Effects of thermal deformation on optical instruments for space application

Abstract

Optical instruments for space missions work in host ile environment, it’s thus necessary to accurately study the effects of ambient parameters variations on the equ ipment. In particular optical instruments are very sensitiv e to ambient conditions, especially temperature. Th is variable can cause dilatations and misalignments of the opti cal elements, and can also lead to rise of dangerou s stresses in the optics. Their displacements and the deformat ions degrade the quality of the sampled images. In this work a method for studying the effects of t he temperature variations on the performance of ima ging instrument is presented. The optics and their mount ings are modeled and processed by a thermo-mechanical Finite Element Model (FEM) analysis, then the output data, which describe the deformations of the opti cal element surfaces, are elaborated using an ad hoc MATLAB routine: a non-linear least square optimizatio n algorithm is adopted to determine the surface equat ions (plane, spherical, n th polynomial) which best fit the data. The obtained mathematical surface representations a re then directly imported into ZEMAX for sequential raytracing analysis. The results are the variations of the Spot Diagrams, of the MTF curves and of the Diffraction Ensquared Energy due to simulated thermal loads. This method has been successfully applied to the St ereo Camera for the BepiColombo mission reproducing expected operative conditions. The results help to design and compare different op tical housing systems for a feasible solution and s how that it is preferable to use kinematic constraints on pr isms and lenses to minimize the variation of the op tical performance of the Stereo Camera.