Yes, size, size, size
Alex, I see where you are coming from. I have a recently acquired an 8 inch, old Orion F4.4..........massive secondary mirror/obstruction.
At the moment with rough (By eye) collimation there is severe astigmatism even at low power. Could be the no brand 25mm plosslyl that came with it(lot's of C.A. also)?
This could be due to the clamps on the mirror which I noticed where tight when I removed the mirror for cleaning, so I did the same.....tightened them against the mirrors surface, firmly.........mistake?
Using higher powers (again with cheap -ass eyepieces) is a joke.
I know the mirror/optics is better than this......surely?