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From: ferguson@gorby.SRC.Honeywell.COM (Dennis Ferguson)
Newsgroups: sci.astro,sci.electronics
Subject: Re: "Big Glass"
Keywords: Discover, Perkin-Elmer, Hubble space telescope mirror
Message-ID: <24798@srcsip.UUCP>
Date: 30 Jun 89 02:23:42 GMT
References: <1479@cbnewsc.ATT.COM>
Sender: news@src.honeywell.COM
Reply-To: ferguson@gorby.UUCP (Dennis Ferguson)
Organization: Honeywell Systems & Research Center, Camden, MN
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In article <1479@cbnewsc.ATT.COM> levy@cbnewsc.ATT.COM (Daniel R. Levy) writes:
>The July, 1989 issue of Discover magazine has an article entitled "The Big
>Glass" by Terry Dunkle, a story about how a small team at Perkin-Elmer built
>the 94" mirror for the Hubble Space Telescope.
>
   (stuff deleted)
>    ...too far into his hat for a _deus_ex_machina_?  (I might also
>mention the part of the story where P-E ruined weeks worth of grinding with
>one smoothing that supposedly flattened the edges too much -- that sure would
>take one hell of a big underestimation of how much material the smoothing would
>remove, I'd think.  Maybe it's not so incredible as the problems with the power
>supplies, but still a stupendous slipup if it DID occur....)

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.

D.Ferguson