I ordered a Nikon to M42 adapter, and it arrived yesterday. It is about as slim as possible, with only 1/2m or less between Nikon lens protrusions and the metal of the adapter.
I'm not exactly sure where to measure from, I'm assuming that the Nikon body metal edge is where I need to achieve 46.5mm back focus.
EDIT: I've checked the flange point, and I was incorrect in my first measurement. So my totals are 9.4mm for the adapter, the EFW is 22.0mm, and the back focus of the QHY9 is 17.3mm. I've the EFW2 adapters arranged in such a way that that add 0.0mm to the connections.
So my total length is 9.4+22.0+17.3mm = 48.7mm. If I add in the 1/3 of the 2.0mm filter thickness this gives me another 0.7mm to spare, which gets me to 48.0mm
However it isnt focusing, but very close. I had hoped that the fact the lens focuses past infinity would give me a little bit to spare but it doesnt give me enough.
Either new filter wheel or new camera by the looks of things as I cant see anywhere to take 1.5mm off it.
What are my options and importantly do I have my calculations right?
Adam.