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Josh.
depends, depends on a lot of things, but you can establish the focus point for yourself quite easily.
Line the scope up on a distant terrestrial object if you have the room, or something like the moon at night if you don't. Take a piece of lunch paper or similar and hold it over the end of the focuser draw tube, and focus. When the image is as close to focus as you can get it, this is the point. I'd suggest you make a measurement from the focuser base, or the outside edge of the tube or something, and record that.
The rest is just mechanical requirements. The QHY9 for example will have a distance of about 15 - 20mm from the chip to the front of the metal section,so your focus point needs to accommodate at least this 20mm.
A DSLR is worse, it needs "about" 45 - 50mm.
Another way is to hold the DSLR body (no lens) at the end of the focuser draw-tube and look through it. If you can focus it, then you are in luck, but I suspect you won't be able to.
Remedies are as you suggest: a lower profile focuser, or shifting the primary mirror towards the focuser. Neither is too daunting, but if you are shortening the tube, remember to measure twice and cut once.
Try shifting the mirror on the collimation bolts/springs first, the little amount you can move it might be all you need. I tried longer collimation bolts for one scope, it worked.
Gary
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