Quote:
Originally Posted by mental4astro
Hi Lumen Miner,
Just a few things on alloy mirrors no longer being used:
*They are not as dimensionally stable as low expansion glass
*Much harder to shape, by ATM's, especially stainless steel
*Would attract dew faster
*Their density is greater than glass (except maybe al), therefore heavier for the same diameter
*When needing to repolish the reflective surface it means regrinding the alloy face, not just a chemical strip & reapplication of al or ag & not touching the shape of the polished glass.
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You have some interesting points there. I do not wish to debate this topic, rather gather input. Could I ask your point of view though on a few points?
When you say not as "dimensionally stable", does that refer to expansion and contraction under extreme conditions, if so are these conditions within our realms or, high and low heat conditions in tests. The data I am reading suggests that under normal use from -10 degrees to 40 degrees celcius, the alloy dimensions would remain similar yet glass would have a higher deformation rate. Perhaps that is normal glass though, not low exp.
It may be just my experience, but a plate of alloy would appear easier to machine / shape as required. As apposed to only a few grinding techniques to achieve the porabolar, alloy's could be machined several ways. CNC, depth controlled trench cutting with an angle grinder, dremel routing with diamond bits, depth controlled drilling, etc. This list could go on and on.
But like I said, that may be just my experience.
Could you explain, the attracting dew faster? That interests me greatly. That would definatley be a neg for alloy's.
Although density is higher, would that not also allow us to manipulate it to suit our parameters? As in could we not cast, parabolas, re-inforced with steel frames for integrity? This would allow us to reduce the overall weight yet retaining a solid none flexing mirror.
I am guessing the alloy would tarnish not rust. Therefore not needing a "regrinding" more a polish. A polishing compound is all that need be applied... I know of a few alloy mirrors still in use in large scopes, in their cleaning proceedure, they polish it, not change / regrind the surface.
I wonder if a clear coat could be applied to inhibit tarnish... I guess the clear coat would distort the image...
Thanks for the input, you have given me much to think about...