7 Conclusion

In Paper I we presented a guideline for a PD approach to calibrate static wavefront aberrations. An extensive investigation of system limitations and error sources has been carried out. This approach was shown to be very flexible and powerful for precise wavefront sensing using experimental data of the first VLT AO system NAOS-CONICA. In Paper II we have given a detailed description of its implementation at the instrument and presented the experimental results of the calibration data for a variety of observing configurations. Especially, we turned our attention to the disentanglement of the measured overall wavefront errors which allows a convenient allocation to the divers optical components and makes the calibration procedure feasible for an instrument with a huge number of possible configurations. The sensed wavefront errors expanded in Zernike coefficients have been used to quantify the image quality in terms of SR and be proven to be consistent with the SRs directly determined with the image data. Finally we gave a striking example of the acquired improvement of optical performance in comparing images with and without AO correction for static wavefront errors. It turned out that even starting with a very good image quality, we still could achieve a significant gain in terms of the SR ratio.

The design of the instrument control software is harmonized with this calibration procedure and the AO loop parameters are automatically updated when the instrument setup changes. This ensures that the utmost optical performance is provided for all the configurations. Additionally, this implementation of wavefront sensing can be used to monitor the optical quality and to alert for small degradations of optical performance. In general, for future high performance AO systems, the presented technique is of great importance to achieve the challenging science goals of the astronomical community. It should be included in the instrumental design at a very early phase.

Acknowledgements

We would like to thank Eric Gendron and Wolfgang Brandner for their patience and assistance in the fine tuning of the instrument and interpretation of the measurements during the commissioning runs. Furthermore, we thank Gert Finger for the fruitful discussions about the detector characteristics. We are thankful to Norbert Hubin for the assistance to this work on the part of the European Southern Observatory. This research was partially supported through a European Southern Observatory contract and the Marie Curie Fellowship Association of the European Community.