Superb New Mirrors Bring Exoplanet Discoveries Nearer

Three unique and remarkably perfect toric mirrors, which are a vital component for SPHERE, a new instrument at the VLT, have been successfully completed and tested. SPHERE is one of the second generation of VLT instruments and it will search for giant Jupiter-sized exoplanets orbiting nearby stars, using direct imaging. The mirrors were manufactured for SPHERE at the Laboratoire d’Astrophysique de Marseille (LAM) in France. The mirrors are called toric because their surfaces have slightly different curvatures in different directions — like the surface of a large torus, or doughnut.

Directly detecting exoplanets is one of the most exciting areas of modern astronomy, but it is also undoubtedly one of the most challenging. Sometimes likened to trying to spot a firefly against the overwhelming light of the Moon; the enormous contrast in brightness between the host star and the orbiting planets means that an extraordinarily sensitive and precise instrument is needed, which is where SPHERE comes in.

This new VLT instrument is equipped with a powerful extreme adaptive optics (XAO) module that corrects for the turbulence of the Earth’s atmosphere and is designed to supply very high quality images in infrared light (at a wavelength of 1.65 µm). The three new and unusually shaped mirrors form part of the XAO system. Their complex toroidal shapes mean that fewer mirrors are necessary. Fewer reflections result in less scattered light and cleaner images, which are absolutely vital for such sensitive observations. These mirrors have been polished under stress using active optics methods, meaning they have extremely smooth, highly polished surfaces — difficult to achieve for such complex shapes.

When SPHERE is fully up and running it will not only produce important scientific data in the field of exoplanets; many other areas of research will also benefit, including the study of protoplanetary discs, brown dwarfs and evolved massive stars.

Courtesy of European Southern Observatory

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