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Proceedings Paper

The infrared imaging spectrograph (IRIS) for TMT: reflective ruled diffraction grating performance testing and discussion
Author(s): Elliot Meyer; Shaojie Chen; Shelley A. Wright; Anna M. Moore; James E. Larkin; Luc Simard; Jerome Marie; Etsuko Mieda; Jacob Gordon
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Paper Abstract

We present the efficiency of near-infrared reflective ruled diffraction gratings designed for the InfraRed Imaging Spectrograph (IRIS). IRIS is a first light, integral field spectrograph and imager for the Thirty Meter Telescope (TMT) and narrow field infrared adaptive optics system (NFIRAOS). IRIS will operate across the near-infrared encompassing the ZYJHK bands (~0.84 - 2.4μm) with multiple spectral resolutions. We present our experimental setup and analysis of the efficiency of selected reflective diffraction gratings. These measurements are used as a comparison sample against selected candidate Volume Phase Holographic (VPH) gratings (see Chen et al., this conference). We investigate the efficiencies of five ruled gratings designed for IRIS from two separate vendors. Three of the gratings accept a bandpass of 1.19-1.37μm (J band) with ideal spectral resolutions of R=4000 and R=8000, groove densities of 249 and 516 lines/mm, and blaze angles of 9.86° and 20.54° respectively. The other two gratings accept a bandpass of 1.51-1.82μm (H Band) with an ideal spectral resolution of R=4000, groove density of 141 lines/mm, and blaze angle of 9.86°. The fraction of flux in each diffraction mode was compared to both a pure reflection mirror as well as the sum of the flux measured in all observable modes. We measure the efficiencies off blaze angle for all gratings and the efficiencies between the polarization transverse magnetic (TM) and transverse electric (TE) states. The peak reflective efficiencies are 98.90 ± 3.36% (TM) and 84.99 ± 2.74% (TM) for the H-band R=4000 and J-band R=4000 respectively. The peak reflective efficiency for the J-band R=8000 grating is 78.78 ± 2.54% (TE). We find that these ruled gratings do not exhibit a wide dependency on incident angle within ±3°. Our best-manufactured gratings were found to exhibit a dependency on the polarization state of the incident beam with a ~10-20% deviation, consistent with the theoretical efficiency predictions. This work will significantly contribute to the selection of the final grating type and vendor for the IRIS optical system, and are also pertinent to current and future near-infrared astronomical spectrographs.

Paper Details

Date Published: 28 July 2014
PDF: 12 pages
Proc. SPIE 9147, Ground-based and Airborne Instrumentation for Astronomy V, 91479C (28 July 2014); doi: 10.1117/12.2056839
Show Author Affiliations
Elliot Meyer, Dunlap Institute for Astronomy and Astrophysics, Univ. of Toronto (Canada)
Univ. of Toronto (Canada)
Shaojie Chen, Dunlap Institute for Astronomy and Astrophysics, Univ. of Toronto (Canada)
Shelley A. Wright, Dunlap Institute for Astronomy and Astrophysics, Univ. of Toronto (Canada)
Univ. of Toronto (Cambodia)
Anna M. Moore, Caltech Optical Observatories (United States)
James E. Larkin, Univ. of California, Los Angeles (United States)
Luc Simard, NRC - Dominion Astrophysical Observatory (Canada)
Jerome Marie, 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)
Jacob Gordon, Univ. of Toronto (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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