Improved performance of the laser guide star adaptive optics system at Lick Observatory Page: 4 of 8
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2. SYSTEM OVERVIEW
2.1. Adaptive optics system
The Lick adaptive optics system3 is mounted at f/17 Cassegrain focus of 3-meter Shane Telescope. The system uses a
deformable mirror built by LLNL with 127 actuators arranged in a triangular pattern. The central 61 actuators are actively
controlled. A separate flat fast steering mirror built by Physik Instrumente is used to control overall image motion. The
wavefront sensor is a Shack-Hartmann design with 40 active sub-apertures (44-cm diameter mapped to the telescope
primary) arranged in a square pattern. The wavefront sensor camera built by camera Adaptive Optics Associates has a frame
rate of up to 1.2 kHz, and uses a 64x64 CCD built by Lincoln Labs with 7 e read noise. A separate quad-cell tip-tilt sensor
built by LLNL using photon-counting avalanche photo-diodes built by EG&G can be used for image motion control with a
natural star when the wavefront sensing is done with the laser guide star. The control computer is a 160-Mflop model built
by Mercury with 4 Intel i860 processors.
In September 1996, the Lick AO system had over 300 nm rms of residual error due to instrumental calibration. These errors
were dominated by aberrations due to alignment of the infra-red camera. New calibration techniques have improved control
of instrumental errors in Lick adaptive optics system. Two high-accuracy in-situ calibration techniques have been
demonstrated: phase diverse phase retrieval with an accuracy of -20 nm rms, and phase shifting diffraction interferometry5
with an accuracy of -10 nm rms. Using these techniques, the instrumental errors have been reduced to the 100 nm rms level.
The remaining instrumental error sources include deformable mirror surface quality (-50 nm rms), non-common path air
turbulence (-50 nm rms), and non-common path flexure (-70 nm rms). The instrumental error can be further reduced by
additional improvement of the system opto-mechanical stability, automation of the calibration procedures to provide periodic
compensation of non-common path flexure during operation, reduction of the non-common path length, improved control of
air flow in the system, and installation of a new deformable mirror with better surface quality. Several of these issues are
being addressed in 1999 with an improved opto-mechanical design for the Lick adaptive optics system.6
2.2. Laser System
The Lick laser system is based on a pulsed dye laser capable of up to 20 W average power output that is tuned to the 589 nm
sodium D2 resonance line. A set of four flash-lamp pumped frequency-doubled solid-state (Nd:YAG) pump lasers are
located in a room below the dome floor, and fiber-optics relay the pump light to the dye laser mounted on the telescope.
Beam control is achieved with active pointing and centering loops. A wavefront sensor and a far-field camera are used to
assess beam quality. A fast up-link tip-tilt control system is used to stabilize the image of the laser guide star in the adaptive
optics wavefront sensor. The beam is launched from a 30 cm refractive telescope.
The Lick laser system is typically operated with an output power of 15 W (nearly always between 13 to 18 W) at 589 nm.
The return signal varies from month to month, ranging from -9.5 to -7.0 equivalent V magnitude. The maximum and
minimum values have been observed in multiple months during the past 4 years of operation. Variation in the laser output
power and wavelength tuning are likely to account for less than a factor of 2 the observed variation in return signal.
Therefore, the sodium density appears to vary by a factor of -5.
The performance of the adaptive optics system is strongly affected by the size of guide star. Theoretically, the error in the
measurement of the wavefront scales linearly with angular size of the reference source as seen by the wavefront sensor
subapertures. The scaling of error with spot size can become more severe due to physical limits in the AO system such as the
field of view of the wavefront sensor subapertures. If a significant fraction of light from the guide star begins to fall outside
of the field of view, the wavefront measurement error rises rapidly.
In 1998, the laser guide star spot size was improved by changing the dye amplifier configuration so that the beam bounces as
it passes through the dye cell.8 This configuration was previously tested in a laser system built by LLNL for the Keck
Observatory.9 The new configuration improved the 80% enclosed-energy diameter from 6.6 arc seconds in September 1997
to 4.2 arc seconds in November 1998. The current spot size is apparently limited by aberrations introduced in the alignment
of the launch telescope. For this system, a beam that is 1.5 times diffraction-limited would have an 80% enclosed energy
diameter less than 3 arc seconds in good (0.75 arc second) seeing. This should be achievable with better launch telescope
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An, J R; Avicola, K; Bauman, B J; Brase, J M; Campbell, E W; Carrano, C et al. Improved performance of the laser guide star adaptive optics system at Lick Observatory, article, July 20, 1999; California. (digital.library.unt.edu/ark:/67531/metadc624162/m1/4/: accessed June 24, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.