Threaded mounts are one of the most common interfaces between optical systems and commercial-off-the-shelf cameras. Popular examples include the established C-mount, as well as the newer TFL-mount which accommodates for larger sensor formats such as the APS-C detector. In all cases, the thread is used to adjust for focus by clocking the optical system with respect to a fixed camera assembly or vice versa. For this reason, the alignment between the datum axis of the optical system and the array detector plane inside the camera depends on both the allowances and tolerances of the thread interface, and on the manufacturing tolerances of the mount components. To highlight how the stack up of these tolerances can affect image quality of an optical system, we first perform an inverse sensitivity analysis to determine the detector alignment specification as a function of system F/#, field of view, and chief ray angle. We then calculate the misalignment contributions of the thread between the optical system and the lock ring that sets the camera axial position for best focus. This optomechanical analysis allows us to determine if thread mounts are appropriate for the specifications of the optical system under consideration and to specify the tolerances of the thread interface when this is the case.
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.
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