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The aim of the Spectro-Polarimetric High-contrast Exoplanet Research (SPHERE) instrument is to detect extremely faint astronomical sources (i.e., giant extra-solar planets, disks etc …) in the vicinity of bright stars. The detection capabilities of an exoplanet hunter are largely controlled by its adaptive optics (AO) system. Better AO correction provides improved coronagraph extinction and fewer starlight residuals. The challenging SPHERE science goals require a very high performance AO system to feed a quasi-perfect flat wave front, corrected for atmospheric turbulence and internal defects, to the scientific instruments.
In May 2014 SPHERE was installed on the third unit telescope (Melipal) of the Very Large Telescope (VLT) in Chile. The results obtained over 3 years of operations essentially nicely confirm the AO predictions made at the time of the design phase and the corresponding performance budget analysis to cover various operation conditions in terms of target brightness and turbulence conditions. This is a strong basis to propose a realistic SPHERE AO upgrade.
The SPHERE upgrade project intends to push the ultimate performance of SPHERE in terms of both final contrast and sensitivity (especially towards redder and fainter stars), thus allowing to address new science cases and to offer new detection or characterisation modes such as the coupling with high spectral resolution spectrographs, either in the infrared or in the visible.
To do so, we have to address several tasks:
• The main AO loop has to be accelerated (up to 3 kHz) and efficient predictive control laws have to be implemented in order to significantly reduce the temporal effects;
• The main wave front sensor scheme has to be revisited in order to make the system more sensitive and make possible to work on very red stars. It will be achieved by adding to the current Visible Spatially Filtered Shack-Hartman an IR-Pyramid counterpart;
• The NCPA correction and the system ability to create and stabilize dark-hole during an entire observation block has to be developed in order to get rid of the residual quasi-static speckles and residual diffraction patterns. This will be done thanks to the combination of very accurate coronagraphic wave front sensors (COFFEE and ZELDA).
This presentation will detail the various system studies and trade-off choices which have led to the new concept of the SPHERE upgrade. A preliminary design of the new AO loop and its main components (IR pyramid, RTC, post-coronographic WFS) will be presented. We will show that the proposed SPHERE upgrade development can be achieved in a timely manner without affecting the current SPHERE configuration and for a reasonable cost. Finally, an AIT concept minimizing the down-time of the instrument will be described. At each stage of the project, special attention will be paid to ensure that the initial capabilities and performance of SPHERE are not be jeopardized by the proposed SPHERE upgrade developments.
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