New techniques in astronomical multi-slit spectroscopy

Abstract

LDSS-2 is a low dispersion survey spectrograph for the William Herschel Telescope. It was constructed to meet an increasing demand for large scale statistical surveys of stellar and galactic populations. This thesis describes its design, construction and installation, together with additional research concerning the fabrication of multi-slit aperture masks.LDSS-2 has a similar optical system to its predecessor LDSS-1, but it can hold greater numbers of aperture masks, filters and grisms, and its control system is fully automated. This is expected to improve its observing efficiency and allow LDSS-2 to be run as a common-user instrument. A range of instrument characterisation tests were performed, and they confirmed that LDSS-2 is capable of meeting its astronomical objectives. Its peak efficiency with the medium-blue grism was found to be 20.4% at 5915A with half-power points at 4135A and 7225A.Six different types of aperture mask were investigated, of which two (photochemically etched and mechanically milled) were tested using LDSS- 2. Work was begun towards the development of laser-cut masks, including the procurement of a 16W carbon dioxide laser and a motorised X-Y table. At present, mechanically milled masks offer the best compromise between cost, lead time, flexibility and quality for an instrument such as LDSS-2. Movable-slit masks may be preferred if access to the focal plane is restricted, but at present they are limited to lower multiplex gains. Laser-cut masks appear to offer the most potential for development in the immediate future, but liquid crystal masks may also become competitive in the longer term.