Spectro-Polarimetric High-contrast Exo-planet REsearch


SPHERE Performance

The figure to the left shows a copmparison of simulated NACO-SDI and SPHERE performances. Although the AO simulation was in this case not completely (or rather, not at all) orthodox, the outcome matches nicely that of full-scale end-to-end simulations. After coronagraphy and spectral differential imaging, SPHERE is predicted to achieve an SNR of ~1 on a 1:105 contrast planet 0.3" from its host star after 10s of integration time. A quick calculation predicts from here, that for a 1:106 contrast planet and one hour of integration time, the SNR will be ~2. This is reasonably close to our top-level requirement of achieving an SNR of 5 after 1 hour at a separation of 0.5" and a contrast of 1:106.

Nicely visible is also the progress from NAOS' to SPHERE's AO systems: The increased number of actuators and loop frequency produce more, smaller speckles in the instantaneous images which average out much better in the long integration. Also, the much better tip-tilt stability allows a much more effective usage of the coronagraph. In this simulation, a so-called "Advanced Lyot Coronagraph", ALC, this one of the options implemented in SPHERE was used.

Note also, that the noise in the SPHERE image after differential imaging appears to be completely dominated by diffraction patterns, whereas in the NACO-SDI case noisy speckle patterns can still be seen. This means, the result can be further improved by obbservations of reference targets.

mfeldt.php, 08/2007