Date: 14 Dec 86 05:04:48 GMT
From: jtk@mordor.s1.gov  (Jordan Kare)
Subject: Re: Mercurial Mirrors

In article <1246@kontron.UUCP> cramer@kontron.UUCP (Clayton Cramer) writes:
>> The 15 Dec 86 issue of TIME magazine (Science, pg 94) has an interesting
>> article about a prototype mirror made entirely of a puddle of liquid mercury
>> in a shallow frame.  The entire device spins slowly (1 revolution/6 seconds)
>> to give the 40 inch diameter mirror the correct concavity. The builder
>> Ermanno Borra, of Quebec's Laval University, emphasizes that he is not the
>> originator of the idea, merely the first to actually make it work. Size is
>> not a problem here, as with greater than 200 inch mirrors: the astrophysicist
>> claims his approach will permit the construction of virtually flawless
>> mirrors at least five times as large as Palomar's.
 
	This seems spectacularly unlikely (that would be an 83 foot mirror, 
remember) but he's certainly welcome to try...  Of course, he'll have to
compete with arrays of ordinary glass mirrors, which are pretty cheap if
they can just lie flat on the ground and not have to adjust for different
pointing angles -- and they're certainly no more sensitive to wind or other
disturbances!

>Sometime during the 1970s Scientific American carried an article about
>infrared astronomy in which they used a similar approach to produce the
>rough blank for a plastic mirror.  They spun up a huge mass of liquid
>resin (about 40 inches across) for three days, then slowly added hardeners.
>(Of course, they still had to figure the mirror by traditional methods
>and aluminize, but the expensive and laborious task of getting a rough
>parabola was all taken care of.)

	J.R.P.Angel and others (at the University of Arizona) are currently
developing honeycomb mirrors, which are cast around blocks of refractory
brick in such a way as to leave thin front and back plates with a connecting
hexagonal web, thus drastically lightening the mirrors.  In order to allow
grinding of the front surface without removing too much of the front plate,
the mirrors will be cast in a rotating furnace to provide the rough figure
for final polishing.  They are currently making 3.5 meter diameter mirrors, 
and are planning to make 8 meter f/1 mirrors in the next few years --
that's a glass furnace close to 30 feet in diameter, all rotating 
continuously for the several-week cooling time.  Incidentally, some
of their preliminary work has been with "assembled" (as opposed to cast)
honeycomb mirrors, made of glass plates, which are fused together
and "slumped" over a concrete form, at just below melting temperature.

	Incidental to all this discussion of really LARGE space 
telescopes is the likelyhood that, for telescopes up to ~100 meters,
the surface of the Moon is a much better site than free space.
One gets "that solid, secure feeling" that only a planetary surface
can provide, and the dust- and trash-clearing effects of a gravitational
field, while retaining the freedom from atmosphere and a g-field
low enough to allow some very large structures.  And your astronomers
can walk around without messing up your observation.  And you can build
on the dark side and have the only site in the entire solar system that
never has to look at (or listen to, for all
you radio astronomers) the Earth.  And there are generous supplies of 
raw materials just lying around.  And other advantages....

	Jordin Kare	jtk@mordor.UUCP