SPHERE – Instrument
SPHERE consists of four main subsystems:
- SAXO: Extreme adaptive optics module used to compensate for atmospheric turbulence at the instrument in real time,
- IRDIS: differential imaging and infrared polarimetry
- IFS: infrared integral field spectrometry
- ZIMPOL: polarimetric differential imaging in the visible
SPHERE combines 3 high contrast imaging techniques:
- Extreme Adaptive Optics (XAO)
- spectral, polarimetric, and angular differential imaging
The Adaptive Optics stage comprises a 40x40 Shack-Hartmann sensor driving a 41x41 actuator deformable mirror at a frequency of more than 1kHz. A suite of coronagraphs is available for attenuation of the star light, the workhorse being the apodized Lyot type. Combined, the two stages provide contrasts down to 10–4 at a separation of a few λ/D. As the third stage, SPHERE provides three focal plane instruments capable of spectral and/or polarimetric differential imaging. In addition, angular differential imaging is applied by default. Angular differential imaging takes into account the fact that at the focus of an alt-azimuth mounted telescope the sky field rotates. We can then discern a real object (which rotates) from an optical defect of the instrument the orientation of which will remain fixed on the detector.
The observing manual of SPHERE can be found here.
The exposure time calculator can be found here: