I have a really strange problem that's causing my brain to hurt. When I set my backfocus correctly then I cannot get focus, but if I remove about 10mm it works (remove the tip/tilt plate):
So here's my setup.
ES 127ED Triplet with a moonlight 4.5" drawtube focuser. I have in my image train: ES 3" Flattener / Reducer, ZWO Tip/Tilt Plate, 20mm spacer, 8slot EFW, ZWO 1600mm-cool.
The Tip/Tilt plate should be 10mm, the spacer is 20mm, the filter wheel is 20 mm and the camera has a 6.5 mm spacing. That should be 56.5 mm of backfocus space (camera says +-5mm is OK).
If I set things up this way my focus point seems to be closer to the end of the scope than my focuser can travel (IE it's bottoming out at 1mm of travel).
If I remove the tip/tilt plate I can get focus with about 15 or 16 mm of draw tube travel.
I'm at a loss.