Friday, November 27, 2015

Four Laser Guide Star Facility Passes Key Milestone

One of the units of the Four Laser Guide Star Facility for the VLT. Credit: ESO/M. Zamani

All four laser guide star units that form the Four Laser Guide Star Facility — a core part of the Adaptive Optics Facility (AOF) for ESO’s Very Large Telescope — have now been accepted and are being shipped to Chile. This is a major step towards establishing VLT Unit Telescope 4 as a fully adaptive telescope with much enhanced image quality.

An adaptive optics system uses sensors to analyse the atmospheric turbulence and a deformable mirror integrated in the telescope to correct for the image distortions caused by the atmosphere. But a bright point-like star very close in the sky to the object being studied is essential, so that the turbulence can be accurately characterized.

Finding a natural star in the right place for this role is unlikely. So, to make the correction of the atmospheric turbulence possible everywhere in the sky, for all possible science targets, an artificial star is needed. Such stars can be created by projecting a powerful laser beam into the sky onto the sodium layer, where it creates a bright glow that appears star-like from the ground.

By measuring the atmospherically induced motions and distortions of this artificial star, and making tiny adjustments to the deformable secondary mirror one thousand times per second, the telescope can produce images with much greater sharpness than is possible without adaptive optics.

The first Adaptive Optics Facility laser guide star unit was installed on the VLT and successfully tested in situ earlier this year. These tests have confirmed the sound design implemented by ESO, in collaboration with European industry and scientific institutes. Tests on VLT Unit Telescope 4 in Chile showed high optical quality, providing an almost perfect artificial star image, and high efficiency of the sodium layer excitation. These successes mean that the team can proceed with preliminary tests with GRAAL, the adaptive optics module feeding HAWK-I, the wide-field imager on Unit Telescope 4; all further steps towards the full commissioning of the Adaptive Optics Facility at Paranal.

The Adaptive Optics Facility will use four lasers simultaneously, which will allow better characterization of the atmosphere’s properties — and hence a larger field of view where the image is corrected — than is possible with just one laser.

When fully installed, the Adaptive Optics Facility will feed light into two instruments, HAWK-I (in conjunction with GRAAL) and the integral field spectrograph, MUSE, (in conjunction with GALACSI).

Credit: ESO

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