum. If it's not - effect known as ringing occurs - it is due to missing high frequencies of signal (cut off by under sampling).

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As @vlaiv noted, it's a stacking artefact. If you have saved the registered images, restacking shouldn't take too long. If you do, save the bright pixel rejection image, and set bright pixel rejection to a higher value. Examine the rejection image, and adjust rejection parameters. You don't need to use all the images when you do the testing.

To fix the image that you have, create a star mask. Apply and use MLT to remove the first and possibly the second layer. The mask must reveal the artefacts, of course.

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