A high resolution fibre-fed echelle spectrograph for the Southern African Large Telescope

The design of the Canterbury Extremely Large Echelle Spectrograph on the Telescope In Africa (CELESTIA) is currently in progress. This high-resolution fibre-fed echelle spectrograph will be used with the Southern African Large Telescope (SALT) which is currently under construction at the South African Astronomical Observatory,near Sutherland, South Africa. CELESTIA uses a mechanically aligned mosaic of two 304 x 408-mm echelle gratings, and cross-dispersion is achieved using two large prisms in double-pass. An extremely fast (f /0.65 in white light, f /2.2 in monochromatic light) camera with all-spherical surfaces based on the design of Epps and Vogt for Keck HIREs has been adapted for this spectrograph. The dispersive elements of CELESTIA may be placed in a helium-filled chamber with the first element of the camera serving as an optical window. A range of resolving powers from 23 000 to 100 000 will be possible with fibres having diameters of 300 to 400 mum and various combinations of fibre-exit micro-slits. It is possible to image the spectrum of a second object, the sky, or a calibration lamp at most resolving powers. Complete spectral coverage from 380 nm to 670 nm, and nearly complete coverage to 880 nm is possible with a mosaic of two 30.7 x 61.4-mm detectors. Some details of the expected performance of CELESTIA are presented. It is expected that the spectrograph will have a maximum efficiency of approximately 20%, not accounting for the atmosphere or telescope. This compares favourably with other existing spectrographs designed for large telescopes.