The UH AO System
The new UH 36-actuator AO system
The University-of-Hawaii "Hokupa'a" adaptive optics
system. Photograph by Richard Wainscoat.
Jpeg, 57 Ko
The system is mounted on a sturdy custom-made optical bench. A
Serrurier-type structure connects the bench to the circular flange seen on
top of the figure. The flange can be bolted either to the UH 88" telescope
or to the CFH telescope. The rectangular box sitting on top of the flange
is normally attached under the bench. It contains the 36 fiber-fed
avalanche photodiodes which sense the wave front. On the left, one can see
part of the large parabolic mirror used to reimage the telescope aperture
onto the deformable mirror. After two symmetrical off-axis reflections on
this mirror, a compensated image forms on the science cameras. A 3-position
beam-splitter wheel (seen in front near the center) is used to select the
beam splitter that will transmit the light to the wave-front sensor. Buzz
Graves (on the right) managed the project. He designed and built the
optical and mechanical parts. The light path is illuminated here by a He/Ne
laser normally used for alignment and calibration purpose.
The 36 element wavefront sensor lenslet array
Jpeg, 16 Ko
Schematic diagram of the Hokupa'a system. I-DEAS image by J.Elon Graves .
Jpeg, 36 Ko
The large orange ring on the upper right part of this figure represents the
Cassegrain flange with which the instrument is attached to the telescope.
Light from the telescope comes from the top through the ring. Near the
focal plane a flat mirror (used for pupil alignment) reflects light to the
left toward a large parabolic mirror (here in pink). A first off-axis
reflection on this mirror produces an image of the telescope entrance
aperture onto a custom-made deformable mirror, called "curvature mirror"
because voltages applied to this mirror change its local curvature. A
second off-axis reflection on the parabolic mirror produces a compensated
image near the middle of the figure. A dichroic mirror (in blue) transmits
the infrared (l>1um) and reflects the visible. The infrared (IR) beam
(colored red) is reflected downward toward the IR camera (blue cylinder).
The visible beam (colored pink) goes through a 3-position beam-splitter
wheel used to reflect part of the visible light toward a CCD camera (orange
cylinder). The beam transmitted by the beam-splitter (colored yellow) is
reflected by a flat steering mirror which forms an image of the guide
source on a thin metal-coated nitrocellulose membrane (small green cylinder
on the right). The membrane reflects light toward a concave mirror (in
green) used to focus a pupil image onto the wave-front sensor (WFS)
detector array. This pupil image can be alternately defocused back and
forth at a 2.6 kHz rate by exciting the fundamental drum-mode vibration of
the membrane.
Olivier Guyon guyon@ifa.hawaii.edu
