Implementation of the chromatic phase diversity method on the SIRIUS test bench

As for large interferometers, the crucial point to solve is the cophasing issue. Indeed, the cophasing device must be both efficient and independent of the number of telescopes allowing a large capture range and accurate piston measurements while being easy to implement. We developed such a cophasing method named the Chromatic Phase Diversity (CPD) method. Actually, using three spectral channels, the CPD method can determine the piston errors without ambiguity and on a range much larger than ± half a wave. This method makes it possible to work whether in coherencing mode or in cophasing mode. We designed and implemented this method on the SIRIUS test bench1 at the Observatoire de la Cote d'Azur, France. We present the instrument design, the results obtained with the CPD method. The performances such as the achieved capture range, the accuracy of the piston values extraction and the attainable magnitude are described and analyzed.