Yes, the 0.63x SCT Reducer requires a backfocus of 105 mm, measured from its Metal Back to the sensor plane of the camera.
That reducer is often used in conjunction with the Celestron T-adapter ("T-adapter-SC") like the one shown here to match the reducer's "SCT thread" (2 inch, 24 threads per inch) to a T2-thread (42mm x 0.75 mm pitch).
https://www.celestron.com/products/t-adapter-for-schmidt-cassegrain-telescopes
This adapter has a optical length of 50mm. If memory serves, it is actually 2 inches long, so more accurately 50.8 mm; I have one in storage somewhere, and can probably dig it out to measure, but ZWO's camera flange focus distance is only accurate to +/- 0.5mm anyway, so you will need to do some fine tuning anyway, and you might as well assume that the adapter is 50mm long).
If you subtract the T2 adpater from the 105mm, you get, ta da, 55mm.
Ever since that time, many manufacturers have been manufacturing reducers and flatteners for a 55mm back focus, since the customers already have adapters for that dimension.
I think I bought my 0.63x and the T2 adapter back about the year 2000 (when I bought a special edition Meade 8" that has native f/6.3 -- with the reducer, it became f/4 -- we did not have the Hyperstar back then). Both components have been in production even before I bought my Meade - my reducer is actually manufactured in Japan, and I will not part with it for any amount of money, since all the current ones are manufactured in China. I think my T2 adapter said it was made in Taiwan.
So, anyhow, 105mm from the reducer is the correct back focus number for the stand-alone 0.63x reducer/flattener. And if you adapt the SCT thread to T2 thread using the above adapter, then the backfocus from that adapter is 55mm.
By the way, you might want to check if that reducer has an image circle that is at least equal to the diagonal of your camera. I don't remember what it is (as they say, memory is the second thing to go when you get old :-). Back in those days, CCD sensors were pretty small. With today's cameras, you may get more vignetting than you expect.
Another by the way, that reducer produces a different reduction factor when used with a 6" SCT. The same is true with the Starizona SCT reducers; they produce different ratios for SCT of different aperture, and even different optimal back-focus :-).
Chen
