ATM Adler Responds to Flex Questions

[Gary Seronik <gseronik@skypub.com>]

Hi All:

Although Alan is not a member of the ATM list, he has been following the
discussion surrounding his November Sky&Telescope article and has the
following comments to offer.

Regards,
Gary Seronik

===============================================================



Date: Mon, 09 Oct 2000 22:05:46 -0700
From: Adler <a1257@worldnet.att.net>

---------
A number of questions have been raised regarding flexed mirror telescopes,
as described in my article in the November Sky & Telescope magazine.  I
welcome discussion and stand ready to assist both amateurs and
professionals who have an interest in flex technology.

Below I address a few of the questions raised to date:

1.  Is the flex method more work than normal parabolizing?

My 8" f6 is the first mirror that I ever ground and polished. With a bit of
patience I made a 1/22 wave (P-V wavefront error) sphere which flexed into
a 1/40 wave paraboloid.  To me, that seems pretty good for a rookie.

While the flex cell may seem complicated compared to three dots of RTV on a
disc of plywood or a piece of carpet, it's really simpler than most
flotation cells.  But even if it were more work, I have a favorite saying
about our hobby, "Remember that astronomy is for passing time, not saving
it".  Wouldn't you be willing to spend some extra shop time to have the
sharpest telescope on the field?

Incidentally, manufacturers will soon be offering flex cell kits for ATM's.
 I expect them to be fairly inexpensive.

2.  Is constant tension adjustment needed?

Here I can answer an unequivocal "no".  I've checked tension (with the
defocused star test) often and it hasn't needed adjustment for many months.
 Read the third column of page 138 and the first column of page 139 for
more on this.

3.  Will the flex cell degrade over time?

Here too, I can confidently answer "no".  A key part of the flex cell is
the coil spring.  It maintains constant tension despite minor creep (due to
time or temperature) in the various cell components.

The stress levels on the flex cell components are very low compared to
their rated strength.  For example, for most flexed mirrors, the adhesive
stress is under 5 PSI, yet the adhesive is rated at thousands of PSI.  The
flex program calculates (and recommends) plate thicknesses.  These
calculations are based on very conservative stress levels.

In closing, I would encourage everyone to read the November article through
carefully and then take a stab at making a flex cell.  It represents
several years of applied research and discusses many of the questions
raised to date.

Alan Adler