Wednesday 4 June 2014
(mise à jour le 24 July 2014)
SPHERE — the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument — has been installed on ESO’s Very Large Telescope (VLT) at the Paranal Observatory in Chile and has achieved first light. This powerful new facility for finding and studying exoplanets uses multiple advanced techniques in combination. It offers dramatically better performance than existing instruments and has produced impressive views of dust discs around nearby stars and other targets during the very first days of observations. SPHERE was developed and built by a consortium of many European institutes, led by the Institut de Planétologie et d’Astrophysique de Grenoble, France, working in partnership with ESO. It is expected to revolutionise the detailed study of exoplanets and circumstellar discs.
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 . 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 the 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 (ann13069, eso0503) .
Extract from ESO Press Release.
Credit : ESO/J.-L. Beuzit et al.
LESIA participated in the SPHERE project at different levels. As responsable of coronagraphic modes, LESIA delivered the coronographic components manufactured by LESIA and the Observatoire de la Côte d’Azur (OCA).
His efforts have focused on the development of an achromatic phase mask (HW4QPM) to mitigate the light from the central star while to observe very similar planets. The images below show the component and its coronagraph test bench installed on the Meudon site.
A mechanical assembly half-wave blade enabled with an accuracy of a few microns to make this achromatic coronagraph component, the HW-4QPM (Half Wave plate 4 Quadrant Phase Mask) based on the concept of 4QPM designed at Meudon in 2000.
Credits LESIA / Observatoire de Paris.
YACADIRE, the IR optical bench that was used for testing of coronographic components made by LESIA and OCA.
Credits LESIA / Observatoire de Paris
In collaboration with the ONERA, the LESIA also participated in the development of :
During the first tests on the sky made in May 2014, several objects of astrophysical interest were observed to validate the different observing modes of SPHERE. Among the results, one of the best images ever made of the dust ring around the star HR 4796A, in which planets were able to form (to be detected yet).
Credits ESO/J.-L. Beuzit et al./SPHERE Consortium
Click to enlarge
The resolution of the ring is exceptional. This image also shows the tremendous capacity of SPHERE to remove light of the bright star at the center of the image, while correcting the effects of atmospheric turbulence. The analysis of such image will tell us about the mechanisms of planets formation in other planetary systems.
The exceptional performance of SPHERE are also illustrated by the image of the star Iota Sagittarii around which a companion (a low-mass star) was observed for the first time.
Images obtained with two instruments of SPHERE, the infrared spectrograph, IFS and the infrared camera, IRDIS. The companion is a low mass star of 9 magnitudes lower than the star and located at a separation of 0.24 arcsec.
Credits ESO / J.-L. Beuzit et al. / SPHERE Consortium
The first light SPHERE obtained at the beginning of May 2014, was used to validate the operation of these coronagraphs on stars. The figure below shows images obtained on the sky of a single star.
Images of a star obtained without (left) and with the coronagraph (right) for two coronagraphs, the HWQ4PM manufactured LESIA (top) and apodized Lyot mask made OCA (APLC, bottom). These coronographic images are consistent with the predictions.
Credits ESO / J.-L. Beuzit et al. / SPHERE Consortium
The instrument will again undergo a series of intensive tests in 2014 to complete the validation of all modes of observation and to be able to be offered to the community at the beginning of 2015. At this time, a great statement for 5 years on hundreds of stars to search for new planetary systems will begin. SPHERE will study the atmosphere of these new giant planets and properties of dust disks in which exoplanets form.
The instrument will be available to the astronomical community in 2015.
SPHERE was developed by a European consortium led by the Institut de planétologie et astrophysique de Grenoble (IPAG, CNRS/Université Joseph Fourier) with the ONERA, the Laboratoire d’astrophysique de Marseille (CNRS/AMU), the LESIA, the laboratoire Lagrange (Observatoire de la Côte d’Azur/CNRS/Université Nice-Sophia Antipolis) and with German, Italian, Switzerland and Dutch institutes, in collaboration with ESO (European Southern Observatory).
 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 (eso0515, eso0842).
 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.