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

The use of adaptive optical systems using electrically deformable mirrors to compensate for turbulence effects is discussed. Since 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 must 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. It is shown 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 current 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 approximately 0.67 and an angular resolution of approximately 0.07 arcsec for an observation wavelength of 0.5 mu m.<
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