Re: [ATM] PLOP Calculations

[Scott Rychnovsky <srychnov@chem.ps.uci.edu>]

Nils,

Thank you for the analysis of the edge support.  This is a topic I 
have been concerned about for a while.  PLOP is very nice but I think 
it leads to over-optimized back support without dealing with more 
serious problems that might arise from faulty edge support.

My most recent Dob cell used wiffle tree support for the edge.  Would 
you like to speculate about the best placement of support for this 
system?  The center of each wiffle tree was +- 45 degrees.

An alternative to the ubiquitous sling (which I don't like very well 
because it isn't clear to me that it positions the mirror precisely) 
might be two short slings. Consider three posts arranged at the 
bottom and +- 60 degrees of the mirror.  Now put a sling over the +- 
60 posts and under the 0 degree posts.  The mirror would rest between 
the two slings.  This may be needlessly complex, but it would hold 
the mirror firmly in the XY plane.  It would be a two point support, 
but a slightly loose sling would give you continuous contact over two 
parts of the mirror's edge.

Andy,

Regarding scheme to reduce friction along the edge due to the sling, 
let me suggest the miracle material, parchment paper.  It you placed 
a sheet of it between the sling and the mirror, I expect it would 
reduce the friction and eliminate forces perpendicular to the mirror. 
It does not stick to anything I have found, and the improvement would 
only run about $3.

Scott

>Andy,
>
>>
>>  I'm also thinking about using a heavy roller chain for the
>>  sling, with the refinement of adding short shafts every
>>  few links in the spaces between the chain links..each shaft
>>  end would carry a small bearing that contacts the mirror
>>  edge..the idea is that if there's differential expansion between the
>>  chain and the mirror, that the bearings would eliminate the stress.
>>
>>  Also, each of the 27 cell support points could be a small
>>  bearing alined so the axle is perpendicular to the radius
>>  of the mirror at that point..thus allowing the supports to expand
>>  or contract relative to the glass without adding stress.
>>
>>  What U guys think about this??
>
>I get the impression you plan to eliminate friction forces in the tangential
>direction - I'm not sure but I believe those forces are pretty innocuous.
>What I am sure isn't as innocuous by far is forces in the axial (parallel to
>the optical axis, pulling part of the edge forward-backward:
>http://w1.411.telia.com/~u41105032/sling/sling.htm
>So I believe the bearings at the edge should roll in the direction
>towards/away from the mirror front.
>
>Perhaps a (moderately but sufficiently heavy) sling of steel wire (easier
>than a chain), strung with small ball bearings at suitable distances (with
>spacers in between) would do the trick - suspended to touch the mirror at
>the level of its COG.
>
>Nils Olof
>
>
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-- 
Scott Rychnovsky
srychnov@chem.ps.uci.edu
ATM Page: http://tlepage.home.mindspring.com/
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