High Resolution Échelle Spectrographs for the Anglo-Australian Telescope Coude Focus and William Herschel Telescope Nasmyth Focus

The échelle spectrographs for the Anglo-Australian Telescope (AAT) and for the William Herschel Telescope (WHT) are being developed following the same basic design, which in many respects, differs from previous echelle instruments. The purpose of this paper is to document some of the more innovative concepts for the benefit of future designers and prospective observers. The instruments were originally conceived around the idea of matching the spectrum format to the features of a particular detector-the University College London Image Photon Counting System (UCL-IPCS). Since construction started, very large CCD's have appeared on the horizon and this has called for certain modifications which are also discussed. Attention is drawn to various tactics for maximising throughput, including the new concept of a variable beam size. The arguments in favor of very large fused silica prisms for cross dispersion are summarised. The echellogram will be displaced in both dispersion directions on the detector and the method of accomplishing this is outlined. A novel automatic check of collimation is also described, together with an overview of important spectrograph functions such as those in the pre-slit area.