A wacky telescope making idea...

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.

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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

"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).

"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.

(Chris.B) wrote in
om:

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.?

Chris.B

Denton Vacuum do this sort of stuff see:

http://www.dentonvacuum.com/coatings/metal.html#one

Don't know if they will do one off jobs for amateurs though.

L.

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