The Anisoplanatic Point‐Spread Function in Adaptive Optics

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Abstract
The effects of anisoplanatism on the adaptive optics point‐spread function are investigated. A model is derived that combines observations of the guide star with an analytic formulation of anisoplanatism in order to generate predictions for the adaptive optics point‐spread function at arbitrary locations within the field of view. The analytic formulation captures the dependencies of anisoplanatism on aperture diameter, observing wavelength, angular offset, zenith angle, and turbulence profile. The predictions of this model are compared to narrowband 2.12 and 1.65 μm images of a 21'' binary (mv = 7.3, 7.6) acquired with the Palomar adaptive optics system on the 5 m Hale Telescope. Contemporaneous measurements of the turbulence profile made with a DIMM/MASS (differential image motion monitor/multiaperture scintillation sensor) unit are used together with images of the primary to predict the point‐spread function of the binary companion. Predicted companion Strehl ratios are shown to match measurements to within a few percent, whereas predictions based on the isoplanatic angle approximation are highly discrepant. The predicted companion point‐spread functions are shown to agree with observations to 10%. These predictions are used to measure the differential photometry between binary members to an accuracy of 1 part in 103, and the differential astrometry to an accuracy of 1 mas. Errors in the differential astrometry are shown to be dominated by differential atmospheric tilt jitter. These results are compared to other techniques that have been employed for photometry, astrometry, and high‐contrast imaging.