The Hobby-Eberly Telescope Mirror Alignment Recovery System Marsha Wolf Graduate Student UT Astronomy Department

The Primary Mirror Telescopes Segmented Primary Entire mirror  11 meters (36 ft)  9 meters (30 ft) used during observations 91 hexagonal segments  1 meter (3.3 ft) across All segments must be aligned to act as a single mirror Alignment is done from the Center of Curvature Alignment Sensor (CCAS) tower CCAS Tower

Burst Stacking Expanding beam sent down to primary from its Center of Curvature Each segment returns its own spot Spots are “burst out” Computer calculates positions of spots, sends them to the center “stacks” them Worked, but not precisely return spots perfectly stacked not stacked spectrograph slit laser projector HET primary mirror in CCAS tower

The Mirror Alignment Recovery System (MARS) Proof-of-Concept instrument installed Summer 2001 Now used for mirror alignment

The Shack-Hartmann Technique Incoming wavefront is sampled by an array of lenses Each lens focuses its portion of the beam Focused spot from a flat wavefront is at the center of its lens Focused spot from a tilted wavefront is translated The amount a spot is translated tells how much its portion of the wavefront was tilted On HET, each lens is one mirror segment evenly spaced unevenly spaced

Shack-Hartmann on HET (MARS in the tower) TO HET shutters reference mirror (perfect sphere) light source beamsplitter cube pinhole knife edge collimating lens WaveScope lenslet array CCD camera fiber optic cable

MARS in Action Reference pupil image HET pupil image Reference spot image HET spot image

Maintaining the Alignment Important for long science observations & efficient use of telescope time Segment Alignment Maintenance System (SAMS) Sensors on edges of all mirror segments More overlap of sensors, means more signal Control system takes sensor inputs and calculates required mirror motions to keep the entire primary aligned as a spherical surface Significantly decreased frequency of alignment during science operations segment 1 segment 2

MARS Performance Stack sizes measured at tower continue to decrease On-sky image quality has largely been limited by “seeing” Final MARS will incorporate improvements and be complete this Summer