Characterizing the Adaptive Optics Off‐Axis Point‐Spread Function. I. A Semiempirical Method for Use in Natural Guide Star Observations

Even though the technology of adaptive optics (AO) is rapidly maturing,calibration of the resulting images remains a major challenge. The AOpoint-spread function (PSF) changes quickly both in time and position on thesky. In a typical observation the star used for guiding will be separated fromthe scientific target by 10" to 30". This is sufficient separation to renderimages of the guide star by themselves nearly useless in characterizing the PSFat the off-axis target position. A semi-empirical technique is described thatimproves the determination of the AO off-axis PSF. The method uses calibrationimages of dense star fields to determine the change in PSF with field position.It then uses this information to correct contemporaneous images of the guidestar to produce a PSF that is more accurate for both the target position andthe time of a scientific observation. We report on tests of the method usingnatural-guide-star AO systems on the Canada-France-Hawaii Telescope and LickObservatory Shane Telescope, augmented by simple atmospheric computersimulations. At 25" off-axis, predicting the PSF full width at half maximumusing only information about the guide star results in an error of 60%. Usingan image of a dense star field lowers this error to 33%, and our method, whichalso folds in information about the on-axis PSF, further decreases the error to19%.Comment: 29 pages, 9 figures, accepted for publication in the PAS