[ATM] Mirror cells - some professional literature and ideas foramateurs

[tkrajci at san.osd.mil (Tom Krajci)]

Here are some professional articles on mirror supports that may be a
source of ideas for us amateurs.  Not every idea can be used by us
(actuators, computer control), but some ideas are worth mentioning:

http://celt.ucolick.org/reports/report16_mirrorsegsupprt_mar01/report16_
mirrorsegsupprt_mar01.htm  (long URL...may need to cut/paste).  Check
out the diagrams CAD drawings for lateral support.  Amateurs use slings.
These folks, on rather large glass, are using only supports from the
rear...and on a small number of points in some cases.  The axial support
uses piano wire compliant hinges, and uses RTV to bond to the glass.)
Lots of good diagrams and ideas here.

The LAMOST project: http://lamost.bao.ac.cn/intro_e.htm
http://lamost.bao.ac.cn/docs/paper/SPIE/Mb.pdf
see fig. 5...central lateral support from rear (single, central post),
whiffletree, and "anti-drop" mechanism...because the mirror is RTV glued
to the support.

Gemini telescope mirror cell:
http://www.gemini.edu/documentation/webdocs/preprints/gpre10.pdf
http://www.gemini.edu/documentation/webdocs/rpt/rpt-o-g0030.pdf
The mirror is very thin....and they had to worry about making the mirror
cell itself rather rigid.  Also, use of four bipods between mirror and
cell for lateral support.

http://www.aura-nio.noao.edu/book/ch4/4_5_C.pdf - especially fig's 9 and
10 show the overall concept very clearly.  Lateral support is only three
bipods.  Axial support is only 18 point whiffletree.

Lots to digest here.

To summarize what I've learned:

The pro's seem to make support schemes that use different mechanical
systems for axial and lateral forces.  Axial support is almost always a
whiffle tree.  Various methods for lateral support:  central disk on
rear face, 3 or 4 bipods on rear face, or various edge support schemes,
etc...and they all seem to be workable...on a professional budget/level
of technology.

However the pro's use actuators/computers to get a working system to get
two support schemes to work more-or-less independently of each other
(axial and lateral).  And they avoid edge support because they make
segmented mirrors...edge supports are not allowed in this case.  At
least the pro's show us that lateral support can be made from the rear
face...and with not too many points either.  This sounds encouraging.

But how do I make a passive (non-actuator/computer) support design that
handles axial and lateral forces on two separate mechanical systems?
Does this mean I'm forced to support both axial and lateral with one and
the same passive system?

My $64 question:  Can I make whiffletree cells for large/thin mirrors
that serve as both axial and lateral support...in other words combine
the two functions of axial and lateral support in one mechanical
structure.  (The pro's choose not to...but can I get away with it?  Hey,
I've already done it with a 16 inch, 1.5 inch thick mirror...but how
big/thin can I go...using a passive system?)

I hope this helps feed some ideas, and also answer some questions.

Tom Krajci
Tashkent, Uzbekistan


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