I just measured my tube I.D. Its 22.9mm. Using the f ratio x tube I.D. = tube length :
10 x 22.9mm = 229mm ( 9inches ). This would place the peephole/pupil ABOVE the Focal Plane of 210mm ( 8.2 inches ). pardon the capitalized Above, just wanted to emphasize...
Patrick, then I'd recommend cutting it at 9" to start. That length would include the distance to the cap with a 2mm pupil drilled at the center. Then go from there.
Remember Nils mentioned "or a little shorter". If it's cut a little shorter, you'll simply have a bit larger "white paper" annulus around the diagonal and bottom of the site tube. That's okay. So you may be able to cut it shorter and get on or inside the focal plane.
I presume you're using PVC that is readily available and low cost. Of course all the PVC pieces will fit the full 1" diameter tube, so don't trim 0.5mm at the very top so the cap fits. If you need to trim the length, take it off the bottom of the site tube (before you install the cross hair).
Go ahead and install the cap/peep hole and trim the rest of tube to fit the 0.96" focuser. The wider cap will rest on the focuser and prevent the tube from sliding down into the focuser. So, that's good. Test fit and cut to length that allows you to see all three collimation signatures.
Yes, trimming 0.5mm is probably not a job for sand paper. There may be a lathe where someone might be able to accurately turn the site tube for you. I've taken small projects to local fabricators before. They were happy to help. Depends on what's available in your area. I don't know how to do it easily by hand, but there may be a way.
Edited by Asbytec, 08 June 2022 - 10:55 PM.