Re: ATM multi-mirror telescope

[Anthony Stillman <atmer@flash.net>]

Holger,

In responce to your post:


>I´m planning to make a multi-mirror-telescope

This is quit a challenge.  Fabrication, mounting and use will all be
difficult.  There are many resources on which to draw.  Your description
for a segmented monolith is similar to experiements conducted by William
Parsons, the third Earl of Rosse in the late 1800's (1)  There have been
many efforts sisnce then.  In the mid and late 1970's the journals "Applied
Optics" and "Optical Engineering"
regularly carried articles detailing various aspects of the MMT, a true
mutil-mirror telescope.  These instruments were recently replaces in their
mount by a single 8m blank.  Guess why.  Though the MMT wasn't a segmented
monolith many of the issues are similar.  Both the Keck I and Keck II are
segmented monoliths, however their segments were seperately fabracated and
then assemblied.  The original contractor went out of business.  Their
process didn't work, but this wasn't the only reason the company failed.
The successor flexed the segments to figure them.  This worked well enough
that a third company could, using ion figuring, bring the segments close to
spec.  It is notable that many of the segments still have permanent flexure
rods adheared to their backs to tweek them into shape.  This is in addition
to their active support system.  Even so, the segment derived wavefronts
are not pretty.  Some edges are up or down several waves.  The instruments
are used mostly for on-axis photometry, the use for which they were
designed, and function well as light buckets.



>...epoxy the mirror blanks

Epoxy, onece set retains internal stress.  This can be relieved by thermal
cycling (sort of like annealing).  Thermal consideration need to be
addressed, with the epoxy, with the backer, with the optical substraight.
Do your reasearch. The optical components of a recently orbited telescope,
which are epoxy bonded to metal, suck because the researchers who speced
the interface didn't do their research and wouldn't listen to the
fabracator.


>- During the process...the holes between

You can fill holes with wax, Plaster of Paris, pitch, ...  Again you must
concern yourself with thermal effects durning fabracation.

>...long-time behaviour...

Well implemented you can expect no more than thirty years of life, likely
less.  You will need to allow for drift adjustments and even rebond from
time to time.


>- Will they fall off during grinding, polishing or aluminizing?

Again, if done correctly, only a debonding agent will seperated the components.


>...a lightweight telescope with an aperture of 12"

Considering that the support system for a single thin mirror is less
involved that what is needed to support seperate segments, I suggest a 1/2
or 3/4 or even 1 inch thick, 12 inch diameter piece of pyrex of soda-lime.




Anthony

PS  Personally I'm intrigued by multi-mirror designs and clearly for some
they will be the future of large apertures.  I encourage you to experiment
and, not to get your hopes up.




1) Unusual Telescopes, Peter L. Manly  p195