ATM "Frozen" liquid mirrors

[CHEN@UIT.GSFC.NASA.GOV]

Hi Folks:
      There seems to be some interest in this subject, so I'll
get on the soapbox and give a quick and brief summary on
spincasting epoxy mirrors.  My purpose is to provide a little
history and background, plus what I think are the potentials and
challenges of this technique.  Then I'll sit back and hopefully
we can toss some ideas around.
    Spincasting is based on the idea that the surface of a
liquid, in uniform  rotation about a vertical axis and under
gravity, assumes a parabolic shape.  The shape of the parabola,
and hence the focal length, is a function of the rotational
speed.  The size, of course, is that of the container.  Therefore
if the liquid can be gradually and uniformly turned into a solid
while rotating, then one can generate parabolas of arbitrary
sizes and focal lengths.  As someone has pointed out (sorry, I
lost you post), this is a very nifty way to generate off-axis
parabolas.  
       The power of spincasting is tremendous. Most of us know of
and are awed by the very large mirrors in use or currently being
produced by Dr. Roger Angel and colleagues at the U. of Arizona's
Mirror Lab.      
    Spincasting epoxy mirrors uses a chemical reaction to
solidify the liquid.  Epoxy is usually chosen because it is easy
to handle, is quite viscous, gives a very smooth surface, and
cures evenly.  (Freezing water won't do.  The ice forms from the
air/water interface and expands downward, causing a bad figure
and very rough texture).  The material is cheap.  A quart of
Shell EPON 828, the preferred medium, costs somewhere between
$15-$20 from an industrial plastics supply shop.    
     The basic process goes as follows: 
1. mix the epoxy with hardening agent according to manufacturer's
recommendation.  
2.  Place mix in an airtight container.  Use vacuum pump to get
out gas bubbles.
3.  Pour mix into rotating container.  Spin overnight until epoxy
hardens.
4.  Take off the hardened epoxy.  Put it in an oven and cure at
low heat.
5.  Vacuum coat with aluminum, or silver, or whatever.
     The process is so simple.  So what are the problems?
    To begin with, the rotational axis must be vertical (to
within a few arcminutes, if I recall correctly).  This is not a
serious problem since a nonvertical axis will cause a small
vortex to form at the center.  On can adjust the tilt of the
apparatus until the vortex disappears.  Of course the chemical
mix must be uniform and be free of bubbles and dust.  Takes work,
but not impossible.     
    The killer problem, which have thwarted previous efforts to
make diffraction limited (oops, I mean Rayleigh criterion)
astronomical mirrors, is uniform rotation.  If the speed is not
held constant, or if the apparatus vibrates, the resulting
surface is covered by ripples.  For those with access to a good
reference library, there is a picture of a 36" f1 mirror made by
this process in Modern Plastics, August 1957, page 116. The
author stated that, despite all efforts, 'the mirrors are not
perfect...ripples 40 millionths of an inch in height are visible
to the naked eye".
     Well, it has been forty years.  Many new technolgies are now
available to solve the problem.  Lasers, quartz oscillator
controlled motor drives, etc.  And we know the problem is
solvable.  After all, Dr. Borra and colleagues at the U. of Laval
(& UBC, UWO, and others) have successfully built and observed
with large diffraction limited rotating mercury mirrors.  And
bear in mind that mercury is very much more sensitive to
vibration (it's not called quicksilver for nothing!).
    So here's the challenge.  Does anyone know how to make a
turntable that is vibration free, cheaply?  The liquid mercury
mirror telescopes use air bearings which are too expensive ($8-
15K), at least for me.  Since epoxy is lighter than mercury and
more viscous (damps out vibrations better), it should be possible
to use a less than perfectly quiet apparatus.  A high grade ball
bearing motor, perhaps?  Come to think of it, something off of an
old jet engine that is not smashed up might be a good candidate.
Hmmmm.  Any suggestions?
    For those who are going to try spincasting, please be sure to
have good ventilation, stay away from food areas, use disposable
gloves and containers, and read the MSDS (material safety data
sheets) for the chemicals.  And have fun.         
    

Peter C. Chen
Astronomer, optics, detectors, and dreamer
chen@uit.gsfc.nasa.gov
homepage http://snoopy.gsfc.nasa.gov/~lunartel/lunar1.html