ALFA: Adaptive optics for the Calar Alto observatory optics, control systems, and performance

The adaptive optics system ALFA differs in some aspects from systems like A DONIS and PUEO which have delivered scientific results since years. Interch angeable lenslet arrays with different numbers of subapertures and a deform able mirror with many more actuators than the number of corrected modes res ult in some peculiarities in the calibration of the system and the reconstr uction of incident wavefronts. We describe the design of ALFA's optics and its modal control architecture with a focus on a comparative study of the p erformance of different mode sets used to correct the wavefront aberrations . An outlook on our plans to improve and simplify the use of ALFA is given. The last section is dedicated to issues related to observing with ALFA in its present state. Expected Strehl ratios for different seeing conditions a nd guide star magnitudes are summarized in a table. AO observations in gene ral, direct imaging and doing spectroscopy with ALFA in particular are disc ussed.