ATM 'Frozen' liquid mirrors

[CHEN@uit.gsfc.nasa.gov]

Hi Folks:
         This is a great discussion.  Thanks for the input.
     For people interested in the nitty gritty aspects of
spincasting epoxy, there is an excellent article in Applied
Optics vol. 18 No. 11, p 1835 (1979) by Y. Ninomiya entitled
"Parabolic mirror made by the rotation method: its fabrication
and defects".  The author was investigating making cheap large
mirrors for projection TV.  This is an interesting and practical
application.  So, if at first you don't succeed in making a
perfect astronomical mirror, perhaps you can put it in front of
your TV and make your family think they are looking at a super
large custom made projection system.  Might help keep the peace,
hmmmm.
    Another interesting potential of spincasting is that one can
just as easily make CONVEX parabolas this way.  Most of us have
heard about the work of Dr. Borra and colleagues in Canada to
develop large (2m+) liquid mercury mirrors.  These mirrors have
been shown to be of diffraction limited quality.  Now, if you can
cure epoxy on top of a liquid mercury layer, the bottom of the
epoxy will be a convex parabola.  This idea isn't mine. It
was developed by folks at the US Army Engineer Research Lab in
Fort Belvoir back in 1958 (Haas and Jenness, J. Optical Soc. Am.
vol 48, No. 2, p 86) 
     Concerning the matter of turntables.  I have, like many
other people, found out that phonograph turntables are not really
suitable.  They are too springy, presumably to damp out small
vibrations and keep the needle from jumping off the platter. 
Also phonograph turntables were not designed to take a large 
(several pounds) axial load.  This is unfortunate, since they do
have the advantage of being cheap and widely available.
      I also tried out an old potter's wheel from a crafts
friend. These can take heavier loads (50 lbs or so), have
segemented elastic belts as Jeff has suggested (Thanks, Jeff),
and have adjustable speeds.  I rewired the speed controller in
the foot paddle and replaced the variable resistor with a fixed
one to keep the speed constant.  Still had problems
with vibrations despite putting everything in a sandbox as my
laser holography friends suggested.  I am not sure whether the
problem is due to inadequate damping from the ground or a worn
bearing in the motor drive. Probably both. 
       The idea of spinning a liquid within a liquid is an
excellent suggestion (Bravo, Bryan!).  The setup is both auto
centering and self damping.  I first came across the idea in an
old US Patent (Bittner, #3,010,153) dated 1961. (I guess the
patent has expired and is now public property).  I tried using
the aforementioned potter's wheel with a 16" punch bowl from the
party store and a couple of gallons of antifreeze.  Can still
see the ripples (in a dish of mercury).  I am thinking of trying
heavy fuel oil next.  Any suggestions?
       Finally, a note on air bearings. For spincasting the
speeds are very slow (a few rpm) and the load is fairly heavy
(50-100 lbs), so I suspect that the setup would be different from
the type of air bearings use in NMR machines.  Also some
provision would be needed to center a fair size load (16 inch+)
very accurately.  You idea of making a custom airbearing is
tempting, Gerry.  I don't have access to a machine shop, which is
fortunate since my skills in that area are not that great.  Are
there any turntable components, with air or high grade bearings,
that one can buy off the shelf?   
      Thanks, everyone.

Peter C. Chen
chen@uit.gsfc.nasa.gov
homepage http://snoopy.gsfc.nasa.gov/~lunartel/lunar1.html