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De-twinkle, de-twinkle little star

Photonics Spectra
Aug 2009
Hank Hogan,

Thanks to what is being billed as the world’s fastest and most sensitive astronomical camera, the European Southern Observatory (ESO)’s Very Large Telescope might be able to spot planets orbiting another star within a few years. Nonastronomical applications for the camera might lie in ophthalmology, combustion analysis or the study of chemical reactions.

Jean-Luc Gach, an instrumentation engineer at Laboratoire d’Astrophysique de Marseille, was responsible for the development of the OCAM camera. He noted that the device could be used where there is a particular need for speed and sensitivity. “You almost don’t need any light,” he said.

The world’s fastest high-precision faint-light camera, the OCAM, was developed in Europe. The highly sensitive device takes 1500 images per second.

Ground-based telescopes, such as ESO’s multioptical instrument that sits on a mountain in Chile, must overcome a major obstacle: The atmosphere is turbulent, which gives rise to temporary optical distortions. One result is that stars twinkle, thereby preventing telescopes from realizing their full potential.

The solution is to employ adaptive optics, a technique that detects and cancels out these atmospheric distortions as they happen. If done properly, the view from the ground can rival that from air-free space.

Currently, such corrections are done hundreds of times a second. But next-generation instruments – such as the exoplanet finder that ESO plans to deploy in 2011 – require corrections more than 1000 times a second. Consequently, the camera capturing the correction-determining data must be both faster and more sensitive.

The greater sensitivity is a result of the need for speed. A shorter exposure time means that there is less light with which to work. On top of that, faster electronics usually create more noise, further increasing the need for sensitivity.

Gach was part of a team – including others from ESO and from Laboratoire d’Astrophysique de Grenoble – that came up with the solution. Working with image sensor manufacturer e2v technologies (UK) Ltd. of Chelmsford, it developed a suitable camera.

The 240 × 240-pixel CCD220 detector from British manufacturer e2v technologies (UK) Ltd. at the core of the OCAM camera has a readout noise 10 times less than that of current detectors. It is used in the faint-light camera systems that will be employed on the second generation of the European Southern Observatory’s Very Large Telescope instruments. Images courtesy of P. Balard/INSU-CNRS/ESO.

In fact, the team achieved a better-than-magnitude increase in performance. The OCAM offers a 1500 fps rate with a half-electron readout noise. That’s three times as fast as the existing camera, with less than 10 per cent of the noise, Gach said. The camera development effort was funded by the European Commission and took five years.

Although the OCAM meets current needs, the technology must be improved to handle tomorrow’s larger telescopes. On the drawing board is the European Extremely Large Telescope, which will cost €1 billion and have a light-gathering area that spans 42 m. Correcting atmospheric distortions for that giant will require further development of the technology, said Norbert Hubin, head of adaptive optics at ESO.

Key will be increasing the number of pixels, from the current camera’s 240 × 240 to 1600 × 1600. The frame rate for the larger detector, however, might be only 700 Hz. The development of a camera for the new telescope, dubbed the E-ELT, is ongoing. But the recently unveiled OCAM might play a role in operation of the large telescope, Hubin said. “We might actually make use of this camera for the E-ELT at first light.”

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