Natural Guide Star Adaptive Optics for the 4.2m William Herschel Telescope

The William Herschel Telescope (WHT) is a 4.2m telescope on La Palma, one of the Canary Islands. The University of Durham (UK) ELECTRA AO system and WHT Natural Guide Star (NGS) AO projects have entered a close collaboration to build a series of AO systems culminating in the WHT NGS system which is specified for common-user (facility) astronomical operation. The collaboration is intended to reduce technical risk through a staged development and to deliver various performance enhancements, based on the ELECTRA segmented mirror design, to the final system. The ELECTRA system, its performance and the project stages are described. The final NGS stage has an opto- mechanical design which permits AO correction to be performed at variable conjugate heights and to dither the science camera field without changing the science pupil. This system and its projected performance are described.