Hi Bob,
I've done some quick back focus calculations for you (see the diagram). Probably best if you check them yourself!
* OAG to QHY9C:
1. Front of OAG to prism centre: 6.7 mm (listed under
Technical details)
2. Prism to OAG camera mount: 4.5 mm
3. (A T2 spacer / extension tube might be needed between the OAG and QHY9C here.): 0 mm
4. Camera mount to main sensor: 29 mm (see
here and
here)
--> OAG to sensor distance: 40.2 mm
* OAG to SSAG:
1. Front of OAG to prism centre: 6.7 mm
2. Prism to OAG T2 mount: 26 - 38 mm (it's adjustable)
3. Orion SSAG mount to sensor: 12.5 mm (see
here - the SSAG is same mechanically as the QHY5)
--> OAG to SSAG distance: 45.2 - 57.2 mm
Okay, so we have two problems to solve now by adding one or two spacers (extension tubes) into the image train:
1. The MPCC-sensor distance for both light paths (SSAG and QHY9C) needs to be 55 mm so that your coma corrector works properly.
2. The two light paths need to be the same length so that both cameras are in focus simultaneously.
Since the OAG path is "within range" of 55 mm, the easiest option is just to add 14.8 mm worth of spacers between the OAG and QHY9C. For example,
this 15 mm extension tube might work.
However, the down side of this is that the OAG pick off mirror is way out to the edge of the field - guide stars will probably be dimmer and more likely to have optical aberrations.
A better approach is to add one extension tube between the OAG and QHY9C (e.g. a
10 mm extension tube), and another between the MPCC and OAG (e.g. a
5 mm extension tube). In theory, this gets you very close to 55 mm between the MPCC and QHY9C sensor, with a bit of room to focus the SSAG too.
Other combinations are possible too - e.g. 3 x
3 mm = 9 mm in front of the OAG, and 2 x
3 mm = 6 mm in front of the QHY9C will let you move the OAG prism even closer to middle. I'm not sure what the ideal distance off-axis is for your prism - but at f/4.7 I'd imagine you wouldn't want it too close.
I hope this helps! (I'm most probably off by a couple of mm here and there for thread connections etc...)