The optical components considered in this chapter do not form images. They are intended either to act as a transparent barrier between the outside environment and the interior of the instrument or, in the case of a filter, to modify the spectral characteristics of the transmitted (or reflected) beam. Typically they have the form of plane parallel plates or meniscus-shaped elements (shells and domes). Special cases are conformal ones, i.e., ones whose contours approximate that of the surrounding skin or structural envelope. Candidate materials for all these optics include optical glasses, fused silica, optical crystals, and plastics. With the exception of filters, they have as their prime purposes the exclusion of dirt, moisture, and other contaminants and/or supporting a pressure differential between interior and exterior atmospheres. Critical aspects of mountings for such components include mechanically or thermally induced surface distortions and stresses, as well as sealing. Since most filters are plane parallel plates, their mountings are usually the same as those for flat windows. The location within the optical system of a window or a filter is important because the tolerances on defects such as surface deformation, wavefront tilt, and homogeneity of the refractive index are more stringent near pupils than near images. Tolerances on component cleanliness, material inclusions, and surface blemishes (scratches and digs) are tighter near images than near pupils. In this chapter, we discuss typical mountings for various configurations of the optics of interest.
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