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Frequency combs make astronomy debut

11 Sep 2008

Laser frequency combs calibrate an astronomical spectrograph to new levels of precision.

“This precision is already beyond the state-of-the-art and its only the first time this technology has been used for this application.”

A group of researchers from Germany and Australia has for the first time used a laser frequency comb for wavelength calibration of an astronomical spectrograph. This has resulted in a calibration precision better than previously attainable using other technologies and brings astronomers one step closer to being able to measure the expansion of the universe in real time. (Science 321 1335)

Spectrographs are used by astronomers to split the light from celestial objects into its component colours, or frequencies. They can then measure the velocities of stars, galaxies and quasars, search for planets around other stars, or study the expansion of the universe.

A spectrograph must be accurately calibrated so that the frequencies of light can be correctly measured. If the laser lines of a laser frequency comb are superimposed on a star's spectral lines (Fraunhofer lines), the latter can be readily measured with the accuracy of an atomic clock.

However, due to the movement of the star, these lines are shifted in frequency with respect to the corresponding lines observed in the laboratory. This is the so-called Doppler effect. Using this effect, the velocity of a star relative to an observer on Earth can be measured.

This has been the aim of astonomers for many years, but would require measurements of Doppler velocity drifts of approximately 1 centimeter per second per year, and astronomical spectrographs have not yet been calibrated to this tolerance.

Now, a team led by researchers at the Max Planck Institute for Quantum Optics (MPQ) in Garching, Germany, has conducted an astronomical laser frequency comb test on the German Vacuum Tower Telescope (VTT) located on the island of Tenerife. The group measured the Sun's radiation at 1.5 microns, and achieved an equivalent Doppler precision of approximately 9 meters per second.

This precision is already beyond the state-of-the-art and this is only the first time this technology has been used for this application.

MPQ's Thomas Udem said: "Even though we've now demonstrated how to use frequency combs to do ultra-precise astronomy, there are many challenges ahead. We want to broaden the comb light to cover more colours, even the entire optical spectrum, from ultra-violet through to infrared light. At the moment, we only have the system working in infrared light. Also, we'd like to make sure we get the same amount of light at all colours; at the moment, there is too much light at some colours, too little at others, for the system to be of general use. But we think these things should be relatively easy to overcome. We aim to have a new way of doing astronomical spectroscopy in the very near future."

Nadya Anscombe is a freelance journalist based in Bristol, UK.

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