I'm struggling to understand back focus/optical path length and it's making me nervous about ordering pieces of equipment that may end up being the wrong size, so I'm here to ask for your expert advice.
My setup is an Astro-tech AT80EDT refractor on an Orion Atlas EQ-G mount + QHY PoleMaster polar scope. I also ordered the Astro-tech 0.8x Reducer/Field Flattener specific to the AT80EDT (on back-order from Astronomics). I'm trying to have the set-up working for both DSLR and CMOS imaging.
For the DSLR prime-focus photography I ordered the appropriate T-ring (three actually, since I'll be working with my students and their cameras). Besides this I'll need a T-mount to 2-inch adapter, correct? Also, according to Astronomics the reducer/flattener can screw straight onto the OTA and has 42mm T-threads to connect to the T-ring on the camera. The reducer/flattener is also listed as having a back focus of 55mm to the image plane, and Astronomics assures me that this is standard for DSLR camera bodies. So I should be good to image directly or through the focal reducer, correct?
For the CMOS imaging, I have a QHY MiniCam5F with built-in filter wheel. This has a CS/C-threads, and a little 1.25 inch barrel adapter. My plan is to ditch the barrel adapter and use a C mount to T mount adapter to attach the MiniCam5F to the focal reducer, but I would need to make sure that it is spaced back by 55mm, correct? My biggest doubts are here: what kind of connectors do I need to get the spacing correct? I know the MiniCam itself has an optical path length of 19.95mm from the front face of the built-in filter wheel to the sensor. Does this mean I need 35 more millimeters of spacers to make sure the CMOS sensor is at the correct 55mm distance from the focal reducer? Maybe something like this:
Or something like this for more precision:
Should I worry about the contribution of the C-mount to T-mount adapter to the optical path length? I don't see this information Amazon, so should I be contacting the manufacturer?
Finally, I realize that I don't have a guide-scope or off-axis guider; those didn't fit in the budget this year but I'm hoping to purchase one for the next schoolyear.
Thanks in advance for your help,