After clouds had passed (it's California. We have clouds?!?) I was able to run some tests. I believe I have discovered the issue and the solution. Here's the basic issue:
I definitely hear what you're saying and agree with you that you could do it as you suggest by using the Optec focuser as a variable spacer, then using the mirror to achieve focus.
I THOUGHT the Optec focuser was built to the same back space dimensions as the Celestron T-adaptor. This allows the user to use the Optec focuser rather than the T-adaptor for the correct spacing when mounting the imaging train.
The focuser is designed to provide 3.5" of back focus from front of the focuser to the mid-point of the focuser position. Converting to millimeters, this is 89mm of back focus at the mid-point of the focuser.
The EdgeHD 9.25 requires 146mm of back focus. 55mm of this are taken up by my camera, EFW, OAG. This leaves 91mm of back focus required of the focuser.
You can see that the focuser at the mid-point of it's travel is 2mm too close which was causing my issues.
The solution is to force the focuser back 2mm from it's mid-point and achieve focus at this point. The focuser defaults to the mid-point every time you turn it on. The focuser has 7,000 steps, so it defaults to position 3,500 every time it turns on. I need to determine what position the focuser needs to be at to achieve the correct back space.
The focuser has 15.2mm of total travel broken into 7,000 steps. This means there are 460.53 steps per millimeter on the focuser. If I need the focuser to move back 2mm, I need to position the focuser to 460.53x2=921.06 steps further back from the mid-point of 3,500. So, I need to have the focuser at position 4,421 for all of my imaging.
I tried this out last night and it was, of course, a BIG help. The downside is that I believe I have now identified some tilt. With this corrected back focus the corners of my image has nice stars EXCEPT for the top left corner. Sigh......
The net result is that I need to add another process to my set up routine where I make sure the focuser is at position 4,421. Then, I can unlock the mirrors, adjust the OTA mirror focuser until I get it in focus, and then lock down the mirrors.
I hope this helps anyone else using this setup.
Edited by dcm_guitar, 29 September 2021 - 06:02 PM.