MIRSI: A Mid-Infrared Spectrometer and Imager

Instrument Description and Specifications

MIRSI offers a large field of view (85 x 64 arcsec), diffraction-limited spatial resolution (0.8'' @ 10 microns at IRTF ), complete spectral coverage over the 8-14 microns and 17-26 microns atmospheric windows for both imaging and spectroscopy, and high sensitivity (20 and 100 mJy for a point source at 10 and 20 microns, respectively, for on-source integration time of 60 seconds) at IRTF . This system offers the unique ability to acquire both spectra and high-resolution, multi-wavelength images of an astrophysical source. This makes it possible to unambiguously correlate the spatial and spectral features observed in astrophysical sources and thereby reveal the key physical and chemical processes at work. MIRSI is uniquely suited to achieve our scientific goals, aimed at understanding young stellar objects and star formation, planetary and protoplanetary nebulae, starburst galaxies, and solar system objects such as planets, asteroids, and comets.

MIRSI characteristics

Spectral Range 2 - 28 microns
Pixel Scale (at IRTF) (diffraction-limited) 0.27 arcsec/pixel
Field of View (at IRTF) 85 x 64 arcsec
Spectral Resolution (imaging mode) up to 1% bandwidth
Spectral Resolution (spectroscopic mode) 200 @ 10 microns
100 @ 20 microns
Nominal point source sensitivity for 1-sigma
detection in 60 sec. integration
20 mJy @ 10 microns
100 mJy @ 20 microns
Optics Reflective

MIRSI sensitivity

Below is a table of the MIRSI sensitivities, see Kassis et al.(2008) for details.

Below is a plot of the Grism sensitivity at 8-14 microns: (also available in: PS PDF formats). The sensitivity estimate is for 1 min. of on-source integration. The sensitivity is likely better, since because of telescope drifts, the FWHM of the standard star during the calibration did not always fill the slit.

My estimate is that an average one sigma is 150 mJy in one minute of on-source integration from 8-13 microns. The noise will decrease with the square of the exposure time. The noise quoted is per pixel. Pixels sizes along the slit are 0.265 arcsec on sky. The R=200 slit width is equal to the diffraction limit at 8.0 microns at the IRTF.


The MIRSI dewar design was developed, under our direction, at Infrared Laboratories, Inc. The dewar was designed to look upward - it attaches directly to the telescope at the Cassagrain focus in order to eliminate intermediate, warm optics from the optical path. The cold plate attached to the optics and the detector is cooled to LHe temperature. The optics chamber is surrounded by LHe- and LN2-temperature shields, as well as a floating shield to minimize heat load on the cryogens. Dewar thermal properties were designed to exceed a hold time of 30 hours. Internal structure design, while mainting low thermal conductance, ensures that mechanical flexure on the telescope results in a shift of only 1/10 pixel per hour of observation.

High school Honors intern Evelyn Aguilar measures cryogen levels in the MIRSI dewar during vacuum pump out and thermal cool down. Optical vacuum chamber and up-looking window (here covered by an aluminum dessicant trap) are located in the box-shaped portion of the dewar. Cryogen vessels and upward-facing fill ports for LHe (left) and LN2 are located in the cylindrical extension.


MIRSI has an upward-looking dewar. The dewar contains a KRS-5 window in the optical path. An entrance aperture wheel allows imaging mode or slit (spectroscopic) mode. After passing through the window and aperture wheel, the beam reflects off a folding flat and an off-axis parabolic collimator mirror and is directed towards a pupil stop. The filter wheel nearest to the pupil holds a CVF and two grisms, while the other holds broad and narrow-band discrete filters. The filter wheels are controlled by cryogenic stepper motors. After passing through the pupil, the beam reflects off two off-axis asperical camera mirrors and is reimaged on the detector array at the desired image scale. The all-reflective design leads to a system that is achromatic over the full 2 - 28 micron range of detector sensitivity.

Filter Wavelength (microns) Width (%)
7.9 - 14.5 CVF-
N-band 10.646
12.28 H21.5
Grism Wavelength (microns)ResolutionSlit Size ('')
8 - 142000.6
17 - 261001.2

The two grisms cover 8 - 14 microns at a resolution of up to 200 (using a 0.6'' slit), and 17 - 26 microns range at a resolution of up to 100 (1.2'' slit). Filters for imaging consist of narrowband filters for both the 10 and 20 micron windows and a CVF operating from 7.9 - 14.5 microns (providing about 5% spectral resolution with our pupil size) to cover both feature and continuum wavelengths over the full spectral range of the instrument. Discrete filters include the stock mid-IR "silicate" set at 8.7, 9.8, 11.7, and 12.3 microns (~10% bandwidths), a filter for the 12.28 microns (1.5%) H2 emission line, and broad filters in N-band (10.6 microns, 46%) and 20.6 microns (37%). The hydrocarbon and fine-structure emission lines in the 10 micron window can be observed with the CVF. Also included are filters at 4.9 microns (21%), 24.8 microns (7.9%), and 18.4 microns (8%) filter. Plots of some of the MIRSI filter transmission curves are also available.

Views inside the optics box, with window looking upward. Note aperture wheel housing (upper left), cryogenic motors, filter wheel housing with pupil (middle box), and detector board (lower left).

Cryogenic motor control and drives for MIRSI aperture wheel and filter wheels.

Detector and Electronics

The MIRSI system is based on a new 320x240 Si:As IBC array recently developed for ground-based astronomy by Raytheon/SBRC (formerly Hughes/SBRC). Astronomical Research Cameras, Inc. implemented the original array controller and software, which we have improved and customized for infrared observations. DSP microprocessors and peripheral components control the array drive clocking and readout electronics. The array reads out in 16 channels simultaneously, with 16 bit A/D resolution. Readout time for a single pixel is ~2 microsec, with total frame time ~18 millisec. We slowed down the readout time to these speeds in order to reduce noise caused by inherent design limitations in the coadder boards. Images can be coadded and stored electronically in 32 bit/pix fast memory before downloading into the host computer. The host computer is a Sun PC, which is connected to the camera electronics via a fiber optic data link receiving at 50 Mbits/s. We have developed Java-based software to interface the camera with the telescope control system and observers.

      Detector array properties

Number of pixels 320 x 240
Pixel pitch50 microns
Material Si:As
Technology IBC
Operating temperature range 6-12 K
Peak quantum efficiency >= 45%
Dark current <= 100 e- (T = 6K)
Frame rates <= 1 kHz
Number of outputs 16
Well depth >= 3 x 10^7 e-
Read noise <= 1000 rms e-
Nonlinearity <= 10%
Operability 99%
Power dissipation ~65 mW

Testing the MIRSI camera preamplifier (black box), and array drive and readout elecronics. The logic analyzer in the Infrared Astronomy Lab displays array clocking voltages.

Related Links

MIRSI homepage
Introduction to MIRSI science
MIRAC: A Mid-Infrared Array Camera
BU Institute for Astrophysical Research