The science potential of ALFA: Adaptive optics with natural and laser guide stars

Adaptive optics with laser guide stars is mandatory in order to make use of the full capabilities of 8-m class telescopes. However, progress has been slow in two particular areas: techniques for spectroscopy at diffraction limited resolution anti wavefront sensing on laser guide stars. ALFA - currently the only European laser guide star adaptive optics system, and the only one in the world open to guest observers - has made significant advances in both of these. In this paper we report on our first results from summer 1998, representing significant improvements over previous performance. We report on observations using natural guide stars which demonstrate that for bright stars (m(V) less than or similar to 8) ALFA can now reach K-band Strehl ratios in excess of 60% and easily resolve binaries at the diffraction limit of the telescope. We then present some of thf first integral field spectroscopy at diffraction limited scales, showing we are able to distinguish spectra of binary stars with a separation of only 0.26". We also discuss results from a wide field image, which indicate that useful correction (allowing binary stars and circumstellar dust shells to be resolved) can be achieved from a relatively faint star to a radius of at least 1'. Our last set of results include a correction on a galaxy using the laser guide star as the reference. The best result to date is of the galaxy UGC 1347 in Abell 262. Correcting tip-tilt on a star 41" away and higher orders on the laser, we achieved an increase in peak intensity of 2.5, and a reduction in FWHM from 1.07" to 0.40".