I noticed some COMA on my astrophotographs lately, that didn't show up last December. Stars near the borders of the image tend to form linear shapes directed towards the center.

My setup is: Skywatcher 80ED with a field flattener and a 450D canon to shoot. The OTA should in fact have some coma, but it should be reduced by the field flattener as it always did... somehow now it doesn't anymore.

I've filmed my attempt to focus a star both in the center of the image and on the borders, and you can download the videos here:

  1. https://www.dropbox.com/s/x7uifouvg33io1f/out_1.avi?dl=0
  2. https://www.dropbox.com/s/cx5iovuh1ocftts/out_2.avi?dl=0

I am also adding a picture I took a couple of days ago, 300 secs of exposure at 400 ISO with decent autoguiding and high light pollution... look on the borders, it seems like coma to me!

example of problem

  • \$\begingroup\$ Are you sure it's coma and not distortion? If the videos are distinctly worse than last year then most likely something is a bit off-axis or tilted. I can't access those links :-( , get a 404 error -- I don't think you pasted them correctly, since "..." is illegal in a URL \$\endgroup\$ Aug 16, 2016 at 11:27
  • \$\begingroup\$ Oh sorry, I've fixed the links now. Please do check them! Especially the second one, which is on the border. By the way, I am also giving you a picture I took a couple of days ago, 300 secs of exposure at 400 iso with decent autoguiding and high light pollution... look on the borders, it seems like coma to me! i.imgur.com/hJiNr0O.jpg \$\endgroup\$ Aug 16, 2016 at 11:37

1 Answer 1


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.

  • \$\begingroup\$ Hey, thanks a lot for your input! Anything helps really, I am a newbie so it's hard to tell oranges from apples. Anyhow here is a picture of M42 I took in December, with 45 secs of exposure, 400 iso in BW and NO autoguiding, so you can tell there are trails. But there is no "coma" or whatever it is! I am also linking you a picture of a few days ago, 300 secs exposure (lots of light pollution) at 400 iso RGB. What do you think I should do? \$\endgroup\$ Aug 16, 2016 at 13:12
  • \$\begingroup\$ I just realized that when I took that picture in December I wasn't using autoguiding so I had only my main scope (80ED) on the Mount. Do you think maybe that the second scope (70mm aperture) on top of the first one is causing some weight issue? If I can I will try tonight without it. But the lens shouldn't be "pushed" by the second scope since it's only mounted on the middle of the tube. \$\endgroup\$ Aug 16, 2016 at 14:10
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    \$\begingroup\$ Distortion is invisible at any one point in the field of view. I believe this is elliptical coma on axis, and a combination of coma and astigmatism off-axis; the caustic or mushroom-cloud type shape that forms very near focus is a classic example of coma. You can go here optikos.com/wp-content/uploads/2013/11/How-to-Measure-MTF.pdf at page 19 for an example, or go here: wyant.optics.arizona.edu/psfMtf/psfMtf.htm and enter e.g. astig:1 coma:1 defocus:0.25. \$\endgroup\$ Aug 16, 2016 at 14:36
  • \$\begingroup\$ @BrandonDube well, yes, but each star's diffraction-limited image is not a (mathematical) point, but rather an extended object. You can certainly see distortion in such an object. That said, I agree with your assessment of coma+astigmatism. \$\endgroup\$ Aug 16, 2016 at 15:37
  • \$\begingroup\$ Do you think the astigmatism could be caused by the guider scope weight? Because a bit of COMA is given with this kind of refractor afaik. \$\endgroup\$ Aug 16, 2016 at 17:08

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