1 Introduction

NAOS-CONICA is the first adaptive optics (AO) system of the Very Large Telescope (VLT) and saw its first light in November 2001 (Brandner et al. 2002). The Nasmyth Adaptive Optics System NAOS (Rousset et al. 1998; Rousset et al. 2000) delivers diffraction-limited images to the Coudé Near Infrared CAmera CONICA (Lenzen et al. 1998; Hartung et al. 2000). To retrieve the maximum possible performance of the system in terms of Strehl ratio (SR) a method has been developed to calibrate the remaining degradation of the image quality induced by its optical components. Defaults of the wavefront attributed to any degradation within the AO loop (common path) are seen directly by the AO wavefront sensor (WFS) and thus the AO system can correct for these aberrations automatically. This is not the case for a degradation of image quality induced by components outside the AO loop. An experimental setup has been applied which allows one to sense the wavefront of the light which has passed the whole system without making use of the AO wavefront sensor. Therefore we draw on a well-known method called phase diversity (Gonsalves 1982; Paxman et al. 1992). It turns out that a number of theoretical and experimental constraints have to be examined before reliable results can be obtained in sensing the wavefront via phase diversity (PD). We focused on this in a precedent paper (Blanc et al. 2003), hereafter Paper I. In this second paper we first give a brief description of the instrument (Sect. 2). Then we focus on the experimental setup which enables us to calibrate the variety of beam splitters, filters and camera objectives. The design constraints for the implementation of PD are illustrated, and the resulting setup as well as the procedure to obtain the appropriate input data for PD are described (Sect. 3).

Because of the huge number of instrument modes it is not feasible to perform the PD calibration for each possible configuration. We explain how the wavefront degradations of the different optical components are disentangled. Then, the individual parts of the optical train can be calibrated separately and it is no longer required to do this for every possible combination. In detail, we will allocate the wavefront error to the dichroic mirrors of NAOS (beam splitter between wavefront sensor and imaging path), to the CONICA filters and camera objectives (Sect. 4).

Thereafter, the sensed wavefront errors are used to calculate the corresponding SRs. These are compared to the SRs directly determined from the images and the consistency is verified (Sect. 5). Finally, after presentation of the complete calibration procedure and its results, the measured wavefront errors are rendered in terms of Zernike coefficients to the AO system to demonstrate the gain in overall performance after closed loop correction (Sect. 6).