...
The mechanical drawing for the flange, showing a back focal length of 10.50 inches = 266.8mm is here (LTS-496.pdf).
That means beam size at mounting flange is 10.50/18 = 0.58 inches in diameter.
The drawing for the flange is NasmythMountFlange.pdf
...
A cable spool is placed at the end of the optical bench, to allows for a draped cable wrap. It mounts to a bar piece that is in turn bolted onto the top of the optical bench.
Uses
This gives us the opportunity to displace a sensor at the focal surface while keeping the position of the secondary fixed. That allows us to make distinct changes to the primary-secondary distance and the primary-instrument distance. The RC telescope needs a specific separation between the primary and the secondary to correct properly for optical aberrations.
We can also put a fast-framing camera on the telescope to look at scintillation effects.
Finally, we can use a pair of apertures as a pupil mask (with wedges) far upstream in the optical system, and turn the Aux Tel into a differential image motion monitor (DIMM) for seeing studies.