Round enough stars?

I guess the question is how round is round enough and what should I expect from the Orion ED80T CF?

With the correct flattener spacing and better flats, I got M42. This is my "middle length" RGB exposure for this target, as I will need some longer for the dim areas and some shorter for the brighter areas. (I was pleasantly surprised with the colors I ended up with when I went back and looked at Jim's recent "dusty" image.) I think more of the stars in this image are rounder, but that upper right corner concerns me. Is this most likely the lens is not square, or is the focuser a bit loose? I must admit I sometimes have issues tightening the focus ring in the grove on the field flattener "enough". The image is here, including a link to the full resolution version:
http://www.fortlewis.edu/observatory/image_detail.asp?ID=256

When I first got the field flattener, I imaged the Soul nebula, but almost under a street light, and with lots of mount and camera fiddling, so getting good flats was almost impossible. The stars in the lower left are much better than the ones in the upper right. With all those issues, I spent way too long processing. That image is here:
http://www.fortlewis.edu/observatory/image_detail.asp?ID=255

Thanks for looking.

Charlie

Charles,
I like the images. I think your M42 is in great shape and your plan for completion looks pretty good. (BTW, you are a brave soul doing M42 with a non-ABG camera)

Regarding the stars, I tend to agree with you in thinking it is a camera orthogonality issue. I have some "middle grade" WO refractors and the flattener does a great job, but the focus mechanisms are somewhat less than robust. I suspect that is the case for the ED80T as well - but I don't have any experience with it. Nosepiece attachment is also problematic at times. I also have to say that I hate grooves in the nosepieces as well - they only cause problems in camera mounting. Top that off with the somewhat heavy ST-10XE and I think orthogonality could be an issue.

The Orion image of mine that you mention was done on an FSQ-106N - Those focusers are truly robust, all camera connections are threaded, and of course the optics are superb. I think the only way I can get other than round stars is to have tracking errors.

Nonetheless, your stars in the upper left can be processed to more "roundness" if you want to do that. You can do that with a corner masked "darken" layer that is radially shrunk a pixel or two (in Photoshop).