SPHERE-Zimpol

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Images of the AU Mic debris disk obtained with SPHERE after unsharp masking, subtraction of the smooth main body of the disk, and stretching in the vertical direction by a factor of two. The same persistent pattern is recovered in all three epochs, though at shifted locations, implying motion away from the star.  © Nature.com / Boccaletti et al.

SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) is the new high-contrast imaging instrument on the ESO Very Large Telescope.  SPHERE was produced by a large international consortium with members from 12 institutes in 6 countries, including NOVA. The consortium was formed by a successful merger of two initially competing consortia. The result was a complex instrument that provides high-order corrected adaptive optics images to three science instruments at optical and infrared wavelengths. The complexity of the instrument has caused some delays on the way to the telescope on Paranal (Chile), but the instrument now offers the currently worldwide best performance for high-contrast imaging.

The main science goal of SPHERE is to provide a statistically sound survey of low mass companions including planets in a 10 arcsecond field of view around bright nearby stars, and to characterize the atmosphere of those planets in terms of clouds, dust content, methane and water absorption lines, effective temperature, radius, and polarization.  SPHERE will also make an attempt to detect planets in reflected light and to test systems that can do so for a larger number of targets in the future. The detection and analysis of scattered light from protoplanetary and debris disk on a spatial scale similar to or better than what can be done at submm wavelengths is an important secondary science case, one SPHERE turns out to excel at. SPHERE is also able to detect and characterize circumstellar material around AGB stars and other bright objects.

SPHERE consists of an extreme adaptive optics system SAXO that uses a deformable mirror with 1681 actuators driven by a Shack-Hartmann wave front sensor to provide a highly stable PSF to the science instruments of SPHERE.  The common path also offers a wide set of coronographs, including classical Lyot coronographs, 4 quadrant phase masks for Y, J, and H bands and an apodized pupil Lyot coronograph.  All science instruments have pupil-imaging lenses. SPHERE contains three science instruments:

IRDIS (InfraRed Dual Imaging System) is a dual-band imager that can be used to simultaneously image an 11×12.5 arcsecond field of view at two different wavelength or two different polarization directions. IRDIS provides spectral and polarimetric differencing modes as a means of enhancing contrast.  The wavelengths coverage of this imager ranges from 0.95 to 2.3 µm (Y to Ks bands).

IFS, the Integral Field Spectrometer, images a small 1.77 x 1.77 arcsecond field into 145 x 145 spectra that cover the wavelength range from 0.95 to 1.35 µm with a spectral resolution of 54.  IRDIS and IFS can be operated together in the default planet-searching mode IRDIFS where the central imaging with IFS is combined with dual band imaging near the methane bands in the H band.  Those bands are common in warm giant planets.

ZIMPOL, the Zürich Imaging Polarimeter, provides high-contrast imaging polarimetry at visible wavelengths (600 – 900 nm) with a field of view of 3.5 x 3.5 arc seconds. ZIMPOL uses a special technique that allows measuring the different polarization directions on the same pixels of the CCD, using a custom CCD design in which every second row is covered and the incoming light is focused using a microlens array onto the uncovered pixels. A fast modulation pushes the electrons in each pixel up and down by one row while the modulated polarization signal is integrated.

The ZIMPOL part of SPHERE has been developed as a collaboration of the ETH in Zürich with API at the UvA and the NOVA Op-IR group.  The basic design is a Zürich development, and the ETH also produced the special CCD cameras used in this system.  The ZIMPOL instrument as a whole was built and integrated at NOVA.

Further information on SPHERE is available from the ESO website.

The NOVA principal investigator for SPHERE is Prof. dr. Carsten Dominik (Anton Pannekoek institute, Amsterdam)