McDonald Observatory has joined in the effort to produce a workable design for a spectroscopic survey telescope utilizing a segmented spherical primary mirror, elaborating on ideas initiated by astronomers at the Pennsylvania State University. The spherical primary figure requires that a secondary focus assembly be driven at the tracking rate in an attitude normal to the spherical focal surface while the telescope as a whole, being tilted at a predetermined angular zenith distance, need only be "set" (and clamped) occasionally in azimuth. The spherical primary mirror segments, all figured to an identical radius of curvature, are potentially simpler to manufacture than off-axis aspheric segments. The overall diameter of the primary (that is, the diameter of the smallest circle that circumscribes the entire set of segments) is approximately 10 m, enough to provide a significant aperture increase for current spec-troscopic research. The radius of curvature is 26 m. The glass segments are supported on a fully triangulated space frame of Invar and 304 stainless steels. A structural analysis using standard techniques of finite elements shows that the expected static performance of both the individual segments and the overall space frame present reasonable goals to be achieved by modern engineering design practice. The design has been implemented in a full-scale prototype consisting of a seven-segment central portion of the telescope's primary mirror structure.
F. B. Ray,
S. V. Krishnamachari,
"Spectroscopic Survey Telescope Design I: Primary Mirror Structure And Support," Optical Engineering 27(9), 279769 (1 September 1988). https://doi.org/10.1117/12.7976759