7" f/12 Istar folded refractor 9: Folding mirror shell backing.

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Progress has been non-existent due to a combination of an extremely painful back, constant sub-zero temperatures and now snow. The forecast is for rising temperatures so I may be able to play in the workshop quite soon.

A new week and a completely new temperature. 43F @ 11.30am! Last week it was -8C, 17F with wind and 3" of lying snow.

A smart, 6" compact, 2-mirror, folded refractor for sale but with a serious problem with the objective. I would have mounted the OTA with the objective at the low side of the main tube and the eyepiece much more readily accessible. In its present arrangement the observer's head will hit the OTA backplate except at very low observing angles.

I was going to rout out the plywood backing disks for the mirror retaining shells. This would leave only a 1/8" 3mm hole in the middle offering some flexibility in layout and mirror support. The problem then is working out of doors in drizzle. It involves carrying a folding workbench outside too which is never kind to my back. The old B&D benches weigh a ton and are an awkward carry. Using electric tools out of doors in the wet is neither safe nor sensible and will rust the cutting bits. Not to mention soggy sawdust collecting everywhere.
 
I'll just have to drill out the center hole and spin the disks on a threaded rod mandrel in the lathe. This also provides fine tuning of disk size and fit in the shells. The router is more hit and miss and can leave a ragged edge which when sanded can cause rounding. Which doesn't provide quite the same support as a nice true parallel/cylindrical edge. Choice of material thickness of the disks suggests extra weight should be avoided. Though hole saws can considerably reduce the volume of heavy Birch plywood. Or I could even use a hand or electric saw to remove material in larger segments.

I'm still undecided as to retaining the shell's rolled rims. The baking tins were 3" deep because I couldn't find any 2" deep in the sizes I wanted. The extra inch adds another inch to the OTA's length unless the collimating cells project. Not a serious problem except that it adds unwanted vulnerability. The OTA cannot be simply put down on one end without considerable care. Better, surely, to sink the collimation screws just beneath the OTA's end faces?  Simple hex socket head screws should suffice to allow occasional, mirror collimation. If it needs collimating every time it is moved I shan't have done a very good job of supporting the tertiary mirrors.

The image shows the shells with carefully turned disk of 12mm [1/2"] birch plywood. I overshot on the first trial despite taking only tiny cuts. So I approached the final sizing on the next two disks by sanding the last ten-thou, or two, with a flat backing plate. The disks were spun on a 10mm stud in the 3-jaw chuck with external jaws fitted. These helped to ensure the disks stayed flat by pressed the plywood hard against the projecting jaws as the nut was tightened. The large washers ensured the nut did not dig into the face of the plywood and made the nut easy to turn. The image is an illusion as the smaller shell is in the foreground.

Click on any image for an enlargement.

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