Re: ATM Adler Responds to Flex Questions

["Stanley A. Gorodenski" <stanlep@extremezone.com>]

I still serioiusly question the longevity of a flexed mirror.  Although
the mirror mentioned below has not needed adjustment for months, still,
what about the long run?  Could such a system be given to a strict
observer, who has no inkling about optics or adjustments, and still
perform 20 years in the future without adjustment?  Would the rubber
puller and pusher last that long, or longer?  To such an individual
having to adjust a mirror may be a real feat.  Also, what about the
amatuer who shelves his mirror for 10 years?  In 10 years he/she may
have forgotten how to adjust it.

The nice thing about conventionally ground mirrors is that they are very
direct and there is no degradation over time one has to worry about. 
Also, there is some pride in owning a good mirror.  For example, I have
a 12.5" newtonian mirror which has about a 1/20th wave made by the then
chief optician at Adler Planatarium, Kenny Woolf(? - my box with the
name on it is 70 miles from this PC and so I cannot immediately verify
the name).  At RTMC one year I met someone from the Chicago area and
discovered there is some notoriety (in the Chicago area) toward owning
such a mirror. 

Stan

Gary Seronik wrote:
> 
> 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