Why are stars stretched toward the edges with an 80ED and field flattener?
Asked 8/16/2016
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My astrophotos used to look fine, but now stars near the borders are stretched into short lines pointing roughly toward the center. The center of the frame still looks fairly round. My setup is a Sky-Watcher 80ED refractor, a field flattener, and a Canon 450D. Since the flattener previously corrected the field well, I’m wondering whether this is a collimation problem or something else in the optics or imaging train.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
9y ago
2 Answers
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Disclaimer: while I have considerable experience with optical systems (and adaptive optic systems :-) ), my preliminary conclusions are based on qualitative observations of your files and not any numeric processing.
The still image shows circular objects near the center, and elliptical objects near the edges; these ellipses appear to be aligned along radial lines (look at items in the corners of the image). This suggests magnification distortion rather than coma.
If I look closely at the first video (image near center of field), I think I see some astigmatism near best-focus. That is, there's some elongation along one direction just before focus, and elongation perpendicular to that direction just after focus.
The second video appears to show similar astigmatism, albeit of larger magnitude.
So, in sum, I'm not convinced there's coma, but there is probably some astigmatism, which, combined with radial distortion, may look like coma. Without seeing data from "when it worked before," it's hard to say what may have changed.
Originally by user25396. Source · Licensed CC BY-SA 4.0
user25396
9y ago
0
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This does not sound like classic coma from collimation. Based on the described star shapes, the more likely causes are field/magnification distortion and some astigmatism in the optical train.
Key clues:
- Stars are fairly round near the center but become elongated near the edges.
- The edge elongation is aligned roughly radially.
- The focus behavior suggests astigmatism: elongation changes direction on either side of best focus.
In a small refractor like the 80ED, true collimation failure is less likely than a spacing, tilt, or accessory issue. The field flattener may no longer be sitting at the correct distance from the camera sensor, or something may be slightly tilted or loose. If anything in the train changed since December—adapters, spacers, camera seating, flattener orientation, or mechanical tightness—that’s the first place to check.
So: probably not a simple collimation issue, and not pure coma. Check backfocus spacing, tilt, and whether the flattener/camera connections are secure and aligned.
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