Near-infrared AO coronagraph design for giant telescopes

We describe the adaptive optical (AO) requirements, optical design, and expected performance of a near-infrared AO coronagraph for a 30 meter class giant telescope. The optical design of the instrument consists of back-to-back finite conjugate relays, each containing a collimated space between a pair of toric mirrors. The first collimated space contains the atmospheric dispersion compensator and the AO components, which are a tip/tilt mirror, a MEMS deformable mirror, and a beamsplitter for the wavefront sensing path. An occulting disk or similar focal plane mask is located at the intermediate image between the two relays, and a Lyot stop is placed at the pupil plane in the second relay. The required AO order of correction for a Strehl ratio of 0.9 at a wavelength of 2.2 microns is about 150 by 150, and the required control bandwidth is 42 Hz. The limiting magnitude at this level of performance is estimated to be 10.4.