The Hobby-Eberly Telescope (HET) is an innovative 10m-class telescope that operates with a fixed-altitude segmented spherical primary mirror (M1) while a tracker, located at the top of the telescope, moves the prime-focus spherical aberration corrector (SAC) and instrument package in order to track the sidereal and non-sidereal motions of celestial objects. In 2016, we completed the wide-field upgrade of the telescope, which includes the Wide-Field Corrector (WFC), the tracker, the prime focus instrument package, and all electromechanical hardware and telescope control software. Post the upgrade, we have been monitoring the imaging performance of the telescope through the in-situ metrology systems. These systems highlight weak spots of the telescope facility in imaging performance and provide quantitative guidance to specific facility repair and/or upgrades to further improve the HET’s overall performance. This paper discusses our long-range (7yrs) datasets, specific analysis techniques used, and current outcomes of this investigative effort in the context of specific facility improvement and upgrade plans.
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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