Improved performance of the laser guide star adaptive optics system at Lick Observatory Page: 3 of 8
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Improved performance of the laser guide star adaptive optics system at
Scot S. Olivier*, Donald T. Gavel, Herbert W. Friedman, Claire E. Max, Jong R. An, Kenneth Avicola,
Brian J. Bauman, James M. Brase, Eugene W. Campbell, Carmen Carrano, Jeffrey B. Cooke, Gary J.
Freeze, Elinor L. Gatesa, Vernon K. Kanz, Thomas C. Kuklo, Bruce A. Macintosh, Michael J. Newman,
Edward L. Pierce, Kenneth E. Waltjen, James A. Watson
Lawrence Livermore National Laboratory, PO Box 808, Livermore, CA 94551
aLick Observatory, Mt. Hamilton, CA 95140
Results of experiments with the laser guide star adaptive optics system on the 3-meter Shane telescope at Lick Observatory
have demonstrated a factor of 4 performance improvement over previous results. Stellar images recorded at a wavelength of
2 tm were corrected to over 40% of the theoretical diffraction-limited peak intensity. For the previous two years, this
sodium-layer laser guide star system has corrected stellar images at this wavelength to -10% of the theoretical peak intensity
limit. After a campaign to improve the beam quality of the laser system, and to improve calibration accuracy and stability of
the adaptive optics system using new techniques for phase retrieval and phase-shifting diffraction interferometry, the system
performance has been substantially increased. The next step will be to use the Lick system for astronomical science
observations, and to demonstrate this level of performance with the new system being installed on the 10-meter Keck Il
Keywords: adaptive optics, laser guide star, sodium layer, imaging
Images of astronomical objects produced by ground-based telescopes are blurred by the Earth's atmosphere. Adaptive optics
can correct for these atmospheric aberrations and provide images with resolution limited only by diffraction from the
telescope aperture.' An adaptive optics system requires a reference source to sense the atmospheric aberrations. The
reference source can be a natural star, but requirements on the brightness and proximity of the reference source limit the
fraction of the sky accessible with natural stars to a few percent for near-infra-red imaging. Use of a laser guide star can
increase the available fraction of the sky since the laser can be directed to any desired location. With a laser guide star the
sky coverage is limited only by the need to use a natural star to control overall image motion. Image motion cannot be
controlled by the laser guide star since its position is randomly varied during propagation up through the atmosphere. Since
the requirements on the brightness and proximity of the reference source for the control of image motion are less severe than
for full correction of the atmospheric aberrations, use of a laser guide star increases the available fraction of the sky by a
factor of -10.
A laser guide star adaptive optics system has been developed for the 3-meter Shane telescope at the University of California's
Lick Observatory, located on Mt. Hamilton near San Jose, California. The laser guide star is formed by resonance
fluorescence of atomic sodium located in a layer near the top of the mesosphere at -95 km altitude. This sodium-layer laser
guide star provides better sampling of the atmospheric turbulence for large telescopes than a guide star formed by Rayleigh
scattering from air molecules at lower altitudes, 5-20 km.
The Lick Observatory laser guide star adaptive optics system first produced significant image correction in September 1996.2
In this paper, we describe the results of experiments with the Lick laser guide star adaptive optics system conducted during
* Correspondence: email: olivierl @llnl.gov; www: http://ep.llnl.gov/urp/science/lgswww/lgs.html; phone: 925-423-6483;
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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/m1/3/: accessed October 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.