The primary mirror (M1) of the Hobby Eberly Telescope (HET) consists of 91 mirror segments in a regular hexagonal shape. The M1’s center of curvature can be accessed from the top of a 90ft-tall tower, located just outside the HET dome. The current alignment metrology system in this tower, called the Center of Curvature Alignment System (CCAS), provides alignment metrology necessary to bring the 91 segments into a single smooth 11-meter diameter spherical surface. This critical function is at risk due to the obsolescence and deficiencies of the aging system. The upgrade includes a suite of advanced metrology instruments to provide improved alignment precision and metrology. In particular, the upgrade features a new dispersed fringe sensing (DFS) device capable of measuring the piston error of all 240 segment edges across the HET’s 11m primary mirror in a single-shot exposure based on wavelength multiplexing, arrayed diffractive components, and spatial filters. This paper summarizes the electro-opto-mechanical design of the system and other design features to be implemented in the upgrade.
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