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In the past, weren't many astronomical telescope mirrors periodically removed and 'resilvered', and wasn't this actually done using silver?

I'm guessing that this is much less common now - new telescopes are so large and complex that routine invasive procedures like that needed for maintenance of a silver coating would not be acceptable, and mirrors in many if not most older telescopes would have been 'updated' to aluminum.

Was silver used in the past because aluminum coating was too technically difficult, or because silver has slightly better reflectivity in the red, or some other reason?

note: I'm only asking about 'optical' telescopes meaning NIR & Vis, not the longer wavelength IR telescopes where IR emissivity is a concern.

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Aluminium coating is a relatively recent process - it became available around the 1920s or 1930s. The Hale telescope arrived just in time to take advantage of this new technology. (It requires a reasonably good vacuum to work, which probably explains why it took a while to come along.)

Before that - around the mid-1800s - various chemical "silvering" processes, usually involving real silver, were used. These weren't ideal as silver tarnished over time - the silver literally oxidised on exposure to air. (Aluminium also reacts with air, but forms a thin oxide layer which doesn't harm the image and prevents further oxidation.) Also some of these silvering processes were either toxic or explosive in application...

Before that - the first telescope mirrors were made of metal that was cast and ground to the required curve then highly polished. The material was "Speculum" a copper-tin alloy. They were poor in quality; far less reflective than later mirrors. This account gives a nice description of casting mirrors for the Parsonstown telescope using over a ton of molten metal. Two mirrors were necessary for the larger telescopes because the mirror in use had to be taken out and re-polished so often...

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  • $\begingroup$ I forgot to mention - the 100-inch Hooker telescope was originally made with a silvered mirror (early 1900s) and was later refurbished to an aluminium coating (1930s?) Hope that gives some idea of the timescale. $\endgroup$ – Andy Jul 12 '16 at 17:12
  • $\begingroup$ Thanks! I know silver was almost universal in the 1800's, and rare - only in special cases now in the 2000's, but when was the transition? Say 1950 to 1970, for smaller to mid-size telescope mirrors (those that could be removed) how common was re-silvering with silver? $\endgroup$ – uhoh Jul 14 '16 at 23:43
  • $\begingroup$ Is it oxygen or sulfur that causes slow degradation of exposed silver coatings? $\endgroup$ – uhoh Jul 14 '16 at 23:45
  • $\begingroup$ @uhoh I believe silver can tarnish from clean air (ie oxygen) too, not just from air with sulphides in it. (In other words there are a couple of reactions which cause the problem.) Also, I get the impression the change to aluminium coatings was around the 1930s. My guess is it's far cheaper than silver which may explain why silver seems so rare these days. I've seen pictures somewhere of amateurs re-aluminising mirrors with home made vacuum chambers - quite a simple process. And I found out recently the Hale telescope is completely recoated every few years! $\endgroup$ – Andy Jul 15 '16 at 9:56
  • $\begingroup$ Thanks! If I had a strong suit, chemistry wouldn't be it. So in the '60s and '70, you think all that routine resilvering was actually aluminum? I'm confused because pure aluminum will form a self-limiting oxide very quickly. In fact if you vent a chamber to air where there is a large area of fresh aluminum you can get an explosion because it oxides so fast. An once you get that first order 10A of oxide it's impenetrable to further oxygen. I don't see why an aluminized mirror needs to be re-coated. Cleaning I can understand. $\endgroup$ – uhoh Jul 15 '16 at 10:04

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