Blured round the edges

Its f10.

I noticed it whilst looking at m37 in Auriga. The cluster filled the field of the eye piece and the stars at the edge seemed to be slightly out of focus and the stars in the centre were in focus.

I am using a diagonal, again the one supplied with the scope.

I have 3 of these eyepieces and one of them is defective.
Next time you're looking at this object, loosen the eyepiece and rotate it, and see if you can see the curvature rotates with the eyepiece.

It might just be that the cluster fits perfectly in the field for your scope and eyepiece combo and is perfect for showing the natural field curvature of your SCT, althoguh I didnt think this was visible in the standard 26mm ep.

When you say its at the edge, do you mean the extreme edge?

I think from memory it was about the final 20% out to the edge. So I suppose a more expensive eyepiece is the real solution :roll:

David --

The field curvature originates in the SCT itself. However, some eyepieces have field curvature of their own, which, depending on the sign, can either add to or cancel out some or all of the SCTs curvature.

That being said, I'm very surprised that you found it with a Plossl of that focal-length. I wasn't aware that Plossls suffered overly from field curvature.

Your eye also plays into to this: some people have more accomodation (ability of the eye to refocus) than others.

Lastly, even Naglers and Pentaxes have field curvature. So try before you buy if you can -- you may need to find a design well-suited to your scope/eyeball combination.

-- Jeff

Hi David

I have noticed this affect myself with my own 8" SCT with an F/6.3 focal reducer and a 26mm eyepiece however it is not serious enough to hamper my own observing.

I am suprised it is prominent in an F/10 instrument as normally it would take a wide field eyepiece in a short focus reflector to see this coma/vignetting in an obvious fashion.

Double check your collimation just to rule that out first and try another low power eyepiece if you have one to see if it is still present and let us know.

Hope you get it sorted...good luck and clear skies to you!

My Nagler is defective?