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Closed loop wave front correction of low order Zernike polynomials has been demonstrated using a phase diversity wavefront sensor. The Lockheed-Martin Advanced Technology Center phase diversity brassboard was used to demonstrate low bandwidth correction of aberrations consisting of the Zernike polynomials describing focus, coma and spherical. The method of Lofdahl-Scharmer is used to estimate and correct fixed aberrations in an optical system. The General Regression Neural Network method is used to estimate slowly varying aberrations in the same optical system. Closed loop experimental results from these tests are presented.

We are developing a technique to measure segment misalignment of large telescopes based on wavefront estimation using phase-diverse images. We report the current results of an experiment to measure piston errors on the Keck II primary segmented mirror, through atmospheric turbulence, using phase-diverse phase retrieval. The segment piston errors are separated from the random turbulence by averaging phase estimates from many frames. Phase estimates from real data collected with segments intentionally moved in piston reproduce the observed speckle patterns well. However, average phase maps do not reveal the segment piston errors. Simulations show that the observed data were collected in a regime of turbulence where the current algorithm often fails, but would be expected to work very well when the adaptive optics system is operating. There is reason to believe that we can eventually make the algorithm work with these or similar data if apparent mismatches between the data and our current imaging model are removed.