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[Phys-L] Re: telescope mirrors



Spun mercury mirrors have failed from persistent vibrations, and spun
mercury mirrors have succeeded.
You may be aware of the float glass method, which makes rather
respectable flat sheets by floating molten glass on molten tin.
The transition to the smoothly rotating frame is best left as an exercise
for the engineer. Still I recall SciAm pieces on the topic, not so long ago.

Mirror materials may be cast into curved surfaces in the bigger sizes
(over 0.7 meter?) with waffle reverse sides, to hold the line on weight.
But dynamic correction and multiple coordinated surfaces, even on long
base lines are growing popular, I hear....

Brian Whatcott

At 04:51 PM 1/27/2006, you wrote:
Sorry, telescope mirrors are almost always made by grinding one piece of
glass over a another piece of glass or softer material with the use of an
abrasive slurry. (Many books on Amateur telescope making)
The use of a rotating mirror (mercury) was proposed several decades ago,
but was not practical because of he necessity of eliminaing all vibrations
which produce ripples on he mercury surface.
to confess my own sins, I once tried to form a parabolic reflector using
epoxy, but ended up spraying epoxy all over the lab.
I believe that there are some experimental mirrors that use a thin film of
mercury over a near prabolic spinning form.

Al Bachman


From: Anthony Lapinski <anthony_lapinski@PDS.ORG>
Reply-To: Forum for Physics Educators <PHYS-L@list1.ucc.nau.edu>
To: PHYS-L@LISTS.NAU.EDU
Subject: telescope mirrors
Date: Thu, 26 Jan 2006 22:22:41 -0500

I'm curious about something. I tell my (high school) astronomy students
that reflecting telescopes are made by melting glass and then rotating it
at high speed while the glass solidifies. The result is always a parabolic
surface. How can one show, in simple terms (no calculus), that the surface
of a spinning liquid is a parabola? I am also wondering what the "limit"
would be as the spin rate becomes"very fast." Any help would be much
appreciated!


Brian Whatcott Altus OK Eureka!
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