The UH AO System
The UH prototype 13-actuator AO system
The 13-actuator AO system. Click on this image to get a larger one as well as an explanation of the AO system components
The 13 elements wavefront curvature sensor
The wavefront curvature sensor consists of 3 main parts,
the flux detector ,
membrane modulator, and the
wavefront sensor steering mirror .
The light detector for the wavefront sensor uses 13 fiber-fed Avalanche
Photo-Diode (APD) modules, each of which receives light from a section
of the telescope pupil.
The pupil sections are shown above, the dotted line representing the
edge of the Telescope pupil.
Light is directed to the APD's using a lenslet array placed approximately
in the pupil plane. The lenslet array is constructed from 13 individual lenses
which have been sectioned and glued together. Each of the 13 fibers
is placed in the image plane of its corresponding lens.
The membrane modulator consists of a small nitro-cellulose pellicle
mirror placed in an image plane of a f/70 beam.
The membrane is attached to the end of a small acoustic cavity, which
when driven at resonance by a "tweater", causes the membrane to rapidly
switch between positive and negative curvatures. We usually chose to
run the membrane at around 2 kHz, at which speed a minimum radius of
curvature of around 25cm can be achieved.
When this membrane is flat a relay lens forms a pupil image upon the
flux detector lenslet array.
On activating the membrane the pupil image on the lenslet array is sequentially defocussed in opposite directions at a 2 kHz rate (suppress towards the image plane).
Synchronous demodulation of the resulting signal gives the wavefront
curvature for the central 7 detectors, and the wavefront radial slope
for the peripheral 6 detectors.
The main image transfer in the AO system takes place in an f/31->f/36
beam. However the wavefront sensor requires an f/70 beam in order
that sufficient signal modulation may be achieved. We convert to
a f/70 beam for the WFS, using a simple 2 lens transfer which
incidentally produces an intermediate pupil image. By placing a
tip-tilt mirror at this pupil image, we are able to move the
WFS with respect to the imaging cameras. This is extremely
useful to allow the use of off-axis guide stars, and/or dithered
imaging techniques.
At the CFHT F/36 focus, the guide star can be moved approximately 30 arc seconds away from the field center, that is about the isoplanatic distance imposed by the atmosphere.
A 13 element bimorph mirror.
The deformable mirror is the active element of the adaptive optics system.
It consists of a 13 electrode bimorph constructed by Laserdot
of France. The bimorph electrodes are arranged approximately as shown
in the above diagram.
In our present system the mirror is placed in a plane conjugate to
the telescope secondary.
The dotted line in the above diagram represents the placement of the
edge of the resulting pupil image.
The seven inner electrodes correct wavefront curvature terms, the
outer six electrodes are designed to impose mainly radial tilt within
the pupil area.
The bimorph mirror itself is constructed of two thin plates of piezo
electric material.
The above electrode pattern is deposited upon each of the plates,
which are then glued together, forming a sandwich of the electrodes.
Connections to these electrodes are then brought out to the rear piezo plate.
Ground planes are applied to both outer sides of the mirror, and a optical
surface constructed on the front plate.
The polarisation of the piezo-electric plate is chosen such that when voltage
is applied to an electrode, one of the plates expands, and the
other contracts. This differential expansion causes the bimorph to bend,
much in the same way as a bi-metallic strip will bend when heated.
These mirrors are relatively simple to construct, and have proved
so far to be highly reliable. You may be familiar with bimorphs
as the active devices in tweater loudspeakers, or the beepers
in modern digital watches. Other non optical uses include ocean
transponders, and some large stroke piezo-electric positioning
devices.
Olivier Guyon guyon@ifa.hawaii.edu