The useful field of view of an adaptive optics system

The corrected field of view of an adaptive optics system is thought to provide a severe limitation on the usefulness of an adaptive optics system. We show that under good seeing conditions, the useful field of view is an order of magnitude larger than the field predicted by the classical isoplanatic angle. Under excellent seeing conditions on Mauna Kea, the corrected field of view in the near-infrared of a high-order adaptive optics system is larger than current near-infrared detector arrays. The corrected field of view is experimentally shown to increase when using spatially degraded wavefront corrections. Two methods of spatially degrading the wavefront were tested: low-order modal wavefront reconstructions and the use of a low-altitude Rayleigh wavefront reference beacon. In both cases, the on-axis performance is sacrificed for an increased field of view. The optimal spatial order of the wavefront correction is shown to depend on the wavelength of the observation. In the K band, the total time to reach a fixed signal-to-noise ratio of a large field of faint objects is nearly independent of the wavefront correction order, while at shorter wavelengths the total exposure time is minimized by using increasingly higher order wavefront corrections. In this study, photometric errors due to changes in the point-spread function across the field are small even when the variations are modeled as a linear variation across the field.