The Hobby-Eberly The Hobby-Eberly Telescope Mirror Telescope Mirror Alignment Recovery Alignment Recovery SystemSystem

Marsha WolfMarsha WolfGraduate StudentUT Astronomy Department

The Primary MirrorThe Primary Mirror-- 91 Telescopes-- 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 mirrorAlignment is done from the Center of Curvature Alignment Sensor (CCAS) tower

CCAS Tower

Burst StackingBurst Stacking Expanding beam sent down to primary from its Center of CurvatureEach segment returns its own spotSpots 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 The Mirror Alignment Recovery System (MARS)Recovery System (MARS)

Proof-of-Concept instrument installed Summer 2001Now used for mirror alignment

The Shack-Hartmann The Shack-Hartmann TechniqueTechniqueIncoming wavefront is sampled by an array of lensesEach 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 Shack-Hartmann on HETHET(MARS in the tower)(MARS in the tower)

TO HET

shutters

reference mirror (perfect sphere)

light source beamsplitter

cubepinhole

knife edgecollimating

lens

WaveScope

lenslet array

CCD camera

fiber optic cable

MARS in ActionMARS in ActionReference pupil image

HET pupil image

Reference spot image

HET spot image

Maintaining the Maintaining the AlignmentAlignment

Important for long science observations & efficient use of telescope timeSegment Alignment Maintenance System (SAMS)Sensors on edges of all mirror segmentsMore overlap of sensors, means more signalControl system takes sensor inputs and calculates required mirror motions to keep the entire primary aligned as a spherical surfaceSignificantly decreased frequency of alignment during science operations

segment 1

segment 2

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