Coma or collimation issue? (SkyWatcher 150PDS) - Forums

Topic starter

Dec 8, 2022

Hello

I'm relatively new to the hobby, started last year with a basic reflector on a Go-To mount.
I'd like to get some advice regarding poor quality of my shots.

My equipment:
SkyWatcher Explorer 150PDS
SkyWatcher EQM-35 Pro mount
Nikon D7200
ZWO EAF focuser
APT for automation and acquisition

1.It is very difficult to get a proper focus even when using the auto-focus feature in APT software. It is so bad plate solving fails most of the time.
2.I see distortions in the corners of every shot, the further from the center the worse they get.

Is this coma, collimation issue or something else?

Some samples below to better explain these issues. 100% crops from corners and center.
Full size photo: https://www.astrobin.com/rkmc2s/

Bottom left corner:

Bottom right corner:

Center:

Top left corner:

Top right corner:

Best regards
Andrzej

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Linus

Dec 8, 2022

Looks to me like it might be Coma + Astigmatism. I see you don't mention any Coma Corrector in your gear, without that the coma will be hard to get rid off. The astigmatism however can be fixed by not overtightening collimation screws and loosen the tension on your primary mirror cell.

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Guillermo de Miranda

Dec 8, 2022

Loosen your mirror clamps. They are tightened down for transport but it warps the mirror ever so slightly. This can cause all sorts of weird optical aberrations. The rubber clamps should not touch the glass.

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Mike Dobres

Dec 8, 2022

As stated above - you need to use a coma corrector: Skywatcher makes one: http://skywatcher.com/product/f5-coma-corrector-2/
I have the same:It works great on both my SW 130PDS and my 350/14 inch SW Dobson.

If you still see problems, then loosen clamps and play with collimation.

Mike

Lorenzo Dal Molin

Dec 8, 2022

Hello,
The coma corrector is mandatory. If you don't use it you will always have deformation. But if that were the only problem you would see a perfectly radial deformation and a perfect photo center. For that, in my opinion you have to check the collimation. The considerations on the tightening of the mirror are correct. You may have slight astigmatism due to this. Something like the coma effect could also result due a wrong distance of the camera sensor from the plane of the corrector lens. I know that all this is complicated and that's also why the problem of deformation is omnipresent in my photos!

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Andy Wray

Dec 8, 2022

I would get the Skywatcher coma corrector and move to Nina