Download The Mirror Alignment Recovery System for the Hobby

The Hobby-Eberly Telescope
Mirror Alignment Recovery
System
Marsha Wolf
Graduate Student
UT Astronomy Department
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
CCAS Tower
Burst Stacking
laser projector
in CCAS
tower
return
spots
HET
primary
mirror
perfectly stacked
not stacked
spectrograph slit
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
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)
light fiber optic cable
source
beamsplitter
cube
lenslet array
pinhole
WaveScope
TO
HET
knife
CCD camera
collimating edge
lens
shutters
reference mirror
(perfect sphere)
MARS in Action
Reference pupil image
Reference spot image
HET pupil 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