Peterpsbradford For my purposes and 12" SCT, it would certainly be nice to be able to reduce the light path length in the physical OAG design so I could utilize the dial (instead of the prism arm length) to focus.
Now that you know how much you had to compensate, you can insert a plate in between the OAG and the filter wheel whose thickness is at least the distance you just had to add to the hypotenuse. Just a large hole with the four smaller bolt holes, and using slightly longer bolts. I don't even own a real drill press, so I have been using a web machine shop in Seattle to fabricate aluminum parts for me. It is a shame ZWO did not include 0.5mm and 1mm shims for that purpose.
Just make sure the hole is smooth and perfectly circular (or make it 0.1mm larger than the existing holes in the filter wheel and OAG), otherwise you will see weird diffraction spikes.
If you are using a reducer that requires an accurate backfocus, make sure you account for this extra shim when calculating spacer lengths (I use PreciseParts to make my spacers).
Oh yeah, you can also add the shim in between the filter wheel and the camera flange, too. Basically, anything to increase the distance of the OAG optical center to the main camera's sensor. Heck, you can experiment with washers to see how thick the shims need to be. But don't use the washers permanently, since different washer thicknesses can give you a tilted sensor, not to mention the inevitable light leak.
Chen
