Re: [ATM] Resilient retention between metal cell and optical glass?

["Chris Dalla Piazza" <dalchri@hotmail.com>]

I am also interested in this.  The corrector cell for my Lurie-Houghton
supports two elements.  When I built it, I used double sided tape between
the front cell ring and the glass.  This has two purposes.  First it
prevents the lens from rotating since it supports the secondary.  Second, I
am hoping that it will squish to accommodate temperature changes.

The other lens is spaced using a poster board paper ring.  The lenses are
not supported at their edges and are clamped down just enough so that they
don't move.

So far so good.  We'll see what happens when winter gets here.

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
David Weinshenker
Sent: Monday, September 10, 2007 2:50 AM
To: atm@atmlist.net
Subject: [ATM] Resilient retention between metal cell and optical glass?

I've been thinking of mounting cell designs for glass elements such 
lenses and "small thick" mirrors which is based on O-rings to provide 
resilience in the clamping of elements in place. (This wouldn't apply 
for "large thin" mirrors needing distributed support.)

For example, consider a glass mounted in a metal cell, with a 
clearance around the element (a few thousandths of an inch perhaps), 
and axial location of the glass between a fixed rim and a threaded 
ring. If the cell is tightened exactly to zero clearance at a given 
temperature, differential expansion would of course result in either 
a pinched or a loose fit at other temperatures. To avoid placing unusual 
stress on the glass, such cells are commonly assembled with a slight 
intentional clearance to allow for thermal changes.

However, this leaves the glass a little loose - it can rattle 
around; it might tilt or shift with different orientations of 
an instrument.

But suppose we place a common industrial O-ring, of perhaps 3/16" 
cross-section, between the glass and one of the locating rims.
This should permit the cell to be tightened enough to place the 
O-ring in light compression, enough to preload the glass against 
the opposite rim (even against the full weight of the glass). The 
edge of the glass would be in axial compression, but this compression 
would be uniform around the circumference, and the actual pressure would 
be much smaller than the "thermal pinch in rigid cell" condition, 
and very nearly independent of temperature.


How well would this work? Can this sort of relatively small, 
controllable, axial clamping force be applied to optics of 
modest size without inducing weird distortions?

-dave w
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