Sharp images at the Large Binocular Telescope – "First Light" for ARGOS

December 18, 2014

With great success, the ARGOS Laser Guide Star Facility at the Large Binocular Telescope (LBT) in Arizona was recently tested. This system allows high-resolution observations at any point of the sky with artificially generated stars in the field of view of the telescope. With such "laser stars" the blurring of images caused by air turbulence is largely corrected.

Until recent years even the largest telescopes in the world had an inevitable problem: Their theoretically available resolution was far from being achieved because stellar images are blurred by turbulence in the Earth's atmosphere. Part of this effect, for example, is the twinkling of stars visible to the naked eye. Through the development of so-called Adaptive Optics (AO), this problem could be solved – particularly in the near-infrared which is very important for astronomical research. With fast deformable mirrors in the telescope beam complementary image errors can be generated that compensate for those errors generated by air turbulence. To achieve this in real time, some bright stars in the field of view – that is, in the vicinity of the astronomical target of interest – have to be permanently analyzed to control the AO. Unfortunately, suitable stars for such an adaptive correction are lacking in many areas of the sky.

Right here ARGOS offers a solution. A total of six powerful pulsed Neodym-YAG-Laser (three laser beams for each of the two 8.4m mirrors of LBT) create artificial stars in the Earth's atmosphere via Rayleigh scattering in the line of sight of the telescope. This configuration allows particularly the measurement of air turbulence at the so-called "Ground layer". In this way, a wide field of view can be corrected – not perfectly but uniform.

In late November 2014, an important milestone in the implementation of the system succeeded: the first three laser stars were used for the AO correction of the right 8.4m mirror and immediately achieved a very good image correction of the near-infrared instrument LUCI2 (a camera and spectrograph). This is shown in the images of the globular cluster NGC 2419 (see Figure 1) – once with and once without adaptive correction. For the best shots in the infrared bands J, H and K, the resolution could be improved by a factor of 4. While the air turbulence allowed images at best with a resolution of about 1 arcsec, a much better image quality with a resolution of 0.2-0.3 arcsec was achieved after switching to the ARGOS/AO mode.

ARGOS is being developed under the direction of MPE (Garching), together with the other LBT-partners MPIA, LSW (both Heidelberg), AIP (Potsdam), MPIfR (Bonn), INAF (Arcetri in Italy) and UoA (USA).

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