ChAOS Sodium Laser Guide Star Experiments

Why use a laser guide star?

In order for a high-order adaptive optics system to perform up to its potential, a key requirement is having a bright star in the adaptive optics system's field of view. This allows the wavefront sensor to sample the atmospheric distortions with enough accuracy to make good corrections. However, since the odds of bright stars naturally occuring very close to interesting objects are very small, a crucial technique is to generate an artificial star for the AO system's wavefront sensor.

The basic requirements for the artificial star are that it be bright for a better signal-to-noise ratio in the wavefront sensor and small in size for better accuracy in determining the wavefront distortions. Furthermore, to better sample the atmospheric turbulence, the artificial star should be put as high up in the atmosphere as possible. All of these requirements can be met by using a laser with its wavelength tuned to the sodium D2 line. With such a laser, a small, bright, artificial "sodium star" can be generated in the mesospheric sodium layer which lies at about 90 kilometers above sea level.

Since April of 1997, our group has conducted a series of experiments on laser guide stars (LGS) using the Vacuum Tower Telescope (VTT) of the National Solar Observatory at Sacramento Peak, New Mexico. During the experiments, the VTT was used to send the laser beam up to the sodium layer and to simultaneously observe the returning light from the artificial star. Artificial stars as bright as about 9th magnitude (visual) and as small as about the size of nearby natural stars have been generated with about 5 watts of sodium laser output. Various factors which affect the quality of the artificial star have been explored.

This photo shows the laser guide star experiment at the National Solar Observatory's Vacuum Tower Telescope (VTT). The photo was taken during the night of Nov 20, 1997. The telescope was lit up by moonlight and the laser beam was launched from the top of the VTT. The constellations Orion and Taurus are also visible in this photo. The exposure time was about one minute using ISO 1000 film.

Sodium laser guide star and the star Eta UMa in the V band. Both images were taken the night of Jan 17, 1998. The LGS is shown on the left. The LGS brightness is V=9.5 mag. Eta UMa is on the right. The two pictures were normalized by their peak brightness and put side-by-side to compare the spot sizes.

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Last modified 30 Mar 1998