Optical instruments for space missions work in hostile environment, it's thus necessary to accurately study the effects of
ambient parameters variations on the equipment performance.
In particular, optical instruments are very sensitive to ambient conditions, especially temperature. This variable can
cause dilatation and misalignment of the optical elements, and can also lead to rise of dangerous stresses in the optics.
Optical elements displacements and surface deformations degrade the quality of the sampled images.
In this work a method for simulating and studying the effects of the thermal deformations, particularly the impact on the
expected optical performance, is presented.
Optical elements and their mountings are modelled and processed by a thermo-mechanical Finite Element Model (FEM)
analysis, reproducing expected operative conditions. The FEM output is elaborated into a MATLAB optimisation code; a
non-linear least square algorithm is used to determine the equation of the best fitting nth degree polynomial, or the
spherical surface of the deformed lenses and mirrors; model accuracy is 10-8 m.
The obtained mathematical surface representations are then directly imported into ZEMAX raytracing software for
sequential raytrace analysis. The results are spot diagrams, chief ray coordinates on the detector, MTF curves and
Diffraction Encircled Energy variations due to simulated thermal loads.
This analysis helps to design and compare different optical housing systems for finding a feasible mounting solution.
The described method has been applied successfully to the optics and mountings of a stereo-camera for the
BepiColombo mission. Different types of lenses and prisms constraints have been designed and analysed. The results
show the preferable use of kinematic constraints, instead of using glue, to correctly maintain the instrument focus in orbit
around Mercury considering an operative temperature range between -20°C and +30°C.