Design and performance analysis of adaptive optical telescopes using laser guide stars

Article 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 (similar to)1 s 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 only to observe 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 sodium layer. The design requirements of a laser-guided adaptive telescope, as well as the expected imaging performance, are discussed in detail in this paper. We show that a 2-m ground-based laser-guided telescope can achieve imaging performance levels at visible wavelengths nearly matching the theoretical imaging performance of the Hubble Space Telescope (HST). The required lasers can be either bought off the shelf or built with today's technology. The laser power requirement for the Rayleigh guide star approach is on the order of 82 W for zenith viewing when the atmospheric seeing cell diameter is 20 cm. For the same conditions the laser power requirement for the Na guide star approach is on the order of 14 W. Both systems will achieve near diffraction limited imaging with a Strehl ratio of (similar to)0.67 and an angular resolution of approximately 0.07 arc-sec for an observation wavelength of 0.5um. (Reprinted by permission of the publisher.)

Design, Optics, Performance Measurement, Telescopes

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Differential absorption lidar sensing of ozone

Article Abstract:

The Differential Absorption Lidar (DIAL) technique is highly accurate in measuring ozone conditions in the atmosphere. NASA's airborne DIAL system, developed at Langley Research Center, was used in four recent experiments: the 1980 EPA Persistent Elevated Pollution Episode (PEPE) Field Experiment, the 1981 Ozone Cloud-Pumping Case Study, the 1984 Tropopause Fold Field Experiment, and the 1985 and 1987 NASA Global Tropospheric Experiment-Amazon Boundary Layer Experiment. These experiments, conducted over differing areas under different seasonal conditions, indicate that the removal of ozone from the lower atmosphere is more efficient during the wet season and that ozone profiles vary according to region.

Chemistry

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Sodium resonance flourescence lidar applications in atmospheric science and astronomy

Article Abstract:

Tunable-dye lasers and lidar measurements are widely used to study atmospheric waves in the sodium layer of the mesosphere. Today's most common lidar configuration is a monostatic resonance fluorescence system. Measurement of the vertical structures of mesospheric waves and tides is a key application of this technology. Lidar systems are also being adapted for use in optical telescopes with a laser technique that creates artificial guide stars. Portability and ease of operation make lidar systems attractive for astronomical and atmospheric research.

Astronomy, Study

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