I built a 16" f/4.3 Newt hexapod and have been chasing my tail regarding primary-to-focuser distance. I know that focus is achieved at 16 x 4.3" from the primary, and that's been set, but then I ran into unexpected difficulties because I have a SIPS coma corrector. I know that by itself, it has a focal effect of 15% (don't recall whether it's 'shortening" or "stretching" the focus). I wondered if it's shortening or stretching the entire focal distance, or just the portion between the SIPS and the eyepiece, but realized that it doesn't matter, it just has to be wherever it has to be.
The SIPS installation instructions say to rack the focuser all the way in, remove the eyepiece, and put tape across the top of the tube. Then, you point the scope at a bright star and adjust the threaded focuser assembly in and out to bring the image on the tape into focus. At that point you're done, supposedly, but I must be doing something wrong. After the SIPS image is in focus and I add an eyepiece, I can't achieve focus - the focuser hits the end of travel. If I screw the focuser assembly in/out to allow focus to be achieved via the eyepiece, when I recheck the SIPS placement, it's now wrong.
Is this even possible? Since I've not seen anyone else post about having this issue, I assume that I'm doing something boneheaded. Once the SIPS is focused per their instructions, does that implicitly mean that any eyepiece will then be able to be focused?