RE: [ATM] Too Small Optics-( and flex produced optics)

["Peter" <peter@kitgear.com>]

Jerry Jerry...

Wow! Such negativity about a technology you've never played with.

Flex scopes are shown at star parties all over the country so go out and
look through one.

What - is a star test not good enough for you? Your point about not being
able to test the mirror on a stand is incorrect. I am sure Bill can supply
Foucault readings if asked. Taking Foucault readings on a flex mirror is a
long involved process due to many iterative puller adjustments which are
necessary until the desired form is achieved. In the field people us a star
test to make the puller adjustments because it is fast and accurate. It has
nothing to do with not being able to take measurements.


Peter

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net]On Behalf
Of Jerry
Sent: Saturday, June 18, 2005 6:15 AM
To: ATM list
Subject: RE: [ATM] Too Small Optics-( and flex produced optics)



	Let me point out that claims of SUPERORITY of flex mirrors over
figured mirrors have been made. They claim FLEX is not just OK. It is
better.

	Not that I don't think that flex might make a serviceable telescope,
but while claims are made of surface smoothness and supposed freedom from
zones, no one seems to want to prove it.

	When I ask about testing data or focograms and ronchigrams the
answer is that you can't test a flexed parabola the same way a figured
parabola is tested. You can't take a focogram of a flexed parabola. I don't
know why you couldn't put the whole flex assembly on a stand and adjust for
total correction from a zone close to the center and a zone close to the
edge and then put the old couder mask on and measure some more zones and see
how well they match the desired numbers, but I've been told by flexers that
you can't do that. I don't know why you can't take a ronchigram of the
mirror un-flexed and flexed for a before and after, but I've been told by a
flexer that you can't do that. But the real test for what I think is
probably the most likely result of flexing would be one to show any
astigmatism. Where are the interferometers that everyone has been making for
years. Well, it probably doesn't matter, I'll bet that a flexed mirror can't
be tested on an interferometer either. And not on any other shop test either
I suppose.

	It seems the only way to test it is at the eyepiece looking at the
sky. 'It's just the air currents, you just can't see how smooth it is under
these conditions.' Or 'That must be tube currents making it look like that.
If you look at the average of the wavy lines they are extremely straight.'

	I thought shop testing should be as easy as having a fish scale in
the puller mechanism. Not a scale of the fish but the kind you would weigh
him with. Calibrate the pull on a star if you wish. I would calibrate in the
shop but non-flexers think backwards. But test it in the shop where the air
currents can be minimized. And take some pictures so we can see the super
smooth surface.

	If someone posted to the list that they were going to mount a mirror
in a cell that would support the mirror perimeter on some type of o-ring on
a backboard, they would be savaged by list members telling them it would
induce astigmatism and he should use PLOP to design a cell. LOOK IN THE
ARCHIVES. But it's for a flex mirror. OH! That's different. Hmmmmm... The
secret must be in the patented plunger.

	Someday I might want to build a flex scope. Or maybe off axis or
de-centered parabolas. How about a 100 foot focal length refractor ... no
make it a reflector. A SCT or better yet a Hootie Flootie. Long ones, short
ones. For the ATM they are all fun and reasonable projects.  Well, maybe not
the Hootie Flootie. Too many surfaces to work.

	I read the SKY & TELESCOPE article and others. Claims of quality are
made but proof is lacking. I find it amusing that some of the same people
who would challenge claims of superior methods, design or materials on most
any other matter by demanding data and proof just jump on board with the
flex thing and accept the claims and defend them no questions asked. I guess
that is because you just can't test a flex mirror like a figured one.

	Knowing there are those who hate to test and hate to judge shadows
and hate to get the zone data and reduce it, I can't help but think there
just might be another reason than quality why "flex superiority" is accepted
so easily. "Just null test it and put the screw to it. That donut in your
mirror is hiding all those zones."

Jerry


-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
mdholm@telerama.com
Sent: Saturday, June 18, 2005 12:12 AM
To: atm@atmlist.net
Subject: RE: [ATM] Too Small Optics-( and flex produced optics)


You need to read Bill Kelly and Alan Alder's Sky & Telescope articles:

This is one of them: I am having trouble locating the other.

Flexing Spheres into High-Quality Telescope Mirror, November 2000,  p.
131-140
(purchase .pdf file at
http://skyandtelescope.com/shopatsky/_additem.asp?aaid=200011131140 )

The short answer is that millionth of an inch machining accuracy is not
needed
if you play the material properties game correctly.  There are some
precision
requirements that are not usually required for mirrors, but they are not
terribly difficult.

Mark Holm




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