The poor sky coverage of NAOMI has limited its uptake in the astronomical community. Fortunately we have recently obtained funding from the NWO in the Netherlands to develop a Rayleigh laser guide star (LGS) beacon called GLAS (see Fig. 4). This system will use an artificial on-axis guide star anywhere in the sky permitting almost full-sky AO observations. GLAS is currently being designed at the ING, Durham AIG and ASTRON in the Netherlands.
The GLAS acronym stands for ground layer adaptive optics system. The system is so named because it only corrects for turbulence in the lower part of the atmosphere close to the telescope itself.
The laser beacon itself consists of a diode pumped Yb-YAG disk laser feeding into a frequency doubling crystal to yield a 30 W beam of 515 nm light. The laser is Q switched and emits a 7 KHz stream of 400 ns wide pulses. It will be mounted on the top-end ring where it will be coupled via an evacuated pipe to a 30 cm diameter refracting beam launch telescope (BLT) mounted behind the secondary mirror. These pulses are approximately 120 m long as they leave the BLT (see Fig. 5). 133 ^s after the launch of each pulse a Pockels cell shutter briefly opens to allow laser light backscattered by the atmosphere to enter the wavefront sensor. The shutter acts as a range gate and effectively defines an artificial star within the atmosphere 20 km from the telescope. Since a Pockels cell must be used in combination with a polarising element it will at most have a 50% transmission. To overcome this loss, two cells will be used, one for each polarisation axis. The LGS light will be directed to each through a polarising beam-splitter. Light exiting the two shutters is then recombined using a second polarising beam-splitter before passing through the SH lenslet. The
SH spots produced on the WFS will be slightly elongated by 0.2" due to parallax effects from the finite laser beacon distance. Positioning the BLT above the secondary acts to minimise this elongation. Associated with the LGS WFS optics are a number of additional small diagnostic cameras to verify the position and intensity of the laser beacon. The LGS is brought to an intermediate focal plane containing a narrow 4" diameter field stop. The plane of this stop is conjugate to the 20 km distant beacon so any light scattered at lower altitudes will be so far out of focus at the pin-hole that only a small fraction of it will pass through - a neat 'lo-tech' solution to improving the shutter contrast ratio.
Whilst improving sky coverage almost 100%, using laser guide stars has certain disadvantages compared to natural guide stars. Firstly, there is the 'cone effect' where the volume of atmosphere through which the laser beacon light returns to the telescope is not the same volume as that traversed by light from the science object (see Fig. 6). The cone effect worsens as the distance of the beacon decreases and the aperture of the telescope increases. Secondly, any turbulence effects that cause a lateral displacement of the science object on the sky, i.e. 'global tip-tilt' terms, will not be detected by the laser beacon due to the double passage of the laser beam through any such turbulence layers. In these cases the lateral deflection of the laser beam in its upward passage will be cancelled by an equal but opposite deflection during its return journey to the telescope. Even when using the laser it is therefore still necessary to observe a natural guide star within 1-2' of the science object to remove these global tip-tilt distortions. This is not such a disadvantage because the tip-tilt guide star can be observed using the whole telescope aperture rather than just a small section as is the case with a Shack Hartmann sensor. Additionally the guide star can be selected from a wider area of sky with longer exposure times. Nevertheless, the availability of a natural guide star still limits sky coverage. If the NGS WFS can reach Mv18, the sky coverage at the Galactic Pole (the sparsest region) will still exceed 60%. The planned start of GLAS operations is October 2006. Until then NAOMI will still be available for observations using its NGS wavefront sensor.
Figure 6. The cone effect.
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