Adaptive optics at the Keck Observatory

In order to conduct the high precision measurements on the sky that enable us to reveal the nature and characteristics of the black hole at the center of our Milky Way and its surrounding we need the world's largest telescopes. But there is a hurdle any observer has to overcome.

The infrared light traveling to us from the center of the Milky Way has to pass trough the turbulent atmosphere of our planet. It suffers from the atmosphere's blurring effects and, as a result, any image, although taken with a 10 m mirror, will be as unsharp as observed with only a 2 m telescope.

A milestone in high resolution near infrared astronomy

In the 1950s an imaging technique - today known as Adaptive Optics (AO) - was proposed for the first time by Horace W. Babcock. But it still took several decades before computational speed and the materials necessary for the optics and infrared detectors reached the level to realize such a complex system. At large telescopes of the 8-10 m class the first AO-systems started operating around the millennium, and one of the first was at the Keck observatory in 1999.

The epochal impact of this new imaging concept on astronomy, and namely on Galactic Center science, can be compared to that of phase contrast microscopy on the life sciences. (Interestingly, the mathematical description of optical aberrations used in the theory of AO-systems has been developed by Frits Zernike, the father of phase contrast microscopy.) The resolution that is achieved with the AO-systems at Keck observatory is about 50 milli-arcseconds in the near infrared. Or in other words: at a distance of the Galactic Center (~25,000 light years) a telescope of this resolution can separate two objects that are as close as about seven times the size of the solar systems. This makes Keck one of the most powerful optical single-dish telescopes in the world.

The concept of an AO-system is surprisingly simple. The optical aberrations responsible for the blurring of the astronomical target are measured on a bright single star, the so-called guide star. A real-time correction loop uses this information to deform a special deformable mirror to correct for the aberrations. In order to keep up with the fast changing turbulence of the atmosphere, all of this has to happen very fast, about 100 times per second. In order to get a good measurement of the aberrations the guide star must be very bright. Such a bright star is not available at all positions on the sky. To correct also on targets outside the vicinity of a suited natural guide star an artificial guide star can be created with a sodium laser that excites atoms in the atmosphere's sodium layer at a height of about 90 km.

The unprecedented accuracy of the Keck telescope in the determination of the relative distance of objects at the Galactic Center and their brightness is the foundation of our research. This is the reason why our research group traditionally is closely related to the technological development of high resolution imaging systems, such as the Keck AO-systems or systems for the future Extremely Large Telescopes. The fruitful collaboration with the instrumentation teams at Keck, UCLA Infrared Lab, Center for Adaptive Optics UC Santa Cruz and the TMT Observatory Corporation is mutually inspiring. On one hand, our research is crucially depending on the performance of the high resolutions systems accessible to us. On the other hand, the Galactic Center is one of the most interesting and most challenging science cases for adaptive optics.

On July 22, 2012 our group conducted measurements of the variable source SgrA* at the direkt location o f the black hole in two infrared bands simultaneously. It was the first time that both Keck telescopes and their AO-lasers pointed simultaneously to the Center of our Galaxy. You can see the spectacular movie of this night here.