Optomechanical analysis of compliant spacers in passively athermalized drop-in optical systems

Passively athermalized optical systems produce high quality images over a large thermal range without actively adjusting focus. This athermalization is achieved through careful selection of the glass for each lens and metal for each mount. For drop-in systems, the material combination for best optical performance often leads to a lens stack with an overall coefficient of thermal expansion (CTE) that is different from the CTE of the barrel that holds it together. Since bulk glass and metal are relatively stiff, this CTE mismatch results in large variations of the preload force retaining the lens stack in compression over the optical system's survival thermal range. For this reason, compliant spacers are commonly added to the lens stack in an effort to attenuate these preload force variations. However, the effect of these compliant spacers on the athermalization of optical systems is seldom analyzed. We perform a first-order calculation of the effective CTE of compliant spacers to assess their impact on optical performance and introduce an optomechanical design approach to reduce the amount of compliance needed by matching the overall CTE of the lens stack to the CTE of the barrel.