I am in the process (still) of building a 12" f6 binocular telescope.
The tubes are finished so now its time to grind the mirrors on a mirrormatic grinding/polishing machine.
These mirrors will be flexed to final shape.
The back face of the mirrors will have a radius of curvature 2.5 inches smaller than the front at 144 inches.
I have two methods to rough grind in mind.
using a swinging pendulum with a diamond grinding disk at the point, swung over the mirror to rough out the curve as its turning on the grinding machine. Both front surfaces can be done this way.
To rough the rear surfaces a plaster tool can be hollowed the same way and lined with steel dumps, re ground to the correct roc and the rear curves can be ground using this tool.
(I have done this and it works very well)
All polishing is carried out in the usual manner with suitable sized pitch laps.
To support the curved rear face of the mirror for polishing I sit the front surface downwards on a smooth flat surface apply a 10mm square ring of rubber around and use an expanding cake ring as a collar then pour in the dental stone.
When set the rubber and cake ring are removed leaving the mirror 10mm proud of the plaster that is now 20mm wider in dia than the mirror and the mirror now supported by the plaster cast is set on the machine and polished in the usual fashion.
Again I have done this with no problems except I accidentally dropped and broke the mirror but early testing showed it was behaving as expected.
The good points I can see are its easy to ensure both mirrors have the same RoC.
The only downside I can see with this method is the the very high tripod required (12 feet)and the inevitable flex in its legs. My experience to date has been on very short focal lengths where its easily managed.
The tripod legs have been braced since this shot.
Spin grind mirror A on top of mirror B.
this will produce the correct curvature in mirror B with a convex bottom surface on mirror A.
now flip them and use the flat bottom of mirror B as the tot tool to grind mirror A front surface which now has a curved back surface so must be supported in plaster and producing the curved back surface to mirror B
I believe it will be possible to make both mirrors this way but ensuring they are the exact same RoC when finished could be harder.
I'm also unsure if the radius of curvature of the back surfaces with be the same ROC as the front.
I guess its not a difficult problem to solve as I can make a concave tool to then grind them to the correct 141.5" back face ROC.
Polishing and testing will be via Foucault and Ronchi as I only want a sphere.
Can anyone see any gotchas that I may not have thought of?