Astigmatism in a refractor

[whzzz28]

I've been under the assumption that my star shape problems were related to my Camera (i self-modded it, thought i stuffed it up), but tonight i borrowed a Canon 7d and the problem is still occurring.

After some more research, the only possible cause i can find is astigmatism.

I was under the belief that refractors did not suffer from astigmatism, at least unless they are not collimated.
This also only seems to affect one side of my image, the right. The left hand side and center of the image are great with nice sharp stars.
I also run a field flattener (Hotech) and this was occurring before the flattener was put in.

Is this astigmatism, or something else?
http://core-au.net/astro/ast.png

If it is astigmatism, can it be corrected?
My wide field shots have to be cropped with a good portion of the right side gone as it is blurry or has elongated stars.

I am concerned that my purchased new scope was in fact a return and the previous owner pulled the scope apart (lenses out) as when i received it,it had a significant amount of dust on the inside of the main lens. It is still there, and short of removing the lens i dont think i can get rid of it.

Might be worth shooting an email to Bintel, although i have had the scope for 5~ months now.

-edit-
Should mention the scope eh;
Meade 5000 ED80 (APO triplet)

Cheers.

[anj026]

If you have an eyepiece you can check the scope for astigmatism by viewing a bright star and racking the focuser in and out. Astigmatism is when the diffraction rings are oval shaped and the orientation of the oval is 90 degrees opposed when inside/outside of focus.

Here is an online article (http://www.cloudynights.com/documents/aberrations.pdf) about optical abberations and how to recognise them.

Refractors can certainly suffer from astigmatism. It depends on the design and quality of the lens. If the elements are misaligned or the retaining ring is overtightened this can cause astigmatism.

The problem you have could be one of alignment. Check the alignment of the camera in the focuser, perhaps a thumbscrew is causing the camera to tilt slightly off axis. Maybe there is an adapter or something that is not concentric with the optical axis. :shrug:

Bintel are very good and you should certainly discuss it with them over the phone initially and see what they suggest.

Andy