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JRS: In article . net,
seen in news:uk.sci.astronomy, Kilolani posted at Sat, 22 Nov 2003 03:28:56 :- If this is true, who don't they make telescope mirrors with silver coatings instead of aluminum. Since we're talking about coatings that are only about 3 molecules thick, it doesn't seem that cost would be a relevant factor. Seek the story of Rutherford and the Platinised Teapot, as given, for example, by A S Russell 1950-12-08, presumed GMT. -- © John Stockton, Surrey, UK. Turnpike v4.00 MIME. © Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links; some Astro stuff via astro.htm, gravity0.htm; quotes.htm; pascal.htm; &c, &c. No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News. |
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Chris.B wrote:
Steve Taylor wrote in message ... The newest big mirrors have a sputtered silver deposition with a nickel-chromium passivating layer I believe Steve Really? I wonder whether this is applicable to smaller apertures? Deposited under high vacuum one assumes, like aluminium coatings? Are there any details on reflectivity, resistance to corrosion etc.? Magentron sputtering is done under quite low vacuum, but after extreme measures are taken to remove water vapour from the chamber. One of the major makers Gencoa have some good information. http://www.gencoa.com/tech/ Steve |
#53
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"Dr John Stockton" wrote in message Seek the story of Rutherford and the Platinised Teapot, as given, for example, by A S Russell 1950-12-08, presumed GMT. Platinised Teapot is very nearly a Googlewhack (2 results). |
#54
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"Michael A. Covington"
wrote in : "Kilolani" wrote in message ink.net... If this is true, who don't they make telescope mirrors with silver coatings instead of aluminum. Since we're talking about coatings that are only about 3 molecules thick, it doesn't seem that cost would be a relevant factor. Silver tarnishes. They used to use silver. Aluminum stays bright essentially forever. It might be time to revisit this, though, and see if a coating can be put on over the silver. Silver has been used for years by Questar. They protect the silver from tarnish by some sort of durable layer. I don't know the details but probably SiO2 - the same as used for protected Al coatings by the likes of Spectrum and others. I vaguely recall that Denton Vaccuum do a "Protected Silver". L. |
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In message , Steve Taylor
writes Chris.B wrote: Chemically deposited silver tarnishes rapidly. Particularly near the sea and air-pulluted areas (sulphur). In a clean inland rural area it might still work. It isn't the amount of metal involved. It is the quantity of silver bearing chemicals required in the wet chemical process to deposit the silver. This makes the process quite expensive. Though I have no idea how it compares with commercial aluminising. The newest big mirrors have a sputtered silver deposition with a nickel-chromium passivating layer I believe There are several technologies about for protecting silver coated astronomical mirrors a detailed description of one is online somewhere as SPIE_vol3352.pdf try links from previous threads in sci.chem Silver coated mirrors are much in demand for their better near IR reflectivity in the 800-2000nm range where aluminium is less than ideal. ISTR there is also a (relatively) cheap and cheerful method using an exotic organic ligand to passivate the silver surface against sulphur and other chemical damage without compromising its optical properties. The catch being that some of the clever passivating layers are so good that cleaning them off for resilvering can be a problem. Regards, -- Martin Brown |
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