To: "Kreig McBride" 
From: ray{at}stabilite.com (Ray Desmarais)
Cc: atm{at}shore.net
Reply-To: ray{at}stabilite.com (Ray Desmarais)


 Hi Kreig,

This is the ticket when it comes to supporting thin optics. I used 8"
diameter donuts to support the 72". they helped a lot when it came to
spreading the load across the mirror. I think that this is probably the
best thing that we can do to use normal cell designes for our mirrors. I
never have liked small contact points in my mirror cells

RAY
>Question:
>
>Why are we using points???
>
>Why not use "large" points such as 1/4" diameter soft
nylon or teflon?
>
>In my 54 point cell, i used the caps found on 35mm film containers. The
>contact points are a ring instead of a point. (The inside of the cap.)
>
>The 54 point cell is not in use yet so dont know how it will perform.
>(experimental)
>
>This is a question, not a suggestion.
>
>Kreig  McBride
>Telescope Optics Workshop
>March 22-23, 2003
>Bellingham, WA
>
>
>-----Original Message-----
>From: Jeff Anderson-Lee 
>To: ATM Mailing List 
>Date: Thursday, February 27, 2003 6:16 AM
>Subject: ATM supporting thin mirrors
>
>
>>
>>With all of the "too big, too thin" talk, it brings me to
a matter of
>>concern for me: how best to support a thin mirror.
>>
>>I have a 16x7/8 plate glass blank that I had Dan Cassaro pre-generate to
>>f/5.  I went with f/5 because Plop seemed to indicate that I could support
>>it with 18 points.  However later analysis shows that may not be so...
>>
>>First, by the time I get a smooth/polished surface I figure [no pun
>>intended] I will more likely be looking at 3/4in of glass, not 7/8in.  That
>>still seems to be doable with 18 points -- at first glance.
>>
>>However being a skeptic of sorts I did a study using Monte Carlo variations
>>to look at the effects of mirror cell fabrication errors.  The results were
>>not very
>>encouraging.
>>
>>You can see the whole study at
>>http://http.cs.berkeley.edu/~jonah/18plus/p18.html but I will summarize the
>>results here.
>>
>>I studied 5 vatiants of an 18-point cell using even angles, varying angles,
>>Plop optimized varying force, 0.8 varying force, and refocusing with
>varying
>>force. Simple Monte Carlo testing on 2mm error showed at least a 3.5x loss
>>in average versus design performance with no refocusing and 8.5x loss in
>>maximum versus design performance on 100 Monte Carlo runs.  When allowing
>>refocusing, the results looked more promising, but that is because simple
>>Monte Carlo testing introduces systemic errors that can be refocused out.
>>
>>Changing the model to reflect more realistic fabrication errors lead to
>>larger changes in performance; I had to reduce the tolerances to 1mm before
>>I began to see "acceptable" levels of performance once again.
>>
>>Using varying force ultimately did not seem to help, in that any seeming
>>performance increase was lost back in increased sensitivity to errors.
>This
>>was especially true with the model optomized with refocusing on.  (I
>>generally optomize with refocusing off.)  The best case still saw a roughly
>>3x average (8x maximum) perfomance loss in implementation (for 1mm errors!)
>>Allowing refocusing after the fact (i.e. design without it, Monte Carlo
>test
>>with it) reduced the spread to about 2x average loss (4x maximum loss).
>>
>>Variation in balance contributed the most to performance loss.  A friend
>has
>>suggested using laser-cut stainless steel parts for the mirror cell for
>more
>>precise fabrication.  However I'm still puzzled at how to join the parts so
>>that the balance points do not shift by more than 1mm!
>>
>>I've heard of using ball-bearing supports for the triangles, but have not
>>found a manufacturer/ part number/ distributor that would seem to supply
>>workable parts at a reasonable price.  (I would need 6 for an 18-point
>>cell).
>>
>>The alternative would seem to be an astatic cell design.  I have a drill
>>press, but no complex machine shop, so any pointers/suggestions for simple
>>astatic cell units would be appreciated.
>>
>>The alternative would be to go for more points of support.  27 or 36 points
>>might give more support, but I'm probably still going to be stuck with
>>fine-tolerances in cell manufacturing--more than a drill-press alone may
>>suffice.
>>
>>For those who are interested, I have a collection of Plop cell designs of
>18
>>or more points as well at http://www.cs.berkeley.edu/~jonah/18plus/ which
>>includes sixteen designs ranging from 18 to 108 point cells.  The overview
>>there describes the collection and notes on the design process used, as
>well
>>as discussions of why I chose to optimize with "Refocus Error
Calculation"
>>turned off and the like.
>>
>>There are two downloads there for the package: a slim 38KB which includes
>>the .gr mesh files, an Excel spreadsheet, and a README file, or a 700KB
>>distribution package with the "full deal" for anyone
wanting all the gifs
>>and html files for private use or redistribution.  The report on 18-point
>>cell optomization is also available there in both .html and .doc format.
>>
>>Phew.  That's a lot!  for those of you who made it this far, thanks for
>>reading.  Any help with cell manufacturing suggestions would be
>appreciated.
>>
>>Jeff Anderson-Lee
>>Sacramento, California; 38.5566N 121.4525W
>>
>>
>
>
>
>
Ray Desmarais
479-890-4918
ray{at}stabilite.com

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