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Silicon immersion gratings (SIGs) offer several advantages over the commercial echelle gratings for high
resolution infrared (IR) spectroscopy: 3.4 times the gain in dispersion or ~10 times the reduction in the
instrument volume, a multiplex gain for a large continuous wavelength coverage and low cost. We
present results from lab characterization of a large format SIG of astronomical observation quality. This
SIG, with a 54.74 degree blaze angle (R1.4), 16.1 l/mm groove density, and 50x86 mm2 grating area, was
developed for high resolution IR spectroscopy (R~70,000) in the near IR (1.1-2.5 μm). Its entrance
surface was coated with a single layer of silicon nitride antireflection (AR) coating and its grating surface
was coated with a thin layer of gold to increase its throughput at 1.1-2.5 m. The lab measurements have
shown that the SIG delivered a spectral resolution of R=114,000 at 1.55 m with a lab testing
spectrograph with a 20 mm diameter pupil. The measured peak grating efficiency is 72% at 1.55 m,
which is consistent with the measurements in the optical wavelengths from the grating surface at the air
side. This SIG is being implemented in a new generation cryogenic IR spectrograph, called the Florida IR
Silicon immersion grating spectrometer (FIRST), to offer broad-band high resolution IR spectroscopy
with R=72,000 at 1.4-1.8 um under a typical seeing condition in a single exposure with a 2kx2k H2RG IR
array at the robotically controlled Tennessee State University 2-meter Automatic Spectroscopic Telescope
(AST) at Fairborn Observatory in Arizona. FIRST is designed to provide high precision Doppler
measurements (~4 m/s) for the identification and characterization of extrasolar planets, especially rocky
planets in habitable zones, orbiting low mass M dwarf stars. It will also be used for other high resolution
IR spectroscopic observations of such as young stars, brown dwarfs, magnetic fields, star formation and
interstellar mediums. An optimally designed SIG of the similar size can be used in the Silicon Immersion
Grating Spectrometer (SIGS) to fill the need for high resolution spectroscopy at mid IR to far IR (~25-300 μm) for the NASA SOFIA airborne mission in the future.
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