Linear image, small stars look like donuts....

R

rroberts

Member
Okay, I don't think I was out-of-focus and nothing else I'm seeing would make me think that, and this is only on the smallest of stars.

The image is from an ASI2600MC, calibrated with flats and darks, cosmetic correction, registered, split into RGB, then normalized scale gradient, recombined, and photometric color calibration. This is NGC 7000, and it looks okay overall, but when I zoomed in to get a sense of how much noise I was going to have, I noticed all the tiny stars look like the've got a hole in the middle. If I turn off the screen stretch, the stars that show up don't show this at all, and you can see from the image that it's really only the small ones and even then not all of them. This is zoomed in at 4:1

ngc7000-donut-stars.png


I don't have that many frames, only 16 (10 minutes each). The only thing I can think of is the cosmetic correction because that's the only thing that would be doing single pixel corrections, but I'm not sure how it would happen so selectively.

In the end, it probably won't matter or be noticeable, but it bothers me to not understand how it got there.
 
Hello @rroberts , I bet that it is your CC. I've been there done that...
I was puzzled about that too and I found that my CC, (Auto 3.0 as I've learned from Adam's tuts) was way to harsh for me.
 
did you use drizzle integration? there are conditions under drizzle where this can happen but i think they might not apply here. otherwise @Euripides is probably right.
 
No drizzle. Technically, I'm probably undersampled, but given typical seeing conditions here in the northeast US, all but the faintest stars span more than one pixel, although they may not span more than one of the same color.

I recently subscribe to Adam Block's Fundamentals and I'm watching his tutorial on CC now. I rushed a bit when I did this and skipped the preview for how CC was behaving on my images.
 
Was definitely the CC. Using a sigma was a bad choice. I had forgotten these images were from a trip to a dark site with better seeing, too, so I really do have undersampled stars that are pretty close to 1-pixel for the faint ones. I switched over to use the master dark and it looks much better now. This is pretty close to the same field as above

ngc7000-no-donut-stars.png
 
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