<title>Closure phase imaging with low-order adaptive optics</title>

Abstract

We examine the effects of low-order adaptive optics on the performance of closure-phase (nonredundant mask) imaging. In particular we investigate a system that uses a small number of plane tip-tilt correcting mirrors. These apply local adaptive corrections to the beams transmitted by a pupil plane mask, but do not correct for piston wave-front errors between the beams. We have identified the optimum sub-aperture size and science channel exposure time for a range of seeing conditions and light levels using numerical simulations. Our calculations predict improvements in the SNR of monochromatic power spectrum and bispectrum measurements by factors in the range 2-15 over their uncorrected values. For optical NRM imaging with large telescopes, this combination of sparse localized wavefront correction, rather than modal correction over the whole telescope aperture, is much more efficient in terms of maximizing the SNR improvement per adaptive degree-of-freedom. Such a zonal AO system represents a cost-effective strategy for extending the utility of closure phase imaging using a very small number of active optical components. Suitable astrophysical sources amenable to such an approach include evolved supergiant stars, long period variables, and other bright compact objects.