tEchnology AdAptivE optics
➤
‘Contrast is vital for these
observations,’ explains Guyon. ‘To identify a planet the size of Earth around these stars, the contrast ratio would have to be at least one billion to one. That’s not possible from the ground with an 8m telescope. Therefore, we’re looking for planets around the size of Jupiter. Depending on the observation wavelength and the planet’s age and mass, these planets can be from 1,000 to 10 million times fainter than the star.’ The AO system being developed
provides around one million to one contrast ratio – it’s able to see objects one million times fainter than the adjacent star. It uses a deformable mirror from Boston Micromachines. ‘The system is extreme in the way it’s designed to clean up the image, but this is only done over a small region of the sky,’ says Guyon. ‘We don’t need big deformable mirrors, but we do need them to be very precise with a lot of actuators. The MEMS technology from Boston Micromachines is well suited for these observations.’ Boston
Micromachines’ MEMS deformable mirrors have no hysteresis – so, when
comments Michael Feinburg, director of product marketing at Boston Micromachines. ‘The more control points you have, the more precisely you can compensate for atmospheric aberrations,’ he says, adding that devices have also become faster.
The AO system developed for the Subaru Telescope measures atmospheric turbulence around 1,000 to 2,000 times per second to achieve the high level of correction required. The system also employs a custom-designed coronagraph to remove the diffraction rings from around a star in the image. ‘Even if you have a perfect wavefront generated by adaptive optics, the star will have diffraction rings around it,’ explains Guyon. ‘Those rings mask the light from planets, so we have to build an optical system that removes them.’
The system has recently been tested on the Subaru Telescope for four nights for troubleshooting and to demonstrate it works. The team
the Ao system being
developed provides one million to one contrast ratio
a voltage is applied to a channel, it results in the same defl ection every time. This compares to piezoelectric devices, in which the positioning of the actuators is dependent on their previous position.
‘The big innovation recently has been increases in actuator counts,’
will begin scientifi c observations later this year, mostly looking at well known stars relatively close to Earth. It doesn’t make sense to build
a large telescope without adaptive optics, because the atmosphere would limit it to the resolution of a small telescope. Smaller telescopes,
CORRECTING LIGHT IN SPACE
Boston Micromachines is currently working on a project funded by a US government grant to develop a deformable mirror (DM) system, the Hex Tip- Tilt Piston (TTP) DM, customised for space-based astronomy. ‘Since there’s no atmosphere in space, you’re not compensating for that,’ explains Michael Feinburg, director of product marketing at Boston Micromachines. ‘Instead you’re compensating for any type of aberration that’s in the optical path, so any system misalignment. No matter how well the system is calibrated and confi gured, it might become misaligned when launched into space.’ The AO system is designed to compensate for these types of misalignments. In addition, the temperature varies widely in
20 ElEctro optics l OCTOBER 2011
space depending on whether the telescope is in the sun or shade and expansion of components in the optical system can cause the image to become blurry. ‘Space-based astronomy requires very high actuator counts for high-order aberrations, similar to ground-based telescopes, but also low-power drive electronics, because of the fi nite amount of power available on satellites,’ Feinburg comments. ‘They need very accurate DMs, but can accept very slow response to minimise the power usage.’ The mirror will have 1,021 tip-tilt piston segments
with a total of 3,063 actuators when complete. The Hex TTP mirror is based on a DM architecture made up of hexagonal segments each with three actuators underneath to piston or tip and tilt it.
laser guide stars are used to measure atmospheric turbulence in areas of the sky with devoid of natural bright stars Image courtesy of Christoph Baranec, Caltech Optical Observatories
though, tend not to use AO. A team from Caltech Optical Observatories in San Diego County, led by Christoph Baranec, has developed an adaptive optics system, called Robo-AO, to increase the power of mid-sized telescopes. Astronomers at Caltech have
access to the Keck telescopes, two 10m machines that are on the leading edge of large telescopes that exist today, as well as the Palomar observatory with the 5m Hale telescope, which for almost 50 years was the largest telescope in the world. There are also several other smaller telescopes – 60-, 48-, 24-, and 18-inch instruments – which are used for other cutting-edge research. The 48- and 60-inch telescopes have recently been used to discover the
supernova PTF11kly as part of the Palomar Transient Factory (PTF) project. A lot of these mid-sized telescopes in the 1-3m range are not utilised as much compared to the large telescopes like Keck, where there is a lot of competition for time, notes Baranec. ‘The idea for Robo-AO was that, if these smaller telescopes can be equipped with adaptive optics, they can be made more powerful,’ he says. ‘Even a small, AO-equipped telescope has a higher resolution than a seeing-limited larger telescope. In addition, by robotising the AO system and the telescope [the 60- inch telescope is robotic and can run autonomously] you can start to do things that you can’t do with the large telescopes – that is, large observational programmes that wouldn’t be practical to do on Keck, for example, because of the restraints on time.
‘By putting an AO-system on a 60-inch telescope we’d have an enormous amount of time and be able to look at tens of thousands, or even hundreds of thousands, of targets. It would be possible to do really large targeted surveys that you’d never be able to do with Keck or with Hubble or with any of the other space telescopes.’ The team is trying to make it cheap, at least relative to the cost of
www.electrooptics.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52