Re: Alternative mirror technologies, WAS ATM Liquid-mirror telescopes (fwd)

[Rick Nelson <richard@netexp.net>]

Fred Floberg wrote:

> You might want to have a look at the archives for May of this list.
> P.C. Chen <CHEN@stars.gsfc.nasa.gov> breifly reviews his work with
> "ultralightweight composite replica mirrors" and provides a URL
> (http://snoopy.gsfc.nasa.gov/~lunartel/lunar1.html) which describes
> the process in more detail.


There is part of an article on
http://home.ust.hk/~westland/astro/material.htm :

The latest developments in graphite composite mirrors were published in
the May/June 1996 issue of High Performance
Composites (pp.64-66). Robert Romeo, president of Composite Mirror
Applications Inc. of Tucson fabricated an all composite space
telescope mirror (with substrate and support structures) of carbon,
glass and Kevlar fibers in epoxy and cyanate ester materices. The
weight is 5% to 10% of pyrex. A 16-inch prototype built for NASA weighs
3 pounds! The mirrors are molded (!) rather than ground
to a mirror surface. 

Romeo claims to be getting quarter wave precision on 10" to 20" mirrors
with a surface roughness of 10 Angstroms (he claims 20
Angstroms is adequate for telescope mirrors). Their 13" prototype for
JPL (which is designed for use in the ultraviolet range) exhibits
a surface accuracy of 1/30th wave in the green range. Since the support
is composite (rather than aluminium or steel) it ceases to be a
source of collimation or stress error. Aluminium has a CTE of 12.7 ppm/
deg F, Pyrex is 1.7, while as mentioned earlier, graphite
composite is near zero. 

Romeo relies heavily on the expertise of the Mirror Lab at the
University of Arizona in Tuscon, where Composite Mirror
Applications is located. He claims that this may be the only source of
large aperture convex mirrors, with surface accuracy in the
1/10th wave range - a requirement for the molding of his composite
meniscii. 

The major problem to be resolved in production stems from the fact that
composites have different mechanical characteristics along
the fiber versus across. Fibers have to be angled in order to create the
desired mirror properties. This is where the proprietary
processes at Composite Mirror Applications Inc. have provided a solution
that allows the inexpensive production of ultra-light,
large aperture mirrors. 

All this is, I think, exciting. It promises mass production of molded,
large sized mirrors that are a small fraction of the weight of
existing mirrors, yet with better temperature stability. Mating such
mirrors to the lightweight, rigid mounts proposed in this article
promises to open up an exiting new level of portability in amateur
astronomy. 

> Also, I'm sure that this topic (mylar film mirrors in a vacuum) was
> brought up not too long ago here. I think Andy Saulietis was interested
> in them, but I can't find the thread in the archives now.

I'm working seriously on a mylar mirror system.  I originally asked the
question to the list 8 weeks ago or so?

Thanks,

Rick