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Preliminary Design Study of the Beam Collapser and Certain Optomechanical Aspects of the Siderostat.

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Four topics were studied during this phase of the design of the USNO astrometric interferometer 1 the optical configuration of the beam collapser 2 the mirror mount for the primary mirror of the beam collapser 3 the beam collapser structure, and 4 bearings for use in the siderostat. A Gregorian beam collapser configuration, with a 750-mm diameter f2.7 primary mirror, a 1 16-mm diameter secondary mirror, with a radius of curvature of 623 mm, and a vertex-to-vertex spacing 2337 mm is recommended. Use of a Gregorian beam collapser reduces optical fabrication costs, with a small impact in overall system length and weight. A solid primary mirror, 100-mm thick and oversized, with a 85 mm diameter, is the simplest and most economical mirror to fabricate and mount. A roller chain would provide radial support for the primary mirror a six-point whiffle tree located outside the optical clear aperture would provide axial support. A modified Serrurier truss connects the secondary mirror to the primary mirror the upper forward section of the truss is removed to allow better sky coverage for the siderostat. Use of a modified Serrurier truss permits testing of the system in the optical axis horizontal position, and without loss of alignment in the 100 down position. In addition, this type of truss is center supported, permitting the use of a tapered pier to minimize concrete volume on site. Spacing from the primary mirror to the secondary mirror, a critical parameter for system performance, is controlled by a pair of metering rods made from the same type of material as the optics. Rolling element bearings, oil pad bearings, and air bearings were considered for use in the siderostat.

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  • Optics

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