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From: mink@cfa.HARVARD.EDU (Doug Mink, OIR)
Newsgroups: sci.astro,sci.electronics
Subject: Re: "Big Glass"
Message-ID: <135@cfa.HARVARD.EDU>
Date: 30 Jun 89 21:05:05 GMT
References: <24798@srcsip.UUCP>
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From article <24798@srcsip.UUCP>, by ferguson@gorby.SRC.Honeywell.COM (Dennis Ferguson):
>>
>    (stuff deleted)
> I didn't read the article in question, but its easy to understand how
> PE could have wrecked a 94" mirror.  Even your garden variety reflector
> telescope has to maintain a mirror flatness of 1/8 wavelength across its
> surface.  To polish a mirror 94" in diameter and keep it flat to less than
> 1/8 wavelength is not exactly easy.  Mirrors comparable to the 200"
> telescope at Palomar are prestressed these days so that they warp under
> gravity to achieve the levels of flatness required.  To achieve extremely
> large reflective surfaces, astronmers have gone to multiple mirror telescopes
> (MMTs) where smaller separately gimbaled mirrors are aligned using lasers.
> The resulting composite mirror is lighter and easier to maintain.

Actually astronomers are going back to the one big mirror concept for
most of the next generation of telescopes.  The MMT, testbed for
multiple mirror concepts, ended up not using the planned laser alignment
systems and is being replaced by a 6-1/2 meter mirror spin-cast by
Roger Angel of the University of Arizona.  Several 8-meter mirrors
are planned using the same technique.  The only multiple mirror
telescopes currently being built are the 10-meter Keck and an array
of telescopes whose size I don't remember at ESO.

Doug Mink

Internet:  mink@cfa.harvard.edu
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