[Stefan Buda]

There was a thread not long ago, started by Joshua Bunn, about proposals for custom adapters, but I can't find it. Peter Ward suggested that an 8 to 12mm nonrotating adjustable spacer would be useful for nailing the optimum spacing between a focal flattener and camera on certain astrographs.
I proposed a design which had some chance of working but looked rather difficult to make.
My problem is that I can't resist a good engineering challenge, so I went ahead and tried to make one.
The result is in the attached images. It works reasonably well, maintaining parallelism within about 25microns over 80mm diameter, but unfortunately it requires the use of a 3mm Allen key for locking. I intended to use an 8mm diameter thumb screw for locking but there is too much play in the threads for that, as the lock relies on radially distorting the red ring.