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Proceedings Paper

The infrared imaging spectrograph (IRIS) for TMT: volume phase holographic grating performance testing and discussion
Author(s): Shaojie Chen; Elliot Meyer; Shelley A. Wright; Anna M. Moore; James E. Larkin; Jerome Maire; Etsuko Mieda; Luc Simard
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Paper Abstract

Maximizing the grating efficiency is a key goal for the first light instrument IRIS (Infrared Imaging Spectrograph) currently being designed to sample the diffraction limit of the TMT (Thirty Meter Telescope). Volume Phase Holographic (VPH) gratings have been shown to offer extremely high efficiencies that approach 100% for high line frequencies (i.e., 600 to 6000l/mm), which has been applicable for astronomical optical spectrographs. However, VPH gratings have been less exploited in the near-infrared, particularly for gratings that have lower line frequencies. Given their potential to offer high throughputs and low scattered light, VPH gratings are being explored for IRIS as a potential dispersing element in the spectrograph. Our team has procured near-infrared gratings from two separate vendors. We have two gratings with the specifications needed for IRIS current design: 1.51-1.82μm (H-band) to produce a spectral resolution of 4000 and 1.19-1.37μm (J-band) to produce a spectral resolution of 8000. The center wavelengths for each grating are 1.629μm and 1.27μm, and the groove densities are 177l/mm and 440l/mm for H-band R=4000 and J-band R=8000, respectively. We directly measure the efficiencies in the lab and find that the peak efficiencies of these two types of gratings are quite good with a peak efficiency of ~88% at the Bragg angle in both TM and TE modes at H-band, and 90.23% in TM mode, 79.91% in TE mode at J-band for the best vendor. We determine the drop in efficiency off the Bragg angle, with a 20-23% decrease in efficiency at H-band when 2.5° deviation from the Bragg angle, and 25%-28% decrease at J-band when 5° deviation from the Bragg angle.

Paper Details

Date Published: 28 July 2014
PDF: 13 pages
Proc. SPIE 9147, Ground-based and Airborne Instrumentation for Astronomy V, 91478X (28 July 2014); doi: 10.1117/12.2055609
Show Author Affiliations
Shaojie Chen, Dunlap Institute for Astronomy and Astrophysics, Univ. of Toronto (Canada)
Elliot Meyer, Dunlap Institute for Astronomy and Astrophysics, Univ. of Toronto (Canada)
Univ. of Toronto (Canada)
Shelley A. Wright, Dunlap Institute for Astronomy and Astrophysics, Univ. of Toronto (Canada)
Univ. of Toronto (Canada)
Anna M. Moore, California Institute of Technology (United States)
James E. Larkin, Univ. of California, Los Angeles (United States)
Jerome Maire, Dunlap Institute for Astronomy and Astrophysics, Univ. of Toronto (Canada)
Etsuko Mieda, Dunlap Institute for Astronomy and Astrophysics, Univ. of Toronto (Canada)
Univ. of Toronto (Canada)
Luc Simard, NRC - Dominion Astrophysical Observatory (Canada)


Published in SPIE Proceedings Vol. 9147:
Ground-based and Airborne Instrumentation for Astronomy V
Suzanne K. Ramsay; Ian S. McLean; Hideki Takami, Editor(s)

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