Practical high-order adaptive optics systems for extrasolar planet searches

Direct detection of photons emitted or reflected by an extrasolar planet is an extremely difficult but extremely exciting application of adaptive optics. Typical contrast levels for an extrasolar planet would be 10 ⁹ - Jupiter is a billion times fainter than the sun. Current adaptive optics systems can only achieve contrast levels of 10 ⁶, but so-called "extreme" adaptive optics systems with 10 ⁴-10 ⁵ degrees of freedom could potentially detect extrasolar planets. We explore the scaling laws defining the performance of these systems, first set out by Angel (1994), and derive a different definition of an optimal system. Our sensitivity predictions are somewhat more pessimistic than the original paper, due largely to slow decorrelation timescales for some noise sources, though choosing to site and ExAO system at a location with exceptional r ₀ (e.g. Mauna Kea) can offset this. We also explore the effects of segment aberrations in a Keck-like telescope on ExAO; although the effects are significant, they can be mitigated through Lyot coronagraphy.