The received wisdom is that a high-resolution spectrograph - R say ∼ 100, 000 or more - should be implemented at the Coude or Nasmyth focus, or fibre-fed in a fixed laboratory. This view evolved due to the large size of earlier classical Coude spectrographs, coupled with arguments of thermal stability and the preference for a fixed gravity vector. This paper documents a radical review of the situation, in the light of technical advances and the requirements of two specific telescope projects. We believe that the results will be widely applicable to other telescope.
We define the stability requirements for a high resolution spectrograph, then show how these can be met at Cassegrain by modern materials, mechanism design, thermal control and passive and active compensation for structural flexure. We consider the optimisation of the 'information-throughput of the spectrograph, in terms of slit-throughput, with the superb imaging performance of modern large telescopes and sites, new developments in image-slicers, the prospects for adaptive-optics feeds for spectrographs, and the internal transmission of the optics. We note that the Cassegrain feed is more efficient than Nasmyth, and retains pure polarisation performance.
We consider in detail the requirements of, and solutions for, high resolution spectrographs for two large telescope projects - the 6.5m MMT conversion and the two Gemini 8-metre telescopes. The converted MMT will have only a Cassegrain focus, and we report on our study of the high resolution spectrograph undertaken for the Harvard/Smithsonian Centre for Astrophysics. In the case of Gemini, the telescopes were previously designed with vertical 4-mirror Nasmyth feeds for the 'HROS' - the High Resolution Optical Spectrograph(s). We show how the studies of EROS provided basic information which opened the way for the Gemini Project radically to re-design the telescopes without Nasmyth foci, with HROS to be mounted at Cassegrain. This new design has resulted in reduced infrared emissivity of the telescope without prejudicing HROS efficiency, plus a major capital saving to the project.
|Title of host publication
|Instrumentation in Astronomy VIII
|David L. Crawford, Eric R. Craine
|Number of pages
|Published - 1 Jun 1994
|1994 Symposium on Astronomical Telescopes and Instrumentation for the 21st Century: Instrumentation in Astronomy VIII - Kailua, United States
Duration: 13 Mar 1994 → 18 Mar 1994
Conference number: 8
|Proceedings of SPIE - The International Society for Optical Engineering
|1994 Symposium on Astronomical Telescopes and Instrumentation for the 21st Century
|13/03/94 → 18/03/94