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tEchnology AdAptivE optics Eyes

robo-Ao, a robotic laser guide star adaptive optics system, with project leader christoph Baranec

greg Blackman discovers that building larger ground-based telescopes is pointless without adaptive optics

H 18 ElEctro optics l OCTOBER 2011

ow do you see further into space, observe the beginnings of the Universe, or pick out

planets orbiting distant stars in which the host star burns a billion times brighter than the planetary system? Part of the answer is simply to build larger telescopes that gather more light – and the largest telescopes in the world, in the eight to 10m class, are certainly employed in these investigations. There are also extremely large telescopes under development, with apertures of more than 20m diameter. These include the European Extremely Large Telescope (E-ELT), the Giant Magellan Telescope (GMT), both planned for construction in Chile, and the Thirty Meter Telescope

(TMT) to be built in Hawaii. Building a larger telescope isn’t the whole story though; the ability to gather more light from the ground is useless without a way to overcome atmospheric turbulence that would otherwise limit the resolution of the instrument. Therefore, most large telescopes use adaptive optics (AO) to improve their resolving power. ‘Without adaptive optics, once the telescope becomes more than about a fifth of a metre in diameter, the resolution of the images doesn’t get any better, the instrument just collects more light,’ says Glen Herriot, systems engineer working on The Narrow-Field Infrared Adaptive Optics System (NFIRAOS) at the National Research Council of Canada. ‘This is because the

atmospheric turbulence blurs the image.’

on the sky

NFIRAOS, pronounced ‘nefarious’, is being designed and built by the National Research Council of Canada for the planned Thirty Meter Telescope (TMT) scheduled for completion in 2019. The AO system is currently in the preliminary design stage. NFIRAOS will take the light

gathered by the TMT’s primary mirror, correct for atmospheric turbulence and some of the imperfections of the telescope itself, and feed the corrected light to three instruments on the telescope. The AO system will deliver a nearly flat wavefront, (191nm rms wavefront error over the 17 arcsecond field of view of the camera fed by NFIRAOS).

The maximum resolution of a telescope is determined by the diffraction limit of the instrument. Space-based telescopes, which don’t have the atmosphere to contend with, get close to this. NFIRAOS is designed to shepherd roughly half the light into the image area that would be produced by a perfect telescope – a Strehl ratio of about 1/2 in the near infrared.

‘Using AO to get close to the

diffraction limit of the telescope means you win two ways,’ says Herriot. ‘Not only do you collect more light, but you also shepherd the light into a smaller spot whose area is proportional to the diffraction limited diameter of the spot. More light is collected and the sensitivity is increased, meaning greater scientific productivity, which increases with the telescope diameter to the fourth power.’

The optics in NFIRAOS operate at -30°C to reduce their thermal background. NFIRAOS therefore achieves 2.5 times the sensitivity of an uncooled AO system for observations in the K-band (2.0- 2.4µm). This is a window in the atmosphere with low water vapour and OH light blockage and a prime observing band. ‘If we didn’t cool the system, a time exposure would take around 2.5 times as long,’

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