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Date: 26 Nov 86 04:51:15 GMT
From:  (Jordan Kare)
Subject: Re: Huge mirrors

In article <8611252302.AA04374@gauss.ECE.CMU.EDU> Hank.Walker@GAUSS.ECE.CMU.EDU writes:
>There is a story in Analog magazine within the past few months describing
>the construction of an 8 km telescope (a la the Grey Lensman) with an 80km
>focal length.  The mirror is made up of millions of optical flats about
>a meter in diameter, which at that focal length, sufficiently approximate
>a parabolic mirror.  The mirrors are adjusted by bimetallic actuators.
>The obvious problem in such a structure is obtaining sufficient stiffness.

The story in question unfortunately contains a prize collection of
physics and engineering blunders.  One of the more glaring ones
is that the giant telescope is rendered much cheaper than the NASA
design (for a smaller telescope) because optical flats are so much
cheaper than concave mirrors (at one point the optics company rep
says something like, "sure, we could make you 8 million optical flats, 
but who would want such junk?").  In reality, it is at least as
hard to make a flat as to make a spherical mirror -- harder, in that
there isn't a convenient focal point for doing optical tests.

Incidentally, the proposals for giant telescopes made from bubbles, etc.
are very nice.  Do keep in mind, though, that inflating such a bubble
takes an enormous amount of gas.  An interesting possibility is using
a (clear) bubble as a refracting lens, which has the advantage of
needing much lower tolerances on surface accuracy.

	Jordin Kare	jtk@mordor.UUCP

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