Temperature compensated lens mount
BACKGROUND AND SUMMARY 
     The difference in the temperature coefficients of expansion between a lens and a frame, called a cell herein, to which it is mounted, can cause stresses resulting in distortion of the lens element. This is especially significant in optical elements such as mirrors which are rigidly mounted to a cell since the cell normally has a greater temperature coefficient of expansion than the mirror and as temperatures increase the cell will expand faster than the mirror. This will effectively stretch the mirror as a &#34;diaphragm&#34; and reduce the curvature of the mirror, degrading it&#39;s optical properties. The difference in the temperature coefficients of expansion between a lens and a cell to which it is mounted can also cause stresses parallel with the contacting surfaces which can cause fractures in noncompliant materials such as lens glass. In optical systems which operate in non-controlled temperature environments, such as surveying instruments, a common method to avoid these undesirable stresses has been to use a pinching technique upon the optical surfaces of the element to hold it in place leaving the end of the optical element free to move. However this method does not rigidly maintain the lens element in a fixed position. A second method especially used in large telescopes has been to elastically mount the element and then readjust the position of the element as required. Again, this relieves stresses upon the optical element but does not rigidly align the element. The present invention allows for the rigid mounting of a lens within a cell to achieve a precise alignment while also eliminating stress between the lens and the cell caused by different temperature coefficients of expansion between the optical element and the cell. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a side view of the present invention used to mount a glass lens to a stainless steel cell. 
     FIG. 2 is a top view of two types of the sandwich of FIG. 1. 
     FIG. 3 illustrates the use of a flat spacer in the invention. 
     FIG. 4 illustrates the use of multiple sandwiches to suspend a lens within a cell. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, a glass lens 10 having a radius equal to OA, is to be mounted within a stainless steel cell 20 having an inner-radius equal to OB. In the preferred embodiment of the present invention the cell is made of stainless steel which has a temperature coefficient of t c1  = 5.5 × 10 -6  inches per inch degree Fahrenheit. The glass lens 10 has a temperature coefficient of approximately t c2  = 3.94 × 10 -6  inches per inch Fahrenheit. Therefore the distance d illustrated in FIG. 1 between the cell and the lens will increase by: 
     
         Δd = OB*t.sub.c1 - OB*t.sub.c2 
    
     for a one degree increase in temperature. The present invention inserts a sandwich 30 between the cell 20 and the lens 10. The sandwich 30 has a net temperature coefficient and a width d such that the expansion of the sandwich 30 due to a temperature change will equal the change in the separation between the cell 20 and the lens 10 due to the temperature change. In the present embodiment the sandwich is made of two layers of epoxy 40 and 60 and an aluminum spacer 50. The epoxy has a temperature coefficient of approximately 81.1 × 10 -6  per degree Fahrenheit at 75°. The aluminum spacer has a temperature coefficient of approximately 13.2 × 10 -6  per degree Fahrenheit. The dimensions are chosen such that: 
     
         OB*t.sub.c1 - OA*t.sub.c2 = t.sub.c3 *[total epoxy thickness] + t.sub.c4 *[aluminum thickness]. 
    
     The use of epoxy in the sandwich 30 allows a rigid bond between the cell and the lens simultaneously with a method for stress free mounting the lens with varying temperatures. The lens can be precisely positioned while the epoxy is in its fluid state, and after the epoxy has hardened the lens is rigidly mounted such that the lens maintains its alignment in varying temperatures and is not subjected to radial stresses. 
     Stresses tangential to the radius of the lens also arise between the cell and the lens when two materials having dissimilar temperature coefficients of expansion are bonded together and subsequently subjected to temperature variations. Some materials, such as the aluminum spacer 50 and the stainless steel cell 20 can tolerate stresses along the bond lines due to their ductile nature. However, excessive tangential stresses applied to a non-compliant and brittle material such as glass lens 10 will cause the glass to fracture along the bond lines causing a partial or complete loss of attachment between the lens 10 and the cell 20. Additionally, the optical characteristics of the glass lens proximate to the fracture are destroyed. I call this phenomena &#34;pullout.&#34; 
     To minimize the tangential stresses applied to the glass lens 10, I have intentionally increased the thickness of epoxy layer 60, proximate to the glass lens. The epoxy is highly compliant and effectively cushions the glass lens 10 from the tangential stresses caused by different temperature coefficients of expansion between the aluminum spacer 50 and the glass lens 10 as temperature varies. Another factor incorporated in the preferred embodiment to minimize tangential stresses is the use of an easily produced flat round aluminum spacer 50 as illustrated in FIGS. 2 and 3. The use of a flat spacer further increases the thickness of cushioning epoxy layer 60 proximate to the glass lens 10 along the periphery of the sandwich 30, where the tangential stresses are maximum. The use of a round spacer minimizes the bond line lengths required and minimizes areas of stress concentration, further reducing the tangential stresses and the possibility of pullout. 
     The present invention uses 6 sandwiches spaced equally about the glass lens attaching it to a cell as shown in FIG. 4. However, it would be obvious to use any number of mounts greater than two to secure a glass lens to a cell.