One of the early motivations of space science was the opportunity for astronomy to use hitherto inaccessible wavelengths, from gamma–and X–rays to infrared, and visible–light astronomy, from orbiting telescopes, was allowed better seeing, free of atmospheric contamination. Meteorological observations and earth remote sensing required orbiting cameras and infrared radiometers. All needed applications of optics.
It will be clear from foregoing chapters that, for space instrumentation, optics should (i) have qualities of rugged mechanical design, (ii) be built of lightweight non–contaminating materials, and (iii) survive years of unattended use in orbit. This chapter offers an introductory account of materials and opto–mechanical design techniques which have been serving these ends. Optical design as such, and physics of sensors, are beyond our scope. The steady improvement of sensor and detector systems, often of great sensitivity, has fostered parallel development of computation and suppression of stray light. As in all space endeavours, pre–launch qualification testing should be carefully and thoroughly conducted. Operation and adjustment in space requires mechanisms whose life may be limited, but in-orbit repair or replacement is either impossible or very costly; hence trade–off decisions may be difficult to make.
Materials for optics
As remarked in paragraph 2.7.12, the concern with glasses and ceramics is their brittleness while exposed to the launch environment. The chart (Fig. 2.31) of fracture toughness versus strength for the diversity of materials shows optical glasses, as a class, to have high strength but low toughness.