Design and performance analysis of adaptive optical telescopes using laser guide stars
Welsh, Byron MacMaster
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https://hdl.handle.net/2142/23503
Description
Title
Design and performance analysis of adaptive optical telescopes using laser guide stars
Author(s)
Welsh, Byron MacMaster
Issue Date
1989
Doctoral Committee Chair(s)
Gardner, Chester S.
Department of Study
Engineering, Electronics and Electrical
Physics, Astronomy and Astrophysics
Discipline
Engineering, Electronics and Electrical
Physics, Astronomy and Astrophysics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Electronics and Electrical
Physics, Astronomy and Astrophysics
Language
eng
Abstract
Atmospheric turbulence severely limits the resolution of ground-based astronomical telescopes. In good seeing conditions at the best observatory sites, resolution at visible wavelengths is typically limited to $\sim$1 sec of arc. During the past 15 years adaptive optical systems using electrically deformable mirrors have been developed to compensate for turbulence effects. Unfortunately, these systems require bright reference sources adjacent to the object of interest and can be used to observe only the brightest stars. Artificial guide stars suitable for controlling an adaptive imaging system can be created in the upper atmosphere by using a laser to excite either Rayleigh backscattering in the stratosphere or resonance backscattering in the mesospheric Na layer. The design requirements of a laser-guided adaptive telescope, as well as the expected imaging performance, are discussed in detail in this thesis.
Analytical expressions giving the performance of a class of adaptive optics systems using slope sensors are derived. The unique analysis takes into account the nonideal characteristics of the wavefront sensor and wavefront correction device, as well as the effects of anisoplanatism. Performance measures include the mean-square residual phase error across the aperture of the optical system and the optical transfer function. We show that a two-meter, ground-based, laser-guided telescope can achieve imaging performance levels at visible wavelengths nearly matching those of the Hubble Space Telescope (HST). The laser power requirement for Rayleigh and Na guide stars is on the order of 33 W and 6 W, respectively, for zenith viewing and r$\sp{\rm o}$ = 20 cm. Both systems will achieve near diffraction limited imaging with a Strehl ratio of $\sim$ 0.73 and an angular resolution of approximately 0.07 arcsec for an observation wavelength of 0.5 $\mu$m.
In the case of guide stars created in the mesospheric Na layer, saturation effects may significantly reduce the backscattered signal expected for resonance fluorescence lidar systems. The level of saturation is determined by the laser's pulse length, pulse energy, beamwidth and linewidth. Design examples, including lidar systems for atmospheric research and laser-guided telescopes, are studied in detail.
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