ATM "frozen" liquid mirrors

[CHEN@uit.gsfc.nasa.gov]

   This is in reference to Tom Warm and Brian Reynolds' post on
spincasting epoxy mirrors.  I have been working on this subject for
a number of years, and it is great to find someone else doing the
same 'non-traditional' mirror making.   Permit me my two bits worth
of comments, if I may.
   The focal length of a spinning liquid, under earth gravity, is
given by:
   f = 17710/(N**2)
   where f is focal length in inches and N is rotational speed in rpm.
Therefore for Tom's turntable at 78 rpm, we get a focal length of
2.9 inches or 7.4 cm, as Tom observed.
    About coating:  epoxy can be vacuum coated if it is the right
type.  I am doubtful that epoxy for auto repair will work in 
vacuum since the stuff was not made for that purpose.  There are 
many types that are vacuum stable.  Try EPON 828 which is available
from plastic supply shops.  They also sell the appropriate curing
agent.
    Mr. Reynold's comment is very important: Get plenty of ventilation,
and keep off food preparation areas.  Also the stuff must be made in
small batches (100 g or so for EPON 828).  Otherwise a runaway
reaction occurs and you get a hard, foamy mess.
     Finally, thought to mention that you gentlemen are following
in a venerable tradition.  Spincast epoxy mirrors were made back
in the fifties and sixties by various groups at the U. of California,
U.S. Army at Fort Belvoir, Birmingham University in the UK, and
astronomers at Cal Tech.  The General Electric Co.'s astrospace
division used spincasting to make mirrors ** 10 meters ** in
diameter!

     Spincasting is a facinating subject.  I'll be happy to go into
more details if anyone is interested.

Cheers,

Peter C. Chen,
chen@uit.gsfc.nasa.gov