Six new stars above LBT

The laser adaptive optics system ARGOS at the LBT telescope in Arizona was pointed at the sky for the first time. The lasers generate the artificial stars that allow, through a complex mechanism fruit, also Italian ingenuity, to correct distortions produced by the atmosphere.



The light sabers are grouped into two bundles, in the night between the 5th and 6th of November. The protagonist of this peaceful assault on the ramparts of heaven is the large binocular telescope LBT. In order to further improve the already excellent view of the LBT is in fact equipping of the adaptive optics system ARGOS , of which the lasers are an integral part and now for the first time are turned towards the sky.

The turbulence in the Earth’s atmosphere “disturb” the light coming from stars and other celestial bodies, preventing from getting astronomical images at full resolution potentially achievable with modern telescopes. To counteract this effect, there are realized adaptive optics systems that measure the disorder through the so-called wave front sensors and the counterbalance in real time by means of a special deformable mirror. The Large Binocular Telescope already has a sophisticated adaptive optics system, FLAO (First Light Adaptive Optics), which was developed by a team of INAF scientists, in particular the Arcetri Astrophysical Observatory in Florence, and Italian industries.

A system that works best when it is able to take as a reference the crisp, bright star near the object wished to observe. If there are no guide stars, or if they are too distorted by atmospheric turbulence, the solution is to use specially designed artificial stars. And that’s what will ARGOS (Advanced Rayleigh Ground Layer Adaptive Optics System) guide to the observations made in wide-field mode with LUCI1 and LUCI2, a pair of twins instruments for imaging and multi-object spectroscopy in the near-infrared installed on LBT. ARGOS is developed by a team of scientists and technologists from the various partner institutions of the LBT. The leadership of the Max Planck Institute for Extraterrestrial Physics, Garching in Germany, but it is significant the participation of the Italian group adaptive optics INAF-Arcetri Astrophysical Observatory led by Simone Esposito, who is also co-head of ARGOS. “The propagation of the laser in the sky is the first and fundamental milestone in the ARGOS project,” says Simone Esposito. “The second milestone of the project is the delivery and installation of the telescope wave front sensors, built in Arcetri, planned for March 2014. It should be emphasized that this milestone is entirely the responsibility of INAF. Later in the period March / April 2014 INAF / Arcetri will undertake to align and calibrate the laser sensors. It will then pass to the final phase of testing of the whole system. It is easy to understand that in these phases INAF plays a key role because it is built of the stars laser sensors for both the experience with the system FLAO. ”

To determine the distortion caused by the atmosphere, ARGOS projected three beams of green laser light perpendicular to each of the mirrors of the LBT, which are seen by the telescope as constellations of three bright spots. The laser light that is pulsed 10,000 times per second is partly reflected by the particles present in the atmosphere. The false glitter of green stars is then analyzed by the wave front sensor.

This type of adaptive optics that uses artificial sources of guidance is called “ground-level” or, in technical terms, GLAO (Ground Layer Adaptive Optics), because it analyzes the atmosphere up to an altitude of 500-1000 meters, a layer in which concentrates most of the turbulence that annoys astronomers. The correction is less than a system, such as that FLAO, which analyzes the entire column of air above the telescope, but the field of view is substantially larger, thanks also to the use of three lasers.

The first screening of the laser beams into the sky is a very important step towards the full functionality of ARGOS. Testing will continue for a few more days to perfectly align the beams and make sure that the alignment is maintained with the telescope tilted up to 45 degrees, to limit the use of the laser.

The next event is important for late spring 2014, when the wave front sensors receive their first artificial light from the stars.

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