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Acquisition and guider unit for the Cassegrain focus of the 4.2-m William Herschel Telescope

The design of the acquisition and guider (A&G) unit in use on the 4.2-m William Herschel Telescope at Observatorio del Roque de los Muchachos since May 1989 is described and illustrated with drawings. The Herschel A&G system is an enlarged and improved version of the A&G unit for the 1.0-m Kapetyn telescope at the same observatory (Ellis, 1983). Facilities provided include full-field guiding with minimal obstruction for spectrographs and wide-field instruments (using a CCD camera to image the guide star), an acquisition facility, spectrograph slit viewing by reflection, a TV camera with a 6-position filter wheel, lamps for spectral comparison and flat fielding, a full-field (15-arcmin) auxiliary focal position at right angles to the beam, polarization calibrators, two 8-position filter wheels, and two 5-filter filter slides. The A&G unit is housed in a cylindrical steel structure of diameter 150 cm and depth 65 cm; the mechanical operation of the components is briefly characterized.