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.
In particular optical instruments are very sensitive to ambient conditions, especially temperature. This variable can cause dilatations and misalignments of the optical elements, and can also lead to rise of dangerous stresses in the optics. Their displacements and the deformations degrade the quality of the sampled images.
In this work a method for studying the effects of the temperature variations on the performance of imaging instrument is presented. The optics and their mountings are modeled and processed by a thermo-mechanical Finite Element Model (FEM) analysis, then the output data, which describe the deformations of the optical element surfaces, are elaborated using an ad hoc MATLAB routine: a non-linear least square optimization algorithm is adopted to determine the surface equations (plane, spherical, nth polynomial) which best fit the data. The obtained mathematical surface representations are 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 Stereo Camera for the BepiColombo mission reproducing expected operative conditions.
The results help to design and compare different optical housing systems for a feasible solution and show that it is preferable to use kinematic constraints on prisms and lenses to minimize the variation of the optical performance of the Stereo Camera.
In this paper the results of the thermo-elastic analysis performed on the Stereo Imaging Channel of the imaging system SIMBIO-SYS for the BepiColombo ESA mission to Mercury is presented. The aim of the work is to determine the expected stereo reconstruction accuracy of the surface of the planet Mercury, i.e. the target of BepiColombo mission, due to the effects of the optics misalignments and deformations induced by temperature changes during the mission lifetime. The camera optics and their mountings are modeled and processed by a thermo-mechanical Finite Element Model (FEM)
program, which reproduces the expected optics and structure thermo-elastic variations in the instrument foreseen operative temperature range, i.e. between -20 °C and 30 °C. The FEM outputs are elaborated using a MATLAB optimization routine: a non-linear least square algorithm is adopted to determine the surface equation (plane, spherical, nth polynomial) which best fits the deformed optical surfaces. The obtained surfaces are then directly imported into ZEMAX raytracing code for sequential raytrace analysis.
Variations of the optical center position, boresight direction, focal length and distortion are then computed together with
the corresponding image shift on the detector.
The overall analysis shows the preferable use of kinematic constraints, instead of glue classical solution, for optical
element mountings, this minimize the uncertainty on the Mercury Digital Terrain Model (DTM) reconstructed via a stereo-vision algorithm based on the triangulation from two optical channels.
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.
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