tEchnology AdAptivE optics
notes Herriot. ‘By cooling it we get tremendous observing efficiency.’ Large telescopes tend to conduct work with adaptive optics in the infrared, because the wavelengths are longer and the same atmospheric perturbation in relative terms is smaller than in the visible. The correction is more accurate in the infrared for a set number of deformable mirror actuators. ‘You can approach the diffraction limit in the near infrared at an affordable cost, which you couldn’t hope to do in the visible with today’s technology,’ states Herriot. ‘In general, astronomers would rather have really good performance in the infrared compared to lower performance in the visible wavelength.’
guiding star
An adaptive optics system is typically comprised of a wavefront sensor, measuring the incoming light and determining the profile of the wavefront. A deformable mirror then alters its actuators according to the data received from the wavefront sensor to change its shape and flatten the wavefront.
Either a natural star is used to
measure atmospheric turbulence, or, if observations are made in an area of the sky with no bright stars, an artificial laser guide star can be utilised. NFIRAOS will have six laser wavefront sensors and three wavefront sensors using natural guide stars to measure tip, tilt, image motion and defocus.
Laser guide stars use laser light to excite a layer of sodium ions at around 90km in altitude, created from micro-meteorites burning up in the atmosphere. The resulting artificial star allows the AO system to assess and correct for atmospheric turbulence. ‘You need to make a lot of higher order wavefront measurements in order to assess the turbulence in the atmosphere and you need to do it quickly,’ says Herriot. ‘There just aren’t enough bright stars providing enough photons to make such detailed measurements in millisecond timescales.’ He adds that calculating
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image motion and defocus does need a natural guide star. Data from the wavefront sensors, all working in slightly different directions, is used for tomography and to calculate the 3D volume of turbulence in the telescope’s field of view. The system then calculates in real time the best shape for the NFIRAOS’ two deformable mirrors (DM) to correct the wavefront over
the field of view. NFIRAOS is unique in the scale of the problem, Herriot says. It will have roughly five times the number of actuators than the AO system for the Hawaii-based 8m Gemini telescope. And the computing problem to take the information from the wavefront sensors and calculate how to shape the DM increases dramatically with the number of actuators.
discovering new planets NFIRAOS and the TMT have a long process ahead until they actually start making scientific observations. At the Subaru Telescope, an 8.2m telescope in Hawaii operated by the National Observatory of Japan, Olivier Guyon and his team are developing extreme adaptive optics to use the telescope to discover and chart planets around other stars.
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