Hi Lee,
don't you just love Newtonians?
So this is some of the ideas I have used to find solutions to problems.
- Check the rotation of the secondary to focuser. That is certainly one cause of tilt or elongation. You don't need to be far off to have this problem.
- Next check the collimation thoroughly. Both the above and collimation have huge effects on the appearance of tilt when combined with a coma corrector.
- Then check the actual corrector (threads etc). These suckers are prone to tilt and paying a lot does not guarantee you don't get tilt. Use a laser through the corrector to see if it is centred. I found it was not the case on the OOUK corrector.
Just remember this is going to be something very minor, only fractions of a millimeter that is causing this.