We are designing a sensitive high resolution (R=60,000-100,000) spectrograph for the Giant Magellan Telescope
(GMTNIRS, the GMT Near-Infrared Spectrograph). Using large-format IR arrays and silicon immersion gratings, this
instrument will cover all of the J (longer than 1.1 μm), H, and K atmospheric windows or all of the L and M windows in
a single exposure. GMTNIRS makes use of the GMT adaptive optics system for all bands. The small slits will offer the
possibility of spatially resolved spectroscopy as well as superior sensitivity and wavelength coverage. The GMTNIRS
team is composed of scientists and engineers at the University of Texas, the Korea Astronomy and Space Science
Institute, and Kyung Hee University. In this paper, we describe the optical and mechanical design of the instrument. The
principal innovative feature of the design is the use of silicon immersion gratings which are now being produced by our
team with sufficient quality to permit designs with high resolving power and broad instantaneous wavelength coverage
across the near-IR.
The Korea Astronomy and Space Science Institute (KASI) and the Department of Astronomy at the University of Texas
at Austin (UT) are developing a near infrared wide-band high resolution spectrograph, IGRINS. IGRINS can observe all
of the H- and K-band atmospheric windows with a resolving power of 40,000 in a single exposure. The spectrograph
uses a white pupil cross-dispersed layout and includes a dichroic to divide the light between separate H and K cameras,
each provided with a 2kx2k HgCdTe detector. A silicon immersion grating serves as the primary disperser and a pair of
volume phased holographic gratings serve as cross dispersers, allowing the high resolution echelle spectrograph to be
very compact. IGRINS is designed to be compatible with telescopes ranging in diameter from 2.7m (the Harlan J. Smith
telescope; HJST) to 4 - 8 m telescopes. Commissioning and initial operation will be on the 2.7m telescope at McDonald
Observatory from 2013.
TEXES is a high resolution mid-infrared spectrograph acting as a visiting instrument at the NASA IRTF and Gemini
North telescopes. Switching from high (R=100,000) to medium resolution (R=15,000), the noise becomes detector
dominated due to charge cascades in the low-background detector. This is remedied by lowering the bias voltage across
the detector, a problem during observing runs as it requires manual adjustment within the electronics. To save telescope
time, a bias selector has been created to allow the bias voltage to be switched from the control room between four
options. The options were chosen based upon signal to noise vs. bias voltage testing.
The Echelon-cross-Echelle Spectrograph (EXES) will provide the Stratospheric Observatory for Infrared Astronomy (SOFIA) with high spectral resolution capabilities in the mid-infrared. EXES will have a maximum spectral resolving power of 100,000 along with lower resolution options (R=10,000; R=3000). EXES on SOFIA will provide sensitivity and spectral resolution never before available from an orbital or sub-orbital platform. Because of the wealth of molecular features in the EXES spectral range, 4.5 to 28.3 μm, and the dramatic reduction in telluric atmospheric interfence provided by SOFIA, EXES will be particularly relevant for studies of the solar system, star formation and the interstellar medium. We report on the EXES design and current status, provide descriptions of observing modes and sensitivity estimates, discuss the calibration and likely data products, and describe the potential gains of incorporating a 1024 × 1024, low-background, Si:As detector array.
A feasibility design study was undertaken to assess the requirements of a mid-infrared echelle spectrograph (MIRES)
with a resolving power of 120,000 and its associated mid-infrared adaptive optics (MIRAO) system on the Thirty-Meter
Telescope. Our baseline design incorporates a 2K×2K Si:As array or array mosaic for the spectrograph and a 1K×1K
Si:As array for the slit viewer. Various tradeoffs were studied to minimize risk and to optimize the sensitivity of the
instrument. Major challenges are to integrate the spectrograph to the MIRAO system and, later, to an adaptive
secondary, the procurement of a suitable window and large KRS-5 lenses, and the acquisition of large format mid-IR
detector arrays suitable for the range of background conditions. We conclude that the overall risk is relatively low and
there is no technical reason that should prevent this instrument from being ready for use at first light on the Thirty-
We present a discussion of the science drivers and design approach for a high-resolution, mid-infrared spectrograph for
the Thirty-Meter Telescope. The instrument will be integrated with an adaptive optics system optimized for the midinfrared;
as a consequence it is not significantly larger or more complex than similar instruments designed for use on
smaller telescopes. The high spatial and spectral resolution possible with such a design provides a unique scientific
capability. The design provides spectral resolution of up to 120,000 for the 4.5-25 μm region in a cross-dispersed format
that provides continuous spectral coverage of up to 2% to 14 μm. The basic concept is derived from the successful
TEXES mid-infrared spectrograph. To facilitate operation, there are separate imaging channels for the near-infrared and
the mid-infrared; both can be used for acquisition and the mid-infrared imaging mode can be used for science imaging
and for guiding. Because the spectrograph is matched to the diffraction limit of a 30-m telescope, gains in sensitivity are
roughly proportional to the square of the telescope diameter, opening up a volume within the Galaxy a thousand times
greater than existing instruments.
TEXES is a versatile mid-infrared spectrograph, which has been used on the NASA IRTF since 2000. It is capable of high spectral resolution (R ≈ 100,000), which is well suited for observations of interstellar and circumstellar molecules and ions, as well as molecules in planetary and stellar atmospheres. It has been installed on Gemini North where its point source sensitivity is expected to improve by a factor of 7, and its angular resolution will improve by 8/3.
TEXES, the Texas Echelon Cross Echelle Spectrograph, is an ideal instrument to study molecular clouds at a spectral resolving power of 100,000 between 5 and 25 μm. In many molecular clouds, high extinction often means that no visible stars are available for off-axis guiding. At a resolving power of 100,000, only the very brightest sources can be observed while guiding on the power in the dispersed IR spectra.
We present the design of a high-speed on-axis guider for TEXES operating at 3.65 μm, a wavelength outside the spectrometer operating band where many of the target sources are still detectable for imaging. We use a new technology gold nanomesh resonant IR filter/mirror from EDTEK, that transmits 3.65 μm light to the guide detector with a peak transmittance of 60% while reflecting light from 5 μm long-ward with 98% efficiency to the dispersing elements in the spectrograph. A PC controls clocking patterns for the CRC-463 detector from Raytheon Infrared Operations and the analog to digital conversion of signals with a 14 bit A/D card. Image centroiding is done in software and then offsets are sent to the telescope for pointing adjustments or tip-tilt corrections when a tip-tilt secondary is available.
This system is a prototype designed to test the feasibility of a similar guider for EXES, the Echelon Cross Echelle Spectrograph, mounted on SOFIA, the Stratospheric Observatory for Infrared Astronomy.
TEXES, the Texas Echelon Cross Echelle Spectrograph, is a versatile mid-infrared (5-25 μm) spectrograph that can be used in several operating modes: high resolution cross-dispersed, with a resolving power of R = 50-100,000; medium resolution long-slit, with R~15,000; low resolution long-slit, with R~3000; source acquisition imaging; and pupil imaging.
The design of TEXES is unique in several respects. The primary disperser is a variation on an echelon, a steeply and coarsely blazed (R10 with 0.3 inch groove spacing), 36 inch long, diamond-machined aluminum grating. The cross disperser is an R4 echelle used in low order at R2, with the grooves acting as corner reflectors. Cold mechanisms allow the echelon to be bypassed to use the cross disperser in long-slit mode. A first-order grating can be inserted in front of the echelle for lower resolution. In addition, the low resolution grating can be turned face-on to act as a mirror allowing source-acquisition imaging and pupil viewing.
TEXES has been used for 8 nights on the McDonald Observatory 2.7m and 45 nights on the NASA IRTF 3m telescopes over the last 2 1/2 years. Sources observed include planets and planetary satellites, stellar atmospheres, circumstellar outflows and disks, and molecular clouds and HII regions in the Milky Way and external galaxies.
The Stratospheric Observatory for Infrared Astronomy (SOFIA) will provide new opportunities for high spectral resolution observations in the mid-infrared. To take advantage of these opportunities, we are developing EXES, the Echelon-Cross-Echelle Spectrograph. EXES will operate from 4.5 microns to 28.5 microns and achieve a velocity resolution of 3 km/s for λ < 10 μm. EXES will be a versatile instrument with three spectroscopic modes: cross-dispersed with R~10<sup>5</sup>; long-slit with R~10<sup>4</sup>; and long-slit with R~3000. The unique aspect of EXES is the high-resolution capability provided by a 1 meter echelon grating and a 256 by 256 low-background Si:As IBC detector. Much of the design and operation of EXES has already been validated by the performance of a very similar ground-based instrument, the Texas Echelon-Cross-Echelle Spectrograph (TEXES). We present here a summary of the EXES design and current status; a brief description of ground-based, high spectral resolution, mid-infrared results; and a look ahead to the possible science using SOFIA and EXES.
We are developing a high spectral resolution grating spectrograph as a PI instrument for the Stratospheric Observatory for Infrared Astronomy. The Echelon-Cross- Echelle Spectrograph (EXES) will operate at 5.5 - 28.5 micrometers in three spectroscopic modes: R approximately 10<SUP>5</SUP>, 2 X 10<SUP>4</SUP>, and 4000. We use an echelon, a coarsely ruled, steeply blazed diffraction grating to achieve high resolution. Cross-dispersion is done with an echelle used at relatively low order. The detector is a 256 X 256 pixel Si:As IBC array. A very similar instrument, the Texas Echelon-Cross-Echelle Spectrograph (TEXES), has had a successful telescope run at the McDonald Observatory. We give here a progress report on the design of EXES and the status of TEXES.
To study narrow features in quiescent molecular clouds, a high spectral resolution, high sensitivity instrument is required. We present the design and capabilities of a mid-IR spectrograph being built as a PI instrument for the Stratospheric Observatory for IR Astronomy. The Echelon- Cross-Echelle Spectrograph (EXES) will operate from 5.5-28.5 micrometers in three spectroscopic modes: R approximately 10<SUP>5</SUP>, 10<SUP>4</SUP>, and 1500. EXES is similar in concept to our ground based instrument, TEXES. EXES consists of three chambers. The first chamber contains focal-reducing optics. The second chamber houses the high resolution grating, an echelon. Discussion of the echelon can be found elsewhere in this volume. The third chamber contains an echelle grating and a first order grating mounted back-to-back. A flip mirror selects operating mode by either directing light into the echelon chamber or allowing the light to pass through to the grating chamber. Re-imaging optics upstream of the detector provide two plate scales. The detector, a 256 by 256 pixel Si:As IBC, is in a baffled subsection of the third chamber. Finally, the low resolution grating serves as a slit positioning camera when it is rotated face on.
A new mid-IR spectrograph, the Texas Echelon Cross Echelle Spectrograph (TEXES) is under construction. The primary motivation for TEXES is to observe interstellar molecules at very high resolution. TEXES will operate at 7-25 micrometers wavelength with three spectrographic modes: a high resolution cross-dispersed mode, with R approximately equals 100,000, a mid-resolution long-slit mode, with R approximately equals 14,000, and a low resolution long-slit mode, with R approximately equals 2000. In hi-res mode, the primary disperser is a 36 inch long, R10 grating with a 7 mm groove spacing. The echelon is cross-dispersed with a 7 in long R2 echelle. In mid-res mode, the echelon is by-passed with an Offner relay, and the echelle is used by itself. In lo-res mode, a first-order grating is inserted over the echelle. For initial test, TEXES will use a Hughes Aircraft 20 X 64 pixel Si:As impurity-band array, which covers only two echelon orders. It will later be replaced with a 256 X 256 pixel array, which will Nyquist sample approximately 10 orders. The spectrograph has been assembled and tested with a partially complete echelon, demonstrating the soundness of the design. When we began this project, we were unable to find a vendor capable of machining or ruling a diffraction grating with the very coarse ruling required. Consequently, we attempted to hand-fabricate the echelon. We have not succeeded in assembling the echelon with the required precision, missing by about a factor of two. Fortunately, Hyperfine, Inc. is now capable of diamond machining the echelon. We are purchasing a machined echelon, and hope to complete the spectrograph by the end of summer 1998.
In March, 1988 the Science Steering Committee of the California Association for Research in Astronomy selected four instruments which were to be built by the time the Keck Telescope received first light. A long wavelength infrared camera was one of the instruments chosen and is described here. The camera was divided into two parts, (1) the detector, the optics and the dewar; (2) the electronics and the computers.
A long-wavelength IR camera which is currently being built for the 10-m Keck Telescope is described, with attentions given to its overall configuration and to the optics, the dewar, the detector, and the electronics. Special consideration is given to the instrument control system and the software architecture for the remote operation of the Keck long-wavelength-camera detector. The instrument parameters and a schematic diagram of the camera are included.