Understanding and correcting low order residual static aberrations in adaptive optics corrected images

In theory, low order static wavefront aberrations should be easily measured and corrected by an Adaptive Optics (AO) system. In practice, there appear to be low order residual quasi-static aberrations in the natural and laser guide star AO corrected images taken with the NIRC2 science imager at the W. M. Keck Observatory. The evidence for these residuals comes mainly from the AO corrected images themselves, which at times include low order structure such as trefoil and astigmatism. Why are these wavefront errors apparently not seen by the wavefront sensor (WFS) and what can be done to correct or characterize them? Several hypotheses exist on the origin of the residuals, including WFS gain errors, structures in the telescope pupil, or variability and frequency of AO system calibration for non-common path aberrations. These hypotheses are discussed, as are experiments that test them. Mitigation strategies are also presented and evaluated in terms of their ability to improve the AO-corrected point spread function (PSF), or characterize the PSF for PSF-reconstruction purposes, while having minimal impact on observing operations.