New telescope traps the stars
A robotically controlled telescope in Chile is helping to identify planets 160 light-years from Earth.
The question of whether life exists, or could exist, on planets outside the Solar System has fired our imagination for generations. But it's only in the past 20 years or so that the technology has advanced to the stage where astronomers can actually detect 'exoplanets', and yet even in that short time more than 460 of them have been discovered.
Most of these planets have been found using a method called radial velocity, also known as Doppler spectroscopy, which relies on the fact that a star with a planet will move in a small orbit of its own in response to the planet's gravity. The method allows variations in the speed with which the star moves towards or away from Earth to be measured, enabling the radial velocity to be deduced from the shift in the parent star's spectral lines - in a sense, the star's electromagnetic 'fingerprint' - due to the Doppler effect.
The method is independent of distance and is good at finding massive planets close to stars that are relatively nearby, about 160 light-years from Earth. But it can only estimate a planet's minimum mass; it can't determine the planet's physical aspects such as its actual size, orbit and whether it has an atmosphere, so it is often used in combination with what's called the transit method.
Here, if a planet passes - or transits - in front of its parent star, the brightness of the star falls slightly, the amount depending on the relative sizes of the star and the planet. Also, during transit, light from the star passes through the planet's upper atmosphere (assuming it has one), making it possible to detect chemical elements in the atmosphere and therefore draw conclusions about the atmosphere's composition. Combining this with radial velocity enables astronomers to determine the density of an exoplanet, and hence learn something about its physical structure.
The European Southern Observatory (ESO) uses this combination of techniques at its various telescopes around the Atacama Desert in Chile. Its latest addition - called Trappist (TRAnsiting Planets and PlanetesImals Small Telescope) - relies on the transit method but whereas the ESO's other telescopes are manned, Trappist is a robotic instrument, controlled remotely. It's also designed to study comets orbiting around the Sun but, whichever the task, it receives its instructions from the University of Liège in Belgium, nearly 7,500 miles (12,000km) away.
As Dr Emmanuel Jehin of the university, and one of the astronomers on the project, explains: 'The various ESO telescopes in Chile work as a team. Trappist has been set up at the La Silla observatory, near the Harps and Coralie instruments, which use the radial velocity method. These identify likely exoplanets. The aim of Trappist is to confirm them quickly and obtain physical data about them. Doing so is a comparatively mundane task, which can be carried out on a daily or even weekly basis, hence the decision to automate Trappist and control it remotely.
'Our 'centre of operations' is actually just a Windows XP Pro computer in the university, but Trappist could be controlled from anywhere in the world via a laptop if need be,' he says. 'We have a VPN between La Silla and Liège for Trappist, making the instrument in effect a node on the network over which we send it encrypted instructions.
'So we send over a script of instructions - as in which targets to scan - for the day and let it carry those out unattended, and accounting of course for the six-hour time difference between us and La Silla. The system contains a map of the southern night sky so Trappist can move on many targets.'
A night's scanning can yield up to 30 Gbytes of data, Dr Jehin says, all of which needs to be processed to make the images useable. 'We can't FTP that much data, the local network isn't up to it, so it's compressed on a small computer at La Silla onto removable disks and sent back to us once a month.'
Trappist is by no means the first robotic telescope - the technology has been around for about 10 years now - but it would not have been possible to deploy the technology on this class of telescope five years ago, says Dr Jehin. At 75kg and with an aperture size of 60cm, it's comparatively lightweight and small. 'One of the main factors for us with Trappist was cost,' says Dr Jehin. 'Because the telescope is lightweight it cost less, but it also allowed us to make its control more precise.
'And on the software side, we developed as little of it ourselves as possible - as much of it as possible is commercial off-the-shelf (COTS), although obviously we've fine-tuned it. This wouldn't have been possible five years ago but advances have made it cheaper and quicker to deploy proven software, and software drivers for astronomy applications have become standardised as well.'
The software comes from DC-3 Dreams for observation scheduling and telescope control, ACE for dome control, Diffraction Ltd for camera control and the standard driver is from ASCOM. Real-time control is not needed for sending observation tasks but it is needed for maintenance and checks that the telescope is working. There is a webcam link inside the observatory to allow for human observation.
The telescope itself and its mount are from Astelco Systems, and the robotics controlling the telescope's movements are COTS too. 'The aim was always to control all the robotic units remotely, so the technology has been imported from elsewhere and is generally of the type you'd find on a modern factory floor,' says Dr Jehin.
'Each device on the telescope - mount, camera, dome and so on - has its own IP address, which allowed us to make the systems as 'smart' as possible so that if, for example, the software were to crash and observations could not continue, we could try restarting the system. If that couldn't be done, however, then the dome would close and the system would switch to safety mode to protect the telescope.'
This smartness extends to other aspects of operation. For example, the software knows the required start and stop times for observations and is designed to tell how much of an allocated task has been carried out and therefore how much (if any) needs to be done the next day.
Also, even though the Atacama is one of the driest places in the world, La Silla is nearly 8,000ft (2,400m) above sea level, so rain, wind and even snow are not unknown up there. The observatory is therefore fitted with an automatic weather station with a rain-snow sensor and automatic dome-shut system from ACE, and a cloud sensor from Diffraction Ltd.
'This is possibly the most important part of the system and is smart enough to close the dome if the weather turns bad or if the sun is rising, making observations impossible,' says Dr Jehin, who adds that there are plans to add an exterior webcam to the system as well.
All this needs a power source that can operate unattended, so while main power comes from La Silla's generators, Trappist also has its own UPS.
At the time of writing, the telescope was still at the testing stage but Dr Jehin says that by the end of June this should have finished and Trappist will be fully automatic. By September, possibly before, there are plans to move the telescope from daily schedules to weekly ones, he says. 'This is a big step though,' he says, 'as the telescope needs to remember which observations it carried out the night before, and that's something we're still working on.'
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