Hobby-Eberly Telescope primary mirror alignment metrology system upgrade

The primary mirror (M1) of the Hobby Eberly Telescope (HET) consists of 91 mirror segments in a regular hexagon shape. The M1’s center of curvature can be accessed from the top of a 90ft-tall tower just outside the HET dome. The current alignment metrology system in this tower, called the Center of Curvature Alignment System (CCAS), keeps the 91 segments into a single smooth 11-meter diameter surface. This critical alignment function is at risk due to the obsolescence and deficiencies of the aging system. There are many reasons to upgrade the CCAS. 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) technique 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.