Using a unique combination of empirical data collected simultaneously by the science camera (INGRID) and the wave front sensor in NAOMI plus the same night profiles of the turbulent layers measured by SLODAR, we discuss the accuracy of the analytic approach to modelling of AO performance. The WFS frames recorded for different atmospheric conditions allow us to make a detailed investigation of the influence of a restricted field of view and sampling of the WFS on the accuracy of the centre of gravity and its propagation to the residual variance. The predictions of Strehl, FWHM and FWHE derived for NAOMI+INGRID using our analytic approach are compared with on-sky performance demonstrated during the commissioning and science observations with NAOMI.
NAOMI is the AO system of the 4.2-m William Herschel Telescope on La Palma. It delivers near-diffraction-limited images in the IR, and a significantly improved PSF at optical wavelengths. The science cameras currently comprise an IR imager (INGRID), an optical integral-field spectrograph (OASIS) and a coronagraph which may be placed in the light path to either instrument. 19 science programmes were observed during 2002-3. Observing overheads are small, with as much as 60% of the night spent integrating on science targets. In late 2004 this year, the WFS will be equipped with a low-noise L3 CCD, giving a gain of a factor of 2 in S:N for faint guide stars. A Rayleigh laser guide star is under development, with first light expected summer 2006, providing a unique facility: AO-corrected optical integral-field spectroscopy anywhere on the northern sky.