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3.1 Mechanical Collimator

  Light entering the instrument passes through a grid collimator that restricts the field of view to a 1° diameter circle (response FWHM) centered on the target star. This collimator is needed to screen out contaminating light from other stars and reduce the illumination from the diffuse Lyman-$\alpha$ geocoronal background. It follows that IMAPS is not suitable for viewing stars that have companions of comparable brightness that are anywhere between about 2 (the detector's resolution limit) and 1° away. The collimator transmits about 62% of the incident flux that passes through its approximately elliptical aperture (semi-axes of 7.4 and 9.6 cm). About 2/3 of the loss comes from mechanical blockage by the grids and the remainder is attributable to wide-angle diffraction. Precautions in the design of the collimator were taken to reduce the confusion that could arise from small angle diffraction spikes. In laboratory tests where extraordinarily strong emission lines are imaged by IMAPS, no such spikes are evident.