Re: ATM Flex-Mirror Notes

[mjc5 <mjc5@psu.edu>]

Me Skeptical? Never!

	My initial thoughts on the whole process.

	Will it work? I think the odd shaped-back mirror will indeed work well.
I have my severe doubts abbot 1/70th wave mirrors. Even if it were
attainable, in practice, I doubt it will happen, unless we can keep the
outside temps exactly stable. If it will, the next Adler project is to
make 1/70th wave secondaries.

	My thoughts on a flat back mirror, which can apparently be stressed to
around 1/12th wave are that it is interesting, but that it is something
attainable by traditional methods. It can be an interesting experiment,
though.

	But that aside, the process of forcing the stress on the mirror to be
applied differently via a back that is shaped to distribute the forces
makes some sense. So you are correct in moving beyond the simple does it
work question (although it would be interesting to follow one of these
scopes in use)

	This brings us to some important questions. The mirror blanks that we
use now, are flat, sometimes pre generated to specific f ratio. In any
event, they are not exceptionally expensive, although that is a relative
term.

	Are these shaped back mirrors going to need to be calculated to their
shape, then formed one off to these specifications? This will be rather
expensive, I would think.

	Is there no way that the amateur can form their own shaped back mirror?
I suspect we are looking at a fairly sophisticated form of slumping the
mirror. Any thoughts on amateur glass machining? Let's not forget the
wedge problems here either. Or does this pretty much take the blank out
of the amateurs hands?

	Assuming that we use a cast mirror with a shaped back of variable
thickness that then becomes a sharply sloped inverted dimple in the
center of the mirror, is the wedge problem easily controlled?

	 I also expect that this is something that will end up being very, very
expensive. Perhaps we can apply for grants to construct our telescopes?
;^) Are there any thoughts on the likely cost of say a 18 inch f5 or 4.5
blank? Yes, that is what I am thinking about making.

	Is there a glass that must be used? Glass can differ a lot between
different types.

	I agree that many mirrors appear overworked. Most amateur scopes should
not be overworked IMHO. This is a problem caused when the worker tries
to continually correct corrections. My rule of thumb has been to try to
sneak up on the parabola, then if I go too far, I take the mirror back
to a sphere, then start over again. Some may think that sounds like a
lot of trouble. Yeah, it is, but it's less trouble than trying to
correct corrections. I've read lots of posts here where a poor person
has just about torn their hair out trying to correct the previous
correction, which may have been correcting an even earlier correction.

	So I think that with the additional work that goes into making sure
that the back is shaped, the preparations for the specialized cell, the
assurance that wedge is minimal, and the other labor that goes into
making a flexed mirror, plus the extra cost of the shaped blank makes
this probably *more* work compared with the labor of correctly making a
parabola, the only *real* reason for making a  flex mirror is superior
performance, or novelty. Pardon that run-on sentence! ;^)

	Now that being said, I still do intend to make one, but I originally
hoped to make a flat back one, but now am wondering if a shaped one will
do better for me.

	- mike -

Gary Seronik wrote:

> In closing, I hope that anyone who makes a flex mirror will offer their
> comments to the list, good or bad. For those of skeptical bent, continue to
> raise questions and try to do some experiments to see if your concerns are
> valid.  I think we can move beyond the "does it work?" phase of discussion
> into the "how can we make it work for a wider range of sizes and
> thicknesses" phase.  The bottom line is that method does work (and works
> well!) if one follows Adler's instructions and heads the warnings and
> caveats outlined in his article.