Well, things are progressing slowly - I have now completed the main mirror cell (except for painting) and test-mounted the mirror. The main features can be seen in the attached images and are as follows:
- Widely spaced push/pull screws as mentioned earlier.
- Three Lateral positioning screws for centration of the mirror in the tube.
- Multiple threaded inserts at the three attachment points to the tube. There are actually 5 threaded inserts at each tube attachment point which will allow 20mm of adjustment once the three holes are drilled (and reinforced) in the tube. I drilled and tapped M8 holes in stainless 1/2" threaded rod and cut to size (15mm long). 1/2" holes were then drilled and tapped in the CF cell and the inserts screwed / epoxied in them. The M8 holes in the inserts will then accept M8 screws through the holes in the tube. This was a LOT of work and was hard to get the positioning perfect in the CF cell such that the spacing of each matched set of three holes was identical to the others. In the end it wasn't perfect (up to 1mm out), so I offset the tapped holes in the threaded inserts to compensate.
I will aim to set the mirror in the tube such that the "middle" insert is used - that way if I am slightly out I have +/- 10mm to play with. It may also be useful in the future for different flatteners / compressors / etc with different backfocus specs.
I'll call it the "SLATO Multi-Point Cell Positioning System" (patent not pending yet 
).