1. Remove everything from the back of the OTA (rings, focuser, OAG, filterwheel, camera, etc.)
2. Place a stiff paper disk with perfectly centered looking hole into the back opening of the OTA.(74 mm, I think for the TS RC8, must be a good fit in any case! I make the disk with a pair of compasses, which gives me the exact center and expand the hole to about 2 mm). In contrast to a cheshire fitted in the focuser, the disk is placed as close to the primary mirror as possible.
3. Lay a little flash light into the front opening of the OTA, shining in direction of the primary.
4. Look through the hole in the paper disk and watch the reflection of the disk on the secondary mirror. You can use a DSLR with live view and zoomed display for this. It takes a bit of fiddling to find the right distance, position and angle.
5. Adjust the secondary mirror until the reflected looking hole appears dead center in the ring mark of the secondary. I consider the secondary now square to the OTA.
6. Remove the paper disk and the flashlight and point the OTA towards the floor (to avoid any potential flex).
7. Attach the focuser with collimation laser. Focuser turned all the way in. No distance rings between focuser and OTA.
8. Watch the reflected beam on the target bullseye of the laser and adjust the primary mirror until the beam hits the center of the bullseye. Done.
To check if there is any flex or "kink" in the optical train, you can put the OTA back in a horizontal position and attach the usual distance rings and pull out the focuser to about 3/4. Only if the laser point on the bullseye would significantly change position, I would attach a tilt plate and compensate. Typically, I do not need to do that.
Edited by pwarborg, 04 December 2020 - 06:28 PM.