Zenith‐Distance Dependence of Chromatic Shear Effect: A Limiting Factor for an Extreme Adaptive Optics System

Consider a perfect AO system with a very fine wavefront sampling interval anda very small actuator interval. If this AO system senses wavefront at awavelength, lambda_{WFS}, and does science imaging at another wavelength,lambda_{SCI}, the light paths through the turbulent atmosphere at these twowavelengths are slightly different for a finite zenith distance, z. The errorin wavefront reconstruction of the science channel associated with thisnon-common path effect, or so-called chromatic shear, is uncorrectable and setsan upper bound of the system performance. We evaluate the wavefront variance,sigma^2(lambda_{WFS},lambda_{SCI},z) for a typical seeing condition at MaunaKea and find that this effect is not negligible at a large z. If we requirethat the Strehl ratio be greater than 99 or 95%, z must be less than about 50or 60 deg respectively, for the combination of visible wavefront sensing andinfrared science imaging.Comment: To appear in 2006/12/01 issue of Ap