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as far as I can tell, the RCC1 has almost no effect on the location of the focal plane in my system - it just fixes coma and curvature and everything else stays the same. ie if you set it up without the RCC1 and then drop it in, the focal plane does not move much.
If you had 40mm spacers and the focuser wound right in, then you need a little bit more spacing. From rusty memory, I needed to cut 55mm off the OTA of my 250/f4 to get the mirror far enough up to get focus on my QHY8/MPCC - that would be consistent with you needing a bit more extension on your mirror spacing (about 20mm possibly). My SW CF 250f4 has the focal plane a long way out by design and I only needed to take about 15mm off that tube. Cutting the tube is the best solution from a stability point of view, and suggest that you consider doing that when you have worked out how much is needed.
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