Guide star

The properties of the design, including its ability to rely on Laser Guide Stars (LGS), have been extensively described in the literature and will not be recalled herein (Dierickx et al., 2000 Dierickx et al., 2000 Dierickx, 2001). 

The idea is to select light beams coming from the elongated LGS in such a way that taken individually, they have the same properties as if they were coming from an infinite distance, as seen by the wavefront sensor. The concept is called PIGS for Pseudo-Infinite Guide Stars and should be tested on-sky in 2003. 

Figure 7. Strehl ratio at 2.2 fim as a function of guide star magnitude for typical system parameters (throughput = 0.4, read noise = 5 e per pixel per frame, etc.) with ro = 0.2 m and To = 5 ms at 0.5 /rm.

In the layer-oriented approach each wavefront sensor sees all the guide stars but each is coupled to a single deformable mirror. Each wavefront sensor... 

The cylinder (cone in the case of LGSs) from a guide star at angle 9 intercepts a circle of diameter at layer 1. The Zemike expansion of the phase on this circle will have coefficients b l) which will be related to the coefficients of the Zemike expansion on the whole meta-pupil by a linear relation ... 

Figure 10. Results of numerical simulations to obtain the Strehl ratio at 2.2 pm over the field of view for a held of diameter 1 arcminute (left) and 1.5 arcminute (right) when using three natural guide stars.

The probability of finding at least 3 guide stars is therefore given by... 

An extension of layer-oriented MCAO has been proposed as a way to get good sky coverage. In principle, LO can combine light from multiple faint guide stars inside the field of view. Even so, the sky coverage will be low for the fields of view over which good Strehl can be obtained (<2 arcmin). The proposed multiple field of view layer-oriented is based on the fact that... 

Keywords lasers, laser guide star, adaptice optics, sum-frequency lasers, fiber lasers, Ra-... 

In order to overcome the requirement of having a bright "natural guide star" (NGS) near an astronomical source, a first generation of laser guide stars (LGS), or laser beacons, has been deployed to produce bright artificial stars from which wavefront distortions can be assessed. LGSs consist of the... 

Sodium Guide Star Brightness. From the properties of the Na atom and the Na layer (Table 1), we can estimate the power needed for the AO system. Assuming isotropic emission and no saturation, the photon flux F of the LGS observed with the WFS is given by ... 

Laser Guide Star Technologies and Experiments 4.1 Rayleigh Guide Star Lasers... 

Figure 14. Layout of the Lick Observatory laser guide star system.

A Systems Design Example The Keck Laser Guide Star. 

Figure 18. Layout of the pulsed dye laser guide star system at the Keck observatory.

Figure 23. First light with the Keck guide star laser, December 2001. (Phototaken by J. MacDonald of the CFHT)...

Fiber laser guide star systems. It is now widely appreciated that the heat- dissipation characteristics of fibers, coupled with the high efficiencies demonstrated (> 80%) and excellent spatial mode characteristics, make fiber lasers a preferred candidate for many high power applications. Based on these features, fiber laser technologies would provide a compact, efficient, robust, turnkey laser source, ideally suited for LGS applications. 

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