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Proceedings Paper

GMTIFS: challenging optical design problems and their solutions for the GMT integral-field spectrograph
Author(s): John Hart; Gabe Bloxham; Robert Boz; Brady Espeland; Robert Sharp
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Paper Abstract

GMTIFS is a first generation instrument for the Giant Magellan Telescope (GMT). It is a combined Imager and Integral Field Spectrograph (IFS) designed to work with the Adaptive Optics (AO) Systems of the GMT. Working at the diffraction limit of the GMT and satisfying the challenging AO interface requirements and constraints results in unique optical challenges. We describe two of these challenges and how we have addressed them. The GMT has a direct feed architecture that maximizes transmission and reduces emissivity. This means that the cryostat window is tilted to reflect visual wavelengths to the external Visual Wave Front Subsystem (VWS). For a plane-parallel window, this tilt causes astigmatism in the transmitted beam that must be corrected. A corrective system using two plates, tilted and slightly wedged in opposite directions, is used. Geometry and performance of the system is described. Another challenging problem is the optical design of the anamorphic field projector. The Integral Field Unit of GMTIFS requires that a small field delivered to it be projected onto an Image Slicer at much larger scale, with the magnification in the spectral direction being twice that in the spatial direction so that the spaxels are square when referred to the sky. Output images must be coincident in the spectral and spatial projections in both the field and pupil domains. Additionally, field and pupil image locations must be independently controllable so that they can be made coincident for interchangeable units that provide a range of output field scales. A two-mirror system that satisfies these requirements is described.

Paper Details

Date Published: 9 August 2016
PDF: 15 pages
Proc. SPIE 9908, Ground-based and Airborne Instrumentation for Astronomy VI, 99089F (9 August 2016); doi: 10.1117/12.2231552
Show Author Affiliations
John Hart, The Australian National Univ. (Australia)
Gabe Bloxham, The Australian National Univ. (Australia)
Robert Boz, The Australian National Univ. (Australia)
Brady Espeland, The Australian National Univ. (Australia)
Robert Sharp, The Australian National Univ. (Australia)

Published in SPIE Proceedings Vol. 9908:
Ground-based and Airborne Instrumentation for Astronomy VI
Christopher J. Evans; Luc Simard; Hideki Takami, Editor(s)

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