Abstract: The main imaging system of the Mars Observer Camera (MOC) is a Cassegrain telescope which will return high resolution images of the surface of Mars as part of the Mars Observer mission to be launched in September 1992. The primary mirror of the telescope is 13.84 inches in diameter and fabricated from fused silica. to make it stiff but lightweight, the mirror was designed with a 'single arch' rear surface contour. After mirror design and fabrication was complete, it was determined through thermal simulation of mission conditions that the primary mirror would experience large radial temperature gradients (as well as small axial and circumferential gradients) during imaging. It was originally thought these gradients would produce uncontrollable aberrations. However, it was determined through finite element analysis that the radius of curvature (ROC) of the reflective surface of this thin mirror changes in a nearly linear fashion with the radial temperature gradient. In addition, a radial gradient only produces a ROC change, without any significant additional surface figure deformation that might cause image degradation. This effect was subsequently utilized in a primary mirror thermal control system which will allow ground control of the ROC, and hence, the focus of the MOC. The effect of radial gradient and the thermal control system were interferometrically tested in a thermal-vacuum chamber (to simulate mission conditions) and the results confirmed the analysis and the visibility of the thermal control approach.!3
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