Hi Lumen Miner,
Mate, do you have a background in metal working? You would be a good fellow to have on side,
.
You make some really good points. Not dealing with alloy mirrors, your question is an interesting one for me.
On dimensional stability, the issue I think would be the conductivity of heat of the alloy creating uneven distortion to the figure. This is the main reason in using low expasion glass where in heating the mirror to remove dew, the figure tends not to distort. Remember we are talking the wavelength of light. These distortions are enough to create distructive interference, hence cancelling light out, something like the multicoloured patter you observe in a thing oil slick. Heating an alloy I think would be dificult to control the uniform expanision to avoid this problem.
Have you ever noticed how much colder a metal surface is in the cold compared to a non-conductive one? They therefore attract dew first- look at where dew settles first of an evening. Alloy mirrors would do the same.
Here the method of 'grinding' an alloy to produce a parabolic surface is my weakest area. I suppose that with the appropriate techniqes is no more or less difficult than figuring glass.
Tarnish, being the oxidation of a metal or alloy, 'rust' in steel, is removed chemically from a glass mirror, & reapplied chemically, all without disturbing the polished glass. An alloy, however, the chemical or mechanical removal of the oxide does affect the shade of the mirror. Polishing is grinding, only at a minute level, using very, very fine grade abrasive, hard or soft. Repolishing a mirror surface is not a task I would be doing too often, it would be inviting trouble. Applying a protective coating is possible, it's done to metal coated glass, but it only slows the process. Applied too thick, which would offer more protection, causes its own problem.
Lumen, I'll be up at Hargraves Lookout tonight with Hickny, join us if you can. We'll be there from about 5pm.
Cheers.