A Re-examination of High Resolution Spectrographs for Large Telescopes - The Cassegrain solution

David D. Walker; Alan S. Radley; Francisco Diego; Andrew Charalambous; Mark Dryburgh; Bruce C. Bigelow

doi:10.1117/12.176800

A Re-examination of High Resolution Spectrographs for Large Telescopes - The Cassegrain solution

David D. Walker, Alan S. Radley, Francisco Diego, Andrew Charalambous, Mark Dryburgh, Bruce C. Bigelow

Research output: Chapter in Book/Report/Conference proceeding › Conference contribution › peer-review

3 Citations (Scopus)

Abstract

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.

Original language	English
Title of host publication	Instrumentation in Astronomy VIII
Editors	David L. Crawford, Eric R. Craine
Publisher	SPIE
Pages	1083-1095
Number of pages	13
Volume	2198
ISBN (Print)	9780819414939, 081941493X
DOIs	https://doi.org/10.1117/12.176800
Publication status	Published - 1 Jun 1994
Externally published	Yes
Event	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

Publication series

Name	Proceedings of SPIE - The International Society for Optical Engineering
Publisher	SPIE
Volume	2198
ISSN (Print)	0277-786X
ISSN (Electronic)	1996-756X

Conference

Conference	1994 Symposium on Astronomical Telescopes and Instrumentation for the 21st Century
Country/Territory	United States
City	Kailua
Period	13/03/94 → 18/03/94

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Cite this

Walker, D. D., Radley, A. S., Diego, F., Charalambous, A., Dryburgh, M., & Bigelow, B. C. (1994). A Re-examination of High Resolution Spectrographs for Large Telescopes - The Cassegrain solution. In D. L. Crawford, & E. R. Craine (Eds.), Instrumentation in Astronomy VIII (Vol. 2198, pp. 1083-1095). (Proceedings of SPIE - The International Society for Optical Engineering; Vol. 2198). SPIE. https://doi.org/10.1117/12.176800

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abstract = "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.",

keywords = "Cameras, Spectrographs, Prisms, Telescopes, Gemini Observatory, Sensors, Large telescopes, Spectral resolution, Mirrors, Vignetting",

author = "Walker, {David D.} and Radley, {Alan S.} and Francisco Diego and Andrew Charalambous and Mark Dryburgh and Bigelow, {Bruce C.}",

year = "1994",

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doi = "10.1117/12.176800",

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Walker, DD, Radley, AS, Diego, F, Charalambous, A, Dryburgh, M & Bigelow, BC 1994, A Re-examination of High Resolution Spectrographs for Large Telescopes - The Cassegrain solution. in DL Crawford & ER Craine (eds), Instrumentation in Astronomy VIII. vol. 2198, Proceedings of SPIE - The International Society for Optical Engineering, vol. 2198, SPIE, pp. 1083-1095, 1994 Symposium on Astronomical Telescopes and Instrumentation for the 21st Century, Kailua, Hawaii, United States, 13/03/94. https://doi.org/10.1117/12.176800

A Re-examination of High Resolution Spectrographs for Large Telescopes - The Cassegrain solution. / Walker, David D.; Radley, Alan S.; Diego, Francisco et al.

Instrumentation in Astronomy VIII. ed. / David L. Crawford; Eric R. Craine. Vol. 2198 SPIE, 1994. p. 1083-1095 (Proceedings of SPIE - The International Society for Optical Engineering; Vol. 2198).

Research output: Chapter in Book/Report/Conference proceeding › Conference contribution › peer-review

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AU - Walker, David D.

AU - Radley, Alan S.

AU - Diego, Francisco

AU - Charalambous, Andrew

AU - Dryburgh, Mark

AU - Bigelow, Bruce C.

N1 - Conference code: 8

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N2 - 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.

AB - 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.

KW - Cameras

KW - Spectrographs

KW - Prisms

KW - Telescopes

KW - Gemini Observatory

KW - Sensors

KW - Large telescopes

KW - Spectral resolution

KW - Mirrors

KW - Vignetting

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M3 - Conference contribution

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SN - 081941493X

VL - 2198

T3 - Proceedings of SPIE - The International Society for Optical Engineering

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BT - Instrumentation in Astronomy VIII

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T2 - 1994 Symposium on Astronomical Telescopes and Instrumentation for the 21st Century

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Walker DD, Radley AS, Diego F, Charalambous A, Dryburgh M, Bigelow BC. A Re-examination of High Resolution Spectrographs for Large Telescopes - The Cassegrain solution. In Crawford DL, Craine ER, editors, Instrumentation in Astronomy VIII. Vol. 2198. SPIE. 1994. p. 1083-1095. (Proceedings of SPIE - The International Society for Optical Engineering). doi: 10.1117/12.176800

A Re-examination of High Resolution Spectrographs for Large Telescopes - The Cassegrain solution

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