X-shooter takes celestial snapshots

The European Southern Observatory's Very Large Telescope now has an X-shooter, a second-generation spectroscopic instrument that can record the entire spectrum of a celestial object in a single shot - from the ultraviolet to the near-infrared - with high sensitivity. This unique new instrument will be particularly useful for the study of distant exploding objects known as gamma-ray bursts.

The VLT is probably the world's most advanced optical instrument. It is an assembly of four 8.2-metre telescopes located on a mountain at the Paranal Observatory in the Atacama Desert, North Chile. Twelve focal stations pipe in the light from the four telescopes and different instruments then carry out imaging and spectroscopic observations.

The first VLT instrument was installed more than a decade ago and was then joined by twelve more instruments. X-shooter represents a second generation of VLT instrumentation and replaces the workhorse-instrument FORS1, used by hundreds of astronomers over the last ten years to observe objects in the far reaches of the universe.

"X-shooter offers a capability that is unique among astronomical instruments installed at large telescopes," says Sandro D'Odorico, who coordinated the Europe-wide consortium of scientists and engineers that built the new instrument. "Until now, different instruments at different telescopes and multiple observations were needed to cover this kind of wavelength range, making it very difficult to compare data, which, even though from the same object, could have been taken at different times and under different sky conditions."

The X-shooter instrument collects the complete electromagnetic spectrum from ultraviolet (UV) wavelengths at 300 nanometres to the near-infrared (NIR) at 2400 nm, D'Odorico explains. "All in all, X-shooter can save us a factor of three or more in terms of precious telescope time and opens a new window of opportunity for the study of many, still poorly understood, celestial sources," he adds.

Importantly, the 2.5-tonne instrument has the capacity to capture data very efficiently from almost any celestial source without having to know in advance the nature and energy distribution of the emissions from that source in advance. This property will be particularly useful for astronomers studying curious objects known as gamma-ray bursts. These are the most energetic explosions known to occur in the universe.

Before X-shooter making observations of sources such as gamma-ray bursts required a rough estimate of the distance of the target, so that an appropriate instrument could be aligned to make the observations. X-shooter avoids that problem meaning such a first observing step is no longer needed. Because gamma-ray bursts can fade very quickly, cutting the time to making detailed observations is key to understanding the nature of these elusive cosmic sources.

"I am very confident that X-shooter will discover the most distant gamma-ray bursts in the universe, or in other words, the first objects that formed in the young Universe," explains François Hammer, who leads the French efforts in X-shooter.

X-shooter was built by a consortium of 11 institutes in Denmark, France, Italy and the Netherlands, together with ESO. In total 68 person-years of work by engineers, technicians and astronomers and a global budget of six million Euros was needed. The development time was remarkably fast for a project of this complexity, which was completed in just over five years, starting from the kick-off meeting held in December 2003.

"The success of X-shooter and its relatively short completion time are a tribute to the quality and dedication of the many people involved in the project," says Alan Moorwood, ESO Director of Programmes.

The instrument made its first full configuration observations on 14 March 2009. It has already proved its worth by acquiring the complete spectra of low metallicity stars, X-ray binaries, distant quasars and galaxies, the Eta Carinae nebulae, and the supernova 1987A. Coincidentally, a gamma-ray burst occurred during this period and that too was recorded. X-Shooter goes online fully for the astronomical community from 1 October 2009 and 150 proposals have already been received amounting to 350 observing nights.