Metrology systems of Hobby-Eberly Telescope wide field upgrade

Abstract

The Hobby-Eberly Telescope (HET) Wide-Field Upgrade (WFU) will be equipped with new closed-loop metrology systems to actively control the optical alignment of the new four-mirror Wide-Field Corrector (WFC) as it tracks sidereal motion with respect to the fixed primary mirror. These systems include a tip/tilt camera (TTCam), distance measuring interferometers (DMI), guide probes (GP), and wavefront sensors (WFS). While the TTCam and DMIs are to monitor the mechanical alignment of the WFC, the WFSs and GPs will produce direct measurement of the optical alignment of the WFC with respect to the HET primary mirror. Together, these systems provide fully redundant alignment and pointing information for the telescope, thereby keeping the WFC in focus and suppressing alignment driven field aberrations. In addition to these closed-loop metrology systems, we will have a pupil viewing camera (PVCam) and a calibration wavefront sensor (CWFS). The PVCam will be used for occasional reflectance measurement of the HET primary mirror segments in the standard R,G,B colors. The CWFS will provide the reference wavefront signal against which the other two WFS are calibrated. We describe the current snapshot of these systems and discuss lab/on-sky performance test results of the systems.