First light for SPHERE exoplanet imager: Revolutionary new VLT instrument installed

SPHERE passed its acceptance tests in Europe in December 2013 and was then shipped to Paranal. The delicate reassembly was completed in May 2014 and the instrument is now mounted on VLT Unit Telescope 3. SPHERE is the latest of the second generation of instruments for the VLT (the first three were X-shooter, KMOS and MUSE).

SPHERE combines several advanced techniques to give the highest contrast ever reached for direct planetary imaging -- far beyond what could be achieved with NACO, which took the first ever direct image of an exoplanet. To reach its impressive performance SPHERE required early development of novel technologies, in particular in the area of adaptive optics, special detectors and coronagraph components.

SPHERE's main goal is to find and characterise giant exoplanets orbiting nearby stars by direct imaging [1]. This is an extremely challenging task as such planets are both very close to their parent stars in the sky and also very much fainter. In a normal image, even in the best conditions, the light from the star totally swamps the weak glow from the planet. The whole design of SPHERE is therefore focused on reaching the highest contrast possible in a tiny patch of sky around the dazzling star.

The first of three novel techniques exploited by SPHERE is extreme adaptive optics to correct for the effects of Earth's atmosphere so that images are sharper and the contrast of the exoplanet increased. Secondly, a coronagraph is used to block out the light from the star and increase the contrast still further. Finally, a technique called differential imaging is applied that exploits differences between planetary and stellar light in terms of its colour or polarisation -- and these subtle differences can also be exploited to reveal a currently invisible exoplanet [2].

SPHERE was designed and built by the following institutes: Institut de Planétologie et d'Astrophysique de Grenoble; Max-Planck-Institut für Astronomie in Heidelberg; Laboratoire d'Astrophysique de Marseille; Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique de l'Observatoire de Paris; Laboratoire Lagrange in Nice; ONERA; Observatoire de Genève; Italian National Institute for Astrophysics coordinated by the Osservatorio Astronomico di Padova; Institute for Astronomy, ETH Zurich; Astronomical Institute of the University of Amsterdam; Netherlands Research School for Astronomy (NOVA-ASTRON) and ESO.

During the first light observations several test targets were observed using the many different modes of SPHERE. These include one of the best images so far of the ring of dust around the nearby star HR 4796A. It not only shows the ring with exceptional clarity but also illustrates how well SPHERE can suppress the glare of the bright star at the centre of the picture.

Following further extensive tests and science verification observations SPHERE will be made available to the astronomical community later in 2014.

Notes

[1] Most of the exoplanets currently known were discovered using indirect techniques -- such as radial velocity variations of the host star, or the dip in brightness of the star caused by a transiting exoplanet. Only a few exoplanets have so far been directly imaged.

[2] A further, but simpler trick employed by SPHERE is to take many pictures of an object, but with a significant rotation of the image in between each. Features in the pictures that rotate are artefacts of the imaging process, and features that stay in the same place are real objects in the sky.