A non-linear curvature wavefront sensor reconstruction speed and the broadband design

Abstract

ABSTRACT In this paper we explain why a non-linear curvature wavefront sensor (nlCWFS) is more sensitivethan conventional wave front sensors such as the Shack Hartmann wavefront sensor (SHWFS) andthe conventional curvature wavefront sensor (cCWFS) for sensing m V < 14 natural guide stars.The non-linear approach builds on the successful curvature wavefront sensing concept but uses anon-linear Gerchberg-Saxton (GS) phase diversity algorithm to reconstruct the wavefront. The non-linear reconstruction algorithm is an advantage for sensitivity but a challenge for fast computation.The current speed is a factor of 10 to 100 times slower than needed for high performance ground-based AO. We present a two step strategy to increase the speed of the algorithm. In the lastpaper 3 we presented laboratory results obtained with a monochromatic source, here we extend ourexperiment to incorporate a broadband source. The sensitivity of the nlCWFS depends on theability to extract wavefront phase from diraction limited speckles therefore it is essential thatthe speckles do not suer from chromatic aberration when used with a polychromatic source. Wediscuss the design for the chromatic re-imaging optics, which through chromatic compensation,allow us to obtain diraction limited speckles in Fresnel propagated planes on either side of thepupil plane.Keywords: wavefront sensitivity, extreme-adaptive optics, non-linear curvature wavefront sensor,Shack-Hartmann wavefront sensor