Polychromatic AO: Wavefront sensing and observing at different wavelengths

When performing adaptive optics (AO) correction for the effect of atmospheric fluctuations, different effects, related to measuring the wavefront at one wavelength and observing at another one, may show up. Some of these effects are related to the dispersive properties of the atmosphere. Others are related to diffraction, both regarding light propagation through the turbulent atmosphere to the entrance pupil of the telescope and regarding the inevitable Fraunhofer diffraction taking place for light propagating from the exit pupil to the image plane of the telescope. In this paper some of these effects are revised and discussed, in particular the way in which uncorrected wavefront errors in the point spread function (PSF) will scale with color, and the way in which dispersion affects the Strehl ratio and the background level of the AO corrected PSF. Also discussed is the trade-off between AO spectral bandwidth and Poisson noise