Adaptive optics enables the use of coronographs with very small mask sizes. We have described a stellar coronograph that we built to work with the ESO COME-ON-PLUS adaptive optics system and we have given its performance in terms of dynamic range and detection capabilities. The detection limit for faint emissions have been discussed and quantified as a function of the distance from the central star. The critical step of the reduction procedure is the remove of the diffracted light from the occulted star. It is very sensitive to the quality of the adaptive optics correction, in terms of width of the PSF but also of regularity and stability of the wings. On this basis, longer total exposure times will enhance the resulting performance; and finally the use of apodizing masks are under study to improve rejection capability of the coronograph. The achieved performance of the present configuration is unprecedented in terms of angular resolution over the whole image and in the vicinity of the star. Both characteristics are critical in the observational study of various astrophysical objects. The first astronomical results will be described in Paper II.
Acknowledgements
We are grateful to all colleagues involved in the development of the adaptive optics coronographic mode, especially P. Kern, C. Marlot, V. Serpette and S. Wang. We also wish to thank N. Hubin, P. Léna, A. Vidal-Madjar, F. Malbet and J.-M. Mariotti for helpful discussions and M. Cullum for comments on the manuscript. Additional thanks are extended to the COME-ON-PLUS team as well as the ESO La Silla staff, especially M. Maugis, E. Prieto, P. Le Saux for their active help during the integration and tests of the coronograph on the 3.6 meter telescope.