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

["Tony Owens" <towens@shibumi-consulting.net>]

There are various solutions the professional industry uses for keeping large
glass in the right place with minimum stress - but one of the most elegant
ones has to be use of a fillet of low modulus elastomer (typically silicone
rubber) injected into a clearance all around the cylindrical edge surface of
the lens within a cylindrical metal cell.
The rubber bonds tenaciously to OD of glass and ID of lens cell.  Normally
the moulding-in-place also requires front and rear rings to be temporarily
attached to the assembly to shape the rubber filler and keep it in the
groove.
The radial thickness of the filler is carefully calculated to that its
thermal contraction on cold nights is compensated for by the difference in
contraction of the metal lens cell and the glass type used.
It is typically injected at several spots around the ring using a syringe
and a series of vent holes.  This can be done in a vacuum if a void-free
fill is required but that is unnecessary.  After cure, the axial former
rings are removed and voila!
The benefits of this method are extensive and can include simplicity, shock
damping, inbuilt thermal compensation, location of the optic to a tolerance
of microns, ability to align the lens radially prior to encapsulation by
'nudging' it using temporary radial setscrews until optical performance of
the instrument is optimal, in the knowledge that the rubber impregnation
will be stress-free and won't disturb the optic.
Those who own largish APO objectives (e.g., LZOS's TMB triplets) may be able
to see evidence of the method by carefully examining the perimeter of their
lens cells.

Tony

-----Original Message-----
From: Chris Dalla Piazza [mailto:dalchri@hotmail.com] 
Sent: Monday, September 10, 2007 1:20 PM
To: atm@atmlist.net
Subject: Re: [ATM] Resilient retention between metal cell and optical glass?

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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