The SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research, Beuzit et al., 2006) project, currently in phase B, is a second-generation instrument for the VLT,
following-on from the NACO (NAOS-CONICA) equipment just described, and which it will replace. It should improve on the performance of NACO by at least an order of magnitude. SPHERE is designed to reach very high Strehl ratios (above 95 per cent), and to suppress the residual speckles to the level of 10~7 to 10~8 with the aim of detecting and carrying out spectral analysis of giant planets. SPHERE will have three scientific modules:
• an infrared module providing differential imagery (IRDIS),
• a module with an integral field spectrograph with a very low resolution (about 20) in the 0.95-1.35-^m spectral band (IFS),
• a module giving differential polarimetry in the visible (ZIMPOL).
The infrared modules with allow the detection of objects by their own emission and are thus particularly suitable for young, hot planetary systems. These modes of operation will occupy 80 per cent of SPHERE's observational time. The visible channel is more suitable for evolved systems, which are cooler, and where the companions are detected by their reflected radiation (light from the star reflected and polarized by the planet's atmosphere). In all cases, SPHERE will be imaging planetary systems around nearby stars, where the star-planet distances are between 1 and 100 AU, and where the contrast is about 12-14 magnitudes. The limiting magnitude for the detection of objects is about 24 in the H band. The aim is to be able to examine about 300-400 target stars. To carry out such a wide-ranging programme, SPHERE will be equipped with several coronagraphs, in particular (cf. Chap. 2) a
4-quadrant half-wave phasemask coronagraph, a classic Lyot coronagraph, and an apodized Lyot coronagraph.
To obtain this performance, SPHERE will be coupled to a powerful adaptive optics system called SAXO (SPHERE Adaptive optics for eXoplanet Observation). This adaptive-optics system contains a deformable mirror 180 mm in diameter, controlled by 41 x 41 actuators with a maximum stroke of ± 3.5 (m. The deformable mirror is controlled by a Shack-Hartman wavefront sensor with 40 x 40 lenslets, and operating in the visible and near infrared (0.45-0.95 |m). A 2-axis tip-tilt mirror with a resolution of 0.5 mas provides precise guidance control. In the coronagraphic mode, an additional tip-tilt mirror, whose sensor is located close to the focus of the coronagraph is used to enable thermo-mechanical drift to be eliminated.
The SAXO-SPHERE instrument should see first light around 2010.
Gemini Planet Imager (GPI)
The Gemini Planet Imager (formerly called ExAOC: Extreme Adaptive Optics Coronagraph) is the Gemini Observatory's analogue of the SPHERE project. It aims to produce near-infrared images with a high dynamical range.
Technically speaking, GPI is an extreme AO system with 2000 actuators (MEMS technology), coupled with an apodised-pupil Lyot coronagraph. The wavefront sensing is performed at the nanometric level by a device that includes an infrared interferometer. The detection is performed by an integral field spectrograph working in the infrared spectral range. The system is designed to give a Strehl ratio better than 0.9 at the observing wavelength of 1.65 |m.
The Keck Precision Adaptive Optics System (KPAO)
The KPAO project aims to provide the Keck Observatory with a new-generation adaptive-optics system. Like SPHERE and GPI, KPAO is to be a 1000-actuator-class AO system. To greatly reduce the residual wavefront errors and increase the image quality, the KPAO project would employ multiple laser guide stars (LGS) to reduce focal anisoplanetism by a factor of 10 compared with present LGS systems. The goal of KPAO is also to reach a Strehl ratio better than 0.9 in the K band.
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