Laser Guide Star for AO Introduction
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Laser guide stars (LGS) projectors have now been deployed on a number of 4 m-class telescopes and are basically part of every 8-10 m-class project. This includes the VLT as well, with the current UT4 (YEPUN) sodium LGS development. The rationale is the much higher sky coverage offered in principle by an LGS - as opposed to the standard Natural Guide Star (NGS)- approach, as can be seen from the set of two curves below. Due to the bright (m~10) artificial star created near the center of the field, cumulative sky coverage, or the probability to achieve a given minimum Adaptive Optics (AO) correction on an arbitrary astronomical target, goes e.g. up to 65% for corrected images with at least an 0.25 K-band Strehl ratio*.

A (possibly fainter/farther from the center of the field) NGS is still needed to correct stellar agitation (tip/tilt), since it cannot be sensed from an LGS; this is why sky coverage is not 100% and goes down for better corrections. Note also that, contrary to the case of NGS-only AO, LGS-based corrections saturate at a K-band Strehl of ~ 0.55 only: this is due to the relatively small altitude (~90 km) of the sodium layer and hence of the artificial star (the so-called cone effect).

Cumulative sky coverage as a function of K-band Strehl ratio (from analytical computations):
- mean Paranal atmosphere parameters (0.66" FWHM seeing & 6 msec. correlation time; 0.4 Strehl anisoplanetism contribution at 20" elongation; all these values at a wavelength of 0.50 micron)
- Sodium LGS (m~10; 1" FWHM spot)
- AO system (80 corrected modes)
Note that these curves would change dramatically for different atmospheric parameters and for different wavelengths.

Both NAOS /CONICA and SINFONI are developed to work both in the NGS only (for better correction when a bright nearby star is available) and the combined LGS/NGS (for extended sky coverage) modes

* defined as the ratio between the peak intensity of the corrected image to that of a pefectly corrected (purely diffraction-limited) one.

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Send comments to <mkissler@eso.org> Last update: March 14, 2001