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ING web news release
5 September, 2016

CANARY's Sodium Laser Guide Star Successfully Commissioned

CANARY is an open-loop Adaptive Optics (AO) system that uses multiple laser-guide stars and deformable mirrors, providing the first on-sky test of combined wide-field Laser Guide Star (LGS) tomography and open-loop AO control. The latest upgrade to CANARY was commissioned between the 19th and 25th July, 2016 with the installation of the European Southern Observatory’s ‘Wendelstein’ sodium Laser Guide Star Unit (WLGSU) near the William Herschel Telescope (WHT).

The 20 Watt 589nm laser is launched at 40 metres from the WHT. Large format: JPEG.

The European Southern Observatory’s ‘Wendelstein’ sodium Laser Guide Star Unit (WLGSU) is installed in a new custom-built laser enclosure next to the WHT. Large format: JPEG.

The new sodium laser is used to excite fluorescence of a thick layer of sodium atoms which are typically observed between 80 and 100km above sea level. The wavefront distortions caused by the turbulent atmosphere in the returned laser light were measured using CANARY 150 times a second, and are compared to the distortions measured from a constellation of background stars.

Unlike other LGS systems in operation, the laser is situated 40m off-axis from the WHT, recreating the LGS geometry expected for the upcoming European Extremely Large Telescope with the WHT simulating a 4.2m diameter segment of this giant telescope. Recreating this geometry is important because the variable thickness and density of the sodium layer means that the LGS suffers from 20-30 arcseconds of perspective elongation in the CANARY wavefront sensors.

LGS systems on existing 8-10m class telescopes typically observe 2-5 arcseconds of elongation, which is inside the isoplanatic patch size of the atmosphere. The first generation of instruments proposed for the E-ELT all rely on adaptive optics system to fulfil their scientific goals, and errors in the wavefront sensing coming from the highly-elongated LGS will affect performance. There are several proposed methods for wavefront sensing from such an elongated LGS, but these have never been investigated on-sky in a real-world environment.

Wavefront images of a natural guide star and the sodium laser plume. Large format: JPEG.

Sodium layer profile obtained using the 6.5 arcminute field of view imager on the INT. Large format: JPEG.

To ensure a measurement of the LGS wavefront that can be disentangled from the effects of density variations in the sodium layer the Isaac Newton Telescope (INT) is also being used to observe the LGS. The INT is 420m off-axis from the LGS launch location resulting in elongations of up to 5 arcminutes. A 6.5 arcminute field of view laser plume imager running at 150Hz (synchronised with the CANARY wavefront sensors) was used to provide a high vertical resolution image of the plume that can be used to calibrate the CANARY wavefront sensor measurements.

The INT telescope control system was integrated with the CANARY systems to allow pointing, focus and rotation commands to be offloaded automatically from either CANARY or the plume imager, turning the INT into a robotic (albeit monitored!) telescope. Simultaneous measurements of the LGS return flux and atmospheric transmission are also provided by the WLGSU via an auxiliary 35cm diameter telescope installed next to the laser enclosure.

Image of the sodium LGS taken using the WLGSU auxiliary 35cm telescope. Photometry and brightness profile have been overplotted. Large format: JPEG.

The CANARY AO demonstrator at the WHT with ESO’s WLGSU is the only facility worldwide capable of performing a comprehensive field investigation of the effects of extreme LGS elongation on AO performance. Results and analysis from CANARY will feed directly into the next generation of ELT scale AO instrumentation.

CANARY Phase D is an international collaboration consisting of Durham University, Observatoire de Paris, European Southern Observatory, INAF - Osservatorio Astronomico di Roma, Instituto de Astrofísica de Canarias (IAC), UK Astronomy Technology Centre and the Isaac Newton Group of Telescopes (ING). CANARY is funded in the UK by STFC through the UK E-ELT program (Ref ST/M007669/1), by CNRS/INSU and Observatoire de Paris in France and by the European Commission OPTICON project (EC FP7 grant agreement 312430). The WLGSU is funded by ESO. The CANARY and LGS teams would like to thank the ING and IAC for their support before and during the run.

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Last modified: 31 August 2016