RE: ATM Epoxy spin-cast mirrors: comments for experimenters

["Andersen, Clark R." <clanders@utmb.edu>]

The references to existing progress with epoxy mirrors have all referred to
mirrors which were cast using an existing glass  surface.  The best figure
which I've encountered for these types of mirrors has been about 1/5 wave,
but for a good reflector scope I understand that you need 1/8 wave.  The
existing designs are typically reinforced with fiberglass cloth (which adds
an astigmatism problem) to achieve lower weight.  The site Mike Gerfen
mentioned http://snoopy.gsfc.nasa.gov/~lunartel/lun7.html has been
particularly successful with this type of mirror, as well as
http://www.abwam.com/opticast/, which is discussed thoroughly here
http://www.taas.org/times/nl/aug98nl.html#atm.

My own interest is more in terms of spin-casting a mirror, which under ideal
conditions will yield a perfect parabola (okay, the curvature of the earth
becomes a significant distortion exceeding 1/8 wave when the radius exceeds
2 meters...).  For more on spin-casting, you might look here
http://www.geocities.com/CapeCanaveral/Lab/7747/epoxy.htm, and there's a
convenient focal length calculator here http://www.atmpage.com/calc.html.
Incidentally, deriving the formula for the surface of a spin-cast fluid is
pretty simple given a knowledge of basic physics & algebra - simply consider
that the equilibrium surface of a fluid is always orthogonal to the sum of
the force vectors (gravity + spin) acting on it.  A 2-D simplification works
fine.

Early experiments with a good epoxy suggested that it had the needed
hardness and stability for a mirror, so I've envisioned simply mixing-up a
batch of epoxy, pouring it into a shallow container, and spinning at a
constant rate (with low vibrations) until it hardens, then silvering the
surface (aluminizing may have problems due to epoxy outgassing).

I figure that if I can achieve an optically flat surface with epoxy (without
spinning), then spin-casting would be a simle follow-on project.  I tried
one experiment to achieve an optical flat.  I mixed a batch of epoxy and
poured it into a shallow mold to a depth of about 2 cm.  I noticed that it
contained bubbles, so I applied an ultrasonic vibration to the container,
which removed many but not all bubbles.  It hardened overnight, and I
reviewed the results.  The surface was very smooth, like a liquid, but even
the tiniest bubble on or near the surface created an obvious distortion.  I
think that a solution to this problem may be to mix and pour the epoxy in a
(near) vacuum chamber, but I have not yet gotten around to trying this.

There are many potential benefits if this can be made to work, principally
including low cost, low weight, and rapid results.  It would also enable the
creation of simple diagonal-free telescopes by spinning about an off-center
axis (aren't these called Herschels?).  I even have this little fantasy of a
ridiculously large pair of binoculars.

Good luck :)