Noise analysis of interferometric piston phasing for adaptive optics with dilute pupil telescopes

The next few years will witness the construction of several new 6.5-10 meter class astronomical telescopes. Imaging with these large instruments not only requires us to gain better understanding of the nature of atmospheric phase distortions over large spatial scales but should also motivate the adaptive optics community to rethink the ways in which we sense wavefront structure. At infrared wavelenghts, direct interference of spatially coherent infrared starlight affords engineers of adaptive optical systems for filled apertures a ruler against which to measure all wavefront sensor designs. Direct interferometry tells us exacdy what we want to know, the relative value of the wavefront phase at various points across the telescope pupil.- Direct interferometry, implemented by splitting off a small fraction of the available photons using a suitable pupil mask, pins down phase difference values that can be used as verification for any wavefront sensing scheme. One technique investigated toward the goal of the application of interferometric techniques to filled aperture telescopes is described below. In this Fourier transform-based technique, direct interferometry provides phase information for the control of adaptive optics systems without the necessity for an independent wavefront sensor. Here we describe and analyze the noise properties of one technique for faint stellar sources.