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The Mirror Alignment Recovery System (MARS) at the Hobby-Eberly Telescope helps align 91 mirror segments to act as one, improving image quality for scientific observations. Using the Shack-Hartmann Technique, the system maintains alignment efficiently. The Segment Alignment Maintenance System (SAMS) ensures long and precise observations. With continuous improvements, MARS enhances telescope performance.
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The Hobby-Eberly Telescope Mirror Alignment Recovery System Marsha Wolf Graduate Student UT Astronomy Department
CCAS Tower The Primary Mirror-- 91 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
laser projector in CCAS tower HET primary mirror perfectly stacked not stacked spectrograph slit 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
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
fiber optic cable light source beamsplitter cube lenslet array pinhole WaveScope TO HET knife edge CCD camera shutters collimating lens reference mirror (perfect sphere) Shack-Hartmann on HET(MARS in the tower)
Reference pupil image Reference spot image HET pupil image HET spot image MARS in Action
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