So, I recently picked up an old Meade 10" SCT (I know I've been talking about refractors a lot but a larger SCT was planned for when one popped up at a decent price, which finally happened). I think it's the model that immediately pre-dated the LX200, the Premiere. Anyways, conditions prevented a really good star test at the time I was buying it (20 degrees outside, scope at room temp, scope was miscollimated, and it was cloudy), but it seemed ok. So I went for it.
I've been star testing it indoors to fix the collimation (which was badly off - why anyone tolerates such bad collimation in an SCT is beyond me, but it's really common) and check out the optical quality. And I notice that outside of focus (I tried to go for the 4.4mm distance recommended in posts here), the star images have a bright outer ring and the secondary shadow is huge, and inside of focus the star pattern is kind of fuzzy and indistinct. I think this would indicate overcorrection. Is this correct?
Also, is there anything in an SCT that can cause overcorrection or other optical problems similar to this, that can be fixed? I accidentally detached the secondary mirror while trying to collimate (I thought it had a collimation screw cover cap - it did not). Fortunately I had the scope pointing downward (since I knew I could be wrong...) and the mirror didn't go anywhere, and was easy to re-attach. It's collimated now and everything seems ok, but I thought I should mention it as a potential source of problems.
I did get to star test it a little last night on an actual star (it was hazy, but the test seemed to still work) and it looked similar.