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TNO is assisting the European Space Agency ESA in its search for extraterrestrial life. ESA is developing technology for the Darwin mission that combines the images from four space-based telescopes to a single shot, at extremely high resolution. The merging of the images takes place in a central satellite. TNO is working on equipment for the heart of this satellite.
Darwin telescope constellation
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The Darwin mission being conducted by ESA will try to detect signs of life on exo-planets (planets outside our own solar system). In recent years astronomers have discovered more than 250 exo-planets. Those planets containing liquid water offer the best chance of finding extraterrestrial life. These habitable planets are relatively warm and close to their star, so the telescope requires extremely high resolution. But there is a further problem: the light from the star is around a million times stronger than that of the planet. It's like looking for a firefly in the glare of a lighthouse... from a distance of 10,000 km! To make the planet visible, the starlight has to be extinguished.
To solve this problem requires combining the signals of two or more telescopes using a technology known as 'Nulling Interferometry'. Nulling Interferometry can generate an extremely high resolution and an artificial eclipse.
For the interferometry to be correct, the length of optical paths from the star, via the telescopes, to the central satellite must be exactly the same. This is effected using an optical delay line developed by TNO. The delay line comprises a set of mirrors that can be shifted with utmost precision to generate optical wavelengths of exactly the same length. The TNO delay line enables the optical path to be adjusted to less than 1 nanometre (some 100,000 times smaller than the thickness of a human hair).
The delay line uses frictionless magnetic bearings for guidance and a voice coil actuator for high-precision positioning. The magnetic bearings and actuator are contact-free and require no lubrication. The Darwin telescopes look into the infrared wavelength range of 4 to 20 micron. To reduce the telescopes' own interference, they are cooled. The delay line operates at a temperature of around -230 °C, and is constructed such that no deformation of the mirrors takes place upon cooling. The mirrors have been manufactured on a diamond lathe at TNO.
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Darwin optical delay line
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Once the optical paths are brought to equal length, a further step must be taken to extinguish the star's light. This is done by shifting the light from one telescope half a wavelength in relation to another telescope. Since the planet is at a small angle, and the planet's light covers a slightly longer distance, the dimming only occurs at the star’s location. This involves more than a single wavelength, a band of 4 to 20 micron, so a broadband shift of light has to be produced. To this end TNO has developed and patented an achromatic phase shifter.
In collaboration with EADS-Astrium, TNO has applied the nulling technology in a near-infrared nulling instrument (at a wavelength of 1.5 micron). A stable broadband dimming of some 100,000 times was achieved during testing. At a wavelength of 12 micron, this generates a dimming of two million times, sufficient to make an exo-planet visible.
