ING's instrument development plan identifies adaptive optics (AO) at short wavelengths (< 1 micron) as one of two areas where the WHT can remain competitive in the era of 8-m telescopes. (The other is wide-field spectroscopy.) Most 8-m and larger telescopes are equipped with AO systems to allow near-diffraction-limited observations, and it is expected that most will be upgraded to use laser guide stars (LGS) as well as natural guide stars, to increase sky coverage with AO to ~ 100%. This should be straightforward in the IR, but at observation wavelengths less than 1 micron, LGS AO on 8-m and larger telescopes is severely affected by the fact that the wavefronts from the science target and from the laser guide star sample different volumes of air (cylinder and cone respectively), and therefore different turbulence. Constellations of laser guide stars are required to sample the wavefront adequately.
On 4-m and smaller telescopes, good AO correction can be achieved with a single guide star even at short wavelengths. The aims of the development program at the WHT are therefore (1) to provide a common-user AO system delivering corrected wavefronts to dedicated optical and IR instruments, and (2) to provide a Rayleigh laser guide star, to raise sky coverage from a few % to ~ 100%.
The WHT's adapative-optics module, NAOMI, is mounted at the WHT's new (2003) AO-dedicated Nasmyth focus (GRACE). NAOMI itself was commissioned in 2000-2002. It delivers near-diffraction-limited images (FWHM ~ 0.15 arcsec, Strehl ~ 0.6 in K, comparable resolution in J, H) to the IR camera INGRID, and is expected to deliver FWHM ~ 0.2" in the optical, to the IFU spectrograph OASIS, commissioned in 2003. A coronograph, OSCA, was commissioned in 2002.
Laser guide stars have been launched through the telescope on several
occasions, and an attempt will be made to close the AO loop on a
Rayleigh laser guide star in Nov 2003.
Funding has been sought for a 25-W 523-nm Rayleigh laser guide star.
The CCI (the La Palma observatory's international governing body)
has welcomed the development of laser beacons on La Palma.
Chris Benn
(crb@ing.iac.es)
2001 Mar 21, last updated 2003 Sep 24