Development of the Mid-Infrared High Dispersion Spectrograph (IRHS)
Y. Hirahara,1 T. Hirao, 1 T. Oka 1, H. Tokoro 2, N. Ebizuka, and K. Kawaguchi 3
1Graduate School of Environmental Studies, Nagoya University, Japan
2
Nano-Optonics Research Institute, Japan3
Department of Chemistry, Okayama University, Japan

High-dispersion infrared observations are useful for the definite identification of molecularspecies in the interstellar medium (ISM) by their vibration-rotation transitions. According toradio wavelength observations, molecular transitions in the ISM are Doppler-broadened, withline widths of 2~20 km/s in typical high-mass star forming regions and circumstellar
envelopes of late type stars. Due to these broadenings, the wavelength resolving power R=λ/Δλ=15,000~150,000 is required for sensitive detection of the spectra. In order toachieve such a high resolution in infrared region, Fourier transform spectroscopy (FTS) has
usually been employed. In the mid-infrared region (8~13μm), however, the dominant noise
source is the background blackbody radiation, so multiplex property of FTS is no longer
advantageous toward fainter astronomical sources.
Recently, we have developed the “mid-InfraRed High dispersion Spectrograph” (IRHS), acryogenic échelle spectrometer designed for Nasmyth focus stage of NAOJ Subaru 8.2m
telescope, which operates at 8-13μm in wavelength with the resolving power λ/Δλ of>50,000 at 10μm. To reduce thermal radiation of the instrument, all optics of IRHS isarranged on the cold optical base plate (~30K) of the cryostat with the diameter of 80cm. In
order to achieve high dispersion, broad bandwidth, and high sensitivity, two key devices areemployed: a single crystal germanium immersion echelle grating (30×30×72mm) and a Si:AsIBC (Impurity Band Conductor) focal plane array (FPA) detector (412×512 pixels, unit pixelsize 30μm) operated at 5 K[1]. The most important key device, germanium immersion
échelle grating for collimated beam size of 28mmφ was fabricated by utilizing ultra precision
micro-grinding method coupled with “ELectrolytic In-process Dressing” (ELID) technique[2].
The immersion grating has 600 μm groove spacing with a 68.75° blaze angle. It is used invery high order, ranging from 332 at 13.5μm to 597 at 7.5 μm. In order to separate theéchelle orders by ~10 pixels on the Si:As array detector, the cross-disperser wheel employs
two first-order echelette gratings: a 100 grooves/mm with a 26.7° blaze angle for shortwavelength (7.5–10 μm) and a 61.97 grooves/mm with a 18.1° blaze angle for long
(10–13.5 μm). The spectral coverage is typically λ/15, which includes 32–56 échelleorders, depending on wavelength. Eight cross-disperser position angles give full continuous
coverage throughout 7.5–13.5 μm.
The optics on the spectrometer was aligned by using an attenuated CO2 laser as a light source.
In the alignment procedures at room temperature condition, an infrared bolometer camera wasreplaced with the Si:As IBC FPA detector. Since the CO2 laser was operated in multi-mode
oscillation, simultaneous observation by using the FT-IR spectrometer (BioRad FTS-6000)
was necessary to assign the laser transitions. After the alignment, placing Si:As IBC arrayback to the original position, we examined the sensitivity tests of IRHS by recording theblackbody radiation around the 10.6 μm wavelength region at the room temperature (~300K). It took approximately 3 seconds to saturate the Si:As IBC detector, which is wellcompared with our design specification and the full-well capacity of the detector. For thecheck of the wavelength resolution, we observed the spacially diffracted light from the CO2
laser source by the #3000 sandpaper. Figure 1 shows the échelle image thus obtained ofIRHS in the region of 904 – 967 cm-1 (10.3-11.1μm in wavelength). The HWHM linewidthfor the most faintly observed P(20) laser transition to 0.018 cm-1, assuming that one pixel

corresponds to 0.0052 cm-1. The currently derived resolving power, ~50,000, is in an
excellent agreement with a design specification of our spectrograph.

411st
412nd
413rd

422nd
423rd

431st

Slow scanner readout direction

421st

432nd

Figure 1: The first échelle image of IRHS for the the CO2 laser, taken with Si:As IBC 512 x 412 FPA
detector cooled to 5.7K. The exposure time for each pixel is uniformly 0.17 second. The abscissa for
the image (412 pixels) represents direction for the fast scanner readout of the detector. The ordinate for theimage (512 pixels) represents the direction for slow scanner readout, and that for dispersion for thecross-disperser. Diffraction orders for the Ge immersion grating separated by the cross-disperser are also
shown. The assignments for the dark spots for the CO2 laser are shown on the image. The backgroundstripes are due to the thermal emission on the room temperature. Note that the laser spots except for P(20)and probably P(26) are heavily saturated and the spot sizes for the laser lines are significantly larger than
the intrinsic resolution of the spectrograph, since the laser power during the measurement is stiil very high.

References[1]
H. Tokoro, M. Atarashi, M. Omori, T. Machida, S. Hirabayashi, H. Kobayashi, Y. Hirahara, T. Masuda,
N. Ebizuka, K. Kawaguchi, “Development of Mid-Infrared High Dispersion Spectrograph (IRHS) for the Subaru
Telescope", Proc. SPIE, 4841, 1016 (2003).
[2]
N. Ebizuka, S. Morita, Y. Yamagata, H. Omori, M. Wakaki, K. Kawaguchi, H. Kobayashi and Y.
Hirahara, “Development of Immersion Gratings for Near and Middle Infrared High Dispersion Spectrograph of8.2 m Subaru Telescope”, Proc. SPIE 4837, 293 (2003).