The objective cell thread matches that of the Tmax of a Solarmax 90! I am not and will not be able to buy a Solarmax 90 for it, but this is a good starting point to verify the thread size of the cell. :-)
I have fine tuned the focuser according to the instruction here:
Now, it's just smoother and I suppose that it should hold more weight as well.
Next, I found that common binoviewer would take up around 4 inches of optical path, so if I cut the OTA by 4 inches, I would probably be able to use binoviewer without any corrector. The focuser travel of the Orion 100ED is more than 5 inches, so it could probably make up the differences. And in the worst case, a very moderate extension tube like 2-3 inches would make it work again for normal operation.
Such a shortened tube should be able to let me focus with a focal reducer, as well as to make it more portable. Not a bad move indeed.
Follow up on the thread size, thank Edd Weninger from CN:-
I just measured the threads on the SM90 T-Max at:
4.25" Outside Diameter
24 Threads per inch
This is not listed as a standard thread size in my Machinery's Handbook so I have no clear tolerances for it.
Noting that it appears to thread into the E100 seems unusual (my ED80 has Metric threads), so it might be possible the thread pitch is 1 mm. There is not much thread length to work with for the thread gauge.
However, the Outside Diameter would then be 108 mm, which would seem odd for a Metric dimension.
I'd suggest that if you are having something custom made, that you provide the mating part to the machinist.
I have checked with all my eyepieces. Assuming that I will be using a 2" mirror diagonal, I could safely cut the OTA by 8.5cm without the need to use of any extension tube to reach focus for all my eyepieces and barlows combinations. To leave some safety margin, cutting 3 inch would be very safe.
Given I have a JMI 2" extension tube which is 1.5 inch in length, cutting 4.5" from that OTA would be no problem. People said binoviewer took around 4-5", so it might work as well.