Searching for planets that lie outside the Solar System is adding impetus to astronomical interferometry, a technique that combines the light-gathering capabilities of an array of telescopes. Jessica Rowbury reports
Looking for exoplanets, planets that lie outside the Solar System, is an area of astronomy requiring the largest telescope apertures available. One of the objectives of the European Extremely Large Telescope (E-ELT), which will have a primary mirror 39 metres in diameter when it is completed in 2023, is to use its massive light-gathering capabilities to look for extrasolar planets
There comes a point, however, where it will be unfeasible to build larger complete telescope primary mirrors. To increase the aperture further, astronomers are turning to interferometry to combine the light from an array of telescopes all acting together.
In September 2013, scientists at the Observatory of Haute-Provence (OHP) in France demonstrated a prototype interferometer called Carlina, or the giant diluted telescope, which has been under development for the last 10 years. And in March 2014, the team reported that the prototype was able to detect fringes on a star (Deneb) – which proves that, conceptually, the system works. ‘It is a very important result, because this new technology opens a new possible way to study the universe,’ said Hervé Le Coroller from Haute-Provence Observatory.
The diluted telescope, which was produced at the OHP by principal investigator Hervé Le Coroller, along with a team including engineer Julien Dejonghe, works by combining a number of small (0.5 to 5m) spherical primary mirrors adjusted along a virtual giant sphere. The goal of this project is to propose an astronomical interferometer that is more sensitive, and has a higher imaging capability, than current systems. The aim is to find a solution for increasing the resolution of telescopes.
‘It is a concept between a regular interferometer and the E-ELT,’ explained Le Coroller. The next logical step after E-ELT might be to build still larger telescopes, but Le Coroller said that a telescope with a full mirror of 100 metres, for instance, might not be possible to make, even with the segmented mirror technology. Something like the Carlina diluted telescope could be an alternative next step, he said – a system with a lot of mirrors, much more than a regular interferometer, which is presently limited to six telescopes, and slightly diluted in comparison to an extremely large telescope.
‘If you want to see the area close to the black hole at the centre of our galaxy, or if you want to study the planet-forming disks, you need telescopes bigger than 100 metres in aperture,’ Le Coroller stated. ‘But we don’t know how to build the telescope with a full mirror of 100 metres or more.’
The primary mirror of the Carlina prototype is made up of relatively small mirrors positioned on the surface of a sphere. In addition, the light from this array of mirrors is combined using interferometry to give a huge baseline area. A major benefit of using this technique is that, unlike the astronomical interferometers today, the prototype doesn’t require delay lines, a technique that is used to equalise the optical paths between the telescopes. This means that it can work with a lot of mirrors as well as being relatively sensitive, Le Coroller noted. Delay lines are complex to build and manage; it is difficult to position a moving mirror with accuracy greater than 1µm. ‘There are no delay lines in Carlina, so we can work with 10 or 100 mirrors or more in the future,’ he said.
There are astronomical interferometers already operational. The European Southern Observatory’s (ESO) Very Large Telescope Interferometer (VLTI) located in Chile is one example, made up of four fixed 8.2m telescopes and four movable 1.8m auxiliary telescopes. The four telescopes can be combined in groups of two or three and the light combined to create an image of much finer detail. Since its formation in 2009, VLTI has observed stellar diameters, the surface of giant stars, binary systems and disks around stars, among other objects.
The goal for astronomical interferometry, Le Coroller pointed out, is to get very high angular resolution to see details. ‘For that we will combine separate telescopes on different baselines. We are not able to have a full mirror, but we will combine separate telescopes,’ explained Le Coroller.
Currently, the maximum number of telescopes that can be combined is six, a number which needs to be increased for higher imaging capability, according to Le Coroller. ‘We need more telescopes,’ he stated. ‘Typically, the number of pixels in the image that you can reconstruct is the square of the number of telescopes.’ There also needs to be further work on applying adaptive optics between the sub-apertures of the interferometers to achieve sharper images, noted Le Coroller. And, coronagraphy systems, designed to block out the direct light from stars so that nearby objects can be resolved, will also have to be developed in order to obtain images of exoplanets.
The next step for the Carlina team will be to construct a larger-scale system, although this will be a lengthy and expensive task. ‘Before building such a system of 100 metres aperture, with tens of mirrors or more, it will require more study,’ commented Le Coroller. ‘Considering the cost, and the complexity of such a project, it will require creating a European or international consortium around this project.’
Measuring mirror segments
The mirror segments for the E-ELT – the first prototype segments were produced and accepted by ESO at the end of last year – have to be made extremely accurately, which in turn is pushing the requirements for the interferometric test methods. ‘This push in telescope optics is requiring us to continually innovate our systems to allow for the next generation of telescopes to be successfully polished, assembled and aligned,’ said Dr Erik Novak, director of business development at 4D Technology, which produces optical metrology equipment, including interferometers.
‘Even though these optics can be multiple metres in diameter, the manufacturers are controlling the surface to a level of a few nanometres,’ explained Novak. ‘So they [telescope optics manufacturers] need extremely capable metrology in order to verify the shape of the telescope optics. Interferometry is really the only technology in the world capable of nanometre precision over larger areas.’
Interferometric measurements are normally taken while a mirror is being polished. ‘Usually the mirror manufacturer will go back and forth multiple times – they’ll measure the telescope optics, and then polish them further, and then they’ll re-measure and keep polishing until they get the proper surface finish and shape,’ said Novak.
To measure large mirrors, the interferometer often must be located metres, or tens of metres, from the test optic. To create such a long measurement path in a small working space the manufacturer will fold back the optical path. Novak described one configuration with multiple reflections in a multi-story arrangement: ‘There might be the telescope mirror on the bottom, a flat mirror near the ceiling, another flat mirror down near the floor, then the interferometer back near the ceiling,’ he said.
In recent years metrology manufacturers have developed techniques to acquire all measurement data simultaneously, making instruments capable of measuring despite vibration and other environmental noise. 4D Technology systems employ dynamic interferometry, in which polarised light is used along with a custom camera with micro-polarisers in front of each pixel to acquire all necessary data in a single frame. The instruments typically include high-powered laser sources which allow for short camera exposure times, so mechanical vibrations won’t degrade the results.
Because of the sheer size of telescope optics and their long focal lengths, there are some challenges that exist for interferometers used in astronomy applications. The biggest challenge, according to Novak, is making a system that is accurate, vibration-insensitive and compact enough to be deployed in these difficult environments – up on scaffolding or on rigging.
Traditional interferometers use a measurement technique known as phase shifting, in which the phase of the test and reference beams are changed over time as multiple frames are imaged. The technique can be highly accurate; however, because the measurement requires time to complete, it is susceptible to both mechanical vibration and air turbulence. Because large mirrors may require long measurement paths, it proves particularly challenging and expensive to isolate the system from vibration in order to acquire accurate data.
‘The test set-ups for large mirrors often require the interferometer to be placed high in a measurement tower, or looking into a vacuum chamber, or placed in an ad-hoc mounting rig,’ Novak said. The systems have to be able to be operated remotely because of where they are positioned, according to Novak. This includes adding motorised control over the reference optic orientation, the intensity of the light, focus, and all of the settings needed for good measurements without having to be physically at the interferometer.
Air turbulence not only distorts images obtained by the telescope when in use, but also can cause errors in measurements during manufacture. ‘We think of air as fundamentally clear and that it doesn’t affect things,’ said Novak. ‘But in these interferometers, the very small differences in air pressure – for example, from an air conditioning vent or general air circulation − easily cause nanometre level errors.’ To counteract this, measurements are normally averaged to remove the effect of airflow from the result.
There is always a need for more accurate measurements. Novak commented: ‘Interferometers are going from tens of nanometres of tolerance on the shape of the [telescope] mirrors to single nanometre tolerances – and that is really pushing interferometers to have even lower noise floors and even higher accuracies.’
In terms of using the technique for astronomy by combining the imaging capabilities of a number of telescopes, Le Coroller commented that greater collaboration within the industry needs to exist to manage such huge science projects. He said that concentrating on exo-planetology is one topic that could add impetus to building a new generation of astronomical interferometers. This was one of the conclusions of a colloquium that was organised at the Haute-Provence Observatory in September 2013.
‘Astronomy is in the [area of] big science, with projects of €1 billion, for example,’ he said. ‘It takes a lot of money; this is the difficulty. We need to increase our community – we need more collaboration with industry, probably all around the world with our colleagues and astronomers that work in exoplanet teams. When you work on a big project, it becomes very expensive, and only a big community can do that.’