Wide-field adaptive optics for deep-field spectroscopy in the visible

We study an adaptive optics (AO) system destined to feed an integral field spectrograph working over a field of view of 1 arcmin in the visible (central wavelength 750 nm). The goal is to increase by a factor of 2 the encircled energy in the corrected point-spread function with respect to seeing, in a 0.2-arcsec diameter. The scattered light due to the guide star(s) must be minimized because very long exposures of about 80 h (cosmological deep fields) are to be obtained. We show that both multi-natural and multi-laser guide star schemes with a single high-order deformable mirror (ground-layer correction) can fulfil the required specifications. However, only the laser-based approach provides high sky coverage at the Galactic poles, where most of the astronomical targets are to be observed. We show that the laser guide star system can be reconfigured to produce diffraction-limited images over a smaller field of view (10 arcsec), but still in the visible, yielding 20 mas spatial resolution. This performance is achieved through a novel AO control algorithm allowing to boost the on-axis performance.