Re: Alternative mirror technologies, WAS ATM Liquid-mirror telescopes

[Chuck Knight <cknight@flash.net>]

John D. Gwinner wrote:
> Heck, just make an epoxy parabola with the turntable technique.  Then,
> vacuum form the mylar.  

Only two little problems with this technique.  I cannot guarantee the
optical quality of the mylar film...its thickness may vary ever so 
slightly.  And, as we all know, this would ruin the mirror.

The other problem is that the surface produced by spin-casting is
not of optical quality...it's a materials problem.  Resins *ALL*
expand or contract while setting, so getting a perfect parabola
with today's materials is virtually impossible.  It'd be best if I
made a casting from an optical quality convex glass mirror...but
for a prototype this won't be necessary.

RANT follows:

If you think about it, the only important part of the mirror is the
reflecting surface...the rest of it is only there to provide a shape,
or physical support.  I'm interested in finding "other" ways to
provide that shape, with accuracy, which use modern materials.  Let's
face it...glass mirrors have been around for centuries, mylar has been
around for only decades.  And, it's cheap enough to warrant some 
experimentation.  BUT, glass is still capable of providing the nearly
perfect optical surface which most polymer mirror research will try
to equal.

Anyway, enough ranting...

I want to create the parabola on a convex form, so that the *reflecting*
surface is the "reference" surface, and the substrate is irrelevant.  I
believe this would provide the best chance of successfully creating an
accurate parabolic surface.  If I spin a mirror for a form, I'd have to
cast a mold from it...a little more complex, but not too bad.  Now,
where
did I put that original spun urethane mirror?  It's small, but it'd 
probably work for a prototype...

Eventually, I'd like to be able to make, economically, a primary
reflector of several meters diameter.  There's no way I'd ever attempt 
that in glass...too much glass to push, not to mention the cost.

> If you are careful with the epoxy, it might be such
> that you could either charge electrostatically the epoxy to repel the mylar
> just a *smidgen* above the epoxy, or just let the Mylar drape realllllly
> close to the epoxy.  I don't know, just a thought.

Not a bad idea, but I don't know what benefit it would provide. 
Assuming
a mold made from an appropriate substance, it shouldn't stick in the
first 
place.  Off the top of my head, I'm thinking of a fine grained plaster,
a 
molding compound, for the convex mold.  Anyone know of a
"super-accurate"
casting compound that's easily available?  Hmmm...I wonder if a simple
spray coating of polyurethane would give me a smooth enough surface? 
Or,
maybe even wax?  Just something with a finer texture than plaster.

     -- Chuck Knight