Extremely Large Telescopes are very promising to detect and characterize Earth-like planets because of their high angular resolution and the increased number of collected photons. We study the impact of aberrations on this detection and the limitations they impose. We consider an extreme adaptive optic device upstream of a perfect coronagraph. Even with the high Strehl ratio provided, the coronagraphic image is not sufficient to detect Earth-like planet. Indeed the contrast between this kind of planet and its star is about 10^-10 in the near infra-red. As a consequence, a calibration device downstream of the coronagraph must be used to reach this contrast. We modelize a realistic system taking into account dynamic aberrations left uncorrected by the adaptive optics, static aberrations of optical system and differential static aberrations due to the calibration channel. Numerical simulations compare the respective assets of a 30 meter telescope in a median site to these of a 15 meter telescope in the dome C. In both cases, we must control common static aberrations at 8 nm and differential aberrations at 0.1 nm. Beyond this limitation due to the speckle noise and despite the great collecting area, another limitation is set by the photon noise. We also compare these results to simulations made with real coronagraphs and with an obstructed pupil.

Date Published: 26 June 2006
PDF: 10 pages
Proc. SPIE 6271, Modeling, Systems Engineering, and Project Management for Astronomy II, 62710I (26 June 2006); doi: 10.1117/12.671662

Published in SPIE Proceedings Vol. 6271:
Modeling, Systems Engineering, and Project Management for Astronomy II
Martin J. Cullum; George Z. Angeli, Editor(s)