Though this is not telescope making, it is a telescope mod. I'm hoping that this post might help someone that has a large SCT the won't maintain "star level" collimation. I noticed on my "OLD" C-14 that the collimation would shift when viewing different parts of the sky. I'm not talking about gross mirror tilt. I'm speaking of the type of primary tilt that shifts the diffraction ring on an in focus star at high power (300x or greater).
My scope has really great optics so this minor tilt became quite annoying to this multiple star observer. When I pulled the primary the retaining ring was still in contact with the primary but was not "tight." I installed 3 nylon tipped set screws (10-24) into the retaining ring. I reinstalled the ring tight then screwed down the set screws firm but not overly tight. The problem appears to be fixed. After star level collimation (in focus) the first diffraction ring has remained centered on the airy disk over the past week of observations. We shall see how well this works long term.
I made this mod because I don't want to have to take this scope apart again for as long as I own it.
40 year old restored C14 with recirculating cooling
w/Skywatcher DOB GOTO Conversion (CPC14)