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[Phys-L] Re: telescope mirrors



Some of the large mirrors have been spin cast. Here's some references.

<http://www.npr.org/templates/story/story.php?storyId=4773461>
The Steward Observatory Mirror Lab recently began casting the first of
seven mirrors -- each 27 feet in diameter -- that will become part of
the Giant Magellan Telescope, creating an enormous reflective surface of
glass.
The facility uses a unique process to cast single-piece mirrors so
large. Twenty tons of glass are spun inside a two-story rotating furnace
to create a naturally concave mirror. The bowl shape is highly effective
at gathering and focusing light. As the glass melts, it flows around
ceramic forms inside the furnace, creating a mirror a few inches thick
on top of a glass honeycomb. That makes it just one-fifth the weight of
a solid mirror.
Cooling the furnace will take almost three months. After that, the
mirror will be moved to another room for polishing, a process that takes
about a year. The finished product will be accurate to within
one-billionth of a meter.

http://cfa-www.harvard.edu/mmt/mmtpr/032599.html
Crews from the MMT Observatory and The University of Arizona Steward
Observatory gathered at 6 a.m. on Mount Hopkins, south of Tucson, for
the last, major, heart-stopping step in an unprecedented telescope
conversion -- the lift of a 21 and 1/2 -foot (6.5-meter) diameter
primary mirror into a pioneering telescope known as the MMT.
The UA Steward Observatory Mirror Lab in 1992 spin-cast the borosilicate
"honeycomb" mirror in the lab's enormous rotating furnace. Lab
scientists since have polished it so perfectly that if the glass were
the size of the United States, no bump on its surface would be higher
than an inch.

Larry Woolf
General Atomics
www.ga.com
www.sci-ed-ga.org

-----Original Message-----
From: ALVIN BACHMAN
Sent: Friday, January 27, 2006 2:52 PM
Subject: Re: telescope mirrors

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)

From: Anthony Lapinski <anthony_lapinski@PDS.ORG>
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.
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