MIRSI - IRTF's Mid-Infrared Imager and Spectrometer
MIRSI is a 2-20 micron camera and grism spectrograph built by Boston University. MIRSI was recently upgraded by IR Labs with a closed-cycle cooler to replace its liquid nitrogen and liquid helium cryostat, allowing MIRSI to be used interchangeably with any other IRTF instrument. It is currently being further refurbished by IRTF with a new array controller and an optical channel called MOC (similar to MORIS on SpeX). MOC will allow simultaneous visible light imaging with MIRSI and serve as a visible light guider for MIRSI.
- Spectral range: 2 to 20 microns
- Plate scale: 0.27 arcseconds per pixel
- Field of View: 85 x 64 arcseconds
- Spectral resolution (imaging): up to 1% of bandwidth with CVF
- Spectral resolution (spectroscopy): R=200 at 10 microns
R=100 at 20 microns
- Point source sensitivity: 20 mJy at 10 microns
(1 sigma in 1 minute) 100 mJy at 20 microns
- Optics: All reflective
- Entrance Window: ZnSe
The capabilities for 2022A are listed here.
MIRSI is in the process of being upgraded. The overall goal of the upgrade is to improve reliability and usefulness of MIRSI, while reducing operating cost. The changes to MIRSI are:
Replacing the LHe cryogen can with a Sumitomo closed cycle cooler. With the LHe cooling, MIRSI would only be cooled for a roughly week long observing run, then allowed to warm until its next run months later. With the close cycler cooler, MIRSI will be kept cold at all times, and be able to be used interchangeably with any other IRTF instrument.
New array controller. The previous controller died, and the PCI card was a pain to deal with. Both have been replaced.
New chopper. The legacy chopper electronics were approaching their 40th birthday, and were becoming difficult to support. A new chopper actuator, based on a piezo actuator, and modern control electronics, are being developed on the existing chopper hardware.
MOC: Visible light CCD camera that allows guiding or simultaneous visible light and mid-IR observations. To support MOC, a cold dichroic was added to the 'nose' of MIRSI.
The entrance window was changed from KRS-5 to ZnSe (see note below), due to the higher transmission and superior durability of ZnSe.
Acknowledgements Observers publishing results obtained with MIRSI are requested to reference the following papers:
MIRSI, A MId-Infrared Spectrometer and Imager: Performance Results from the IRTF
M. Kassis, J. Adams, J. Hora, L. Deutsch, E. Tollestrup, 2008, PASP, 120, 1271
MIRSI: A MId-Infrared Spectrometer and Imager
L. Deutsch, J. Hora, J. Adams, M. Kassis, 2003, SPIE, 4841, 106
The original Boston University web page for MIRSI
Filters and Sensitivity
The filters in MIRSI are listed in the following table. The center wavelength, short wavelength, long wavelength, bandwidth, and peak transmission were measured in 2018 at IfA.
We anticipate that the sensitivity of MIRSI post upgrade will be similar to MIRSI before it came off of the telescope (with an important caveat due to the entrance window, see below). This is Table 5 from Kassis et al. (2008)
This is Table 3 from Kassis et al. (2008)
Typical Exposure Times
This is Table 4 from Kassis et al. (2008)
MIRSI Optical Camera (MOC)
MOC is a visible light CCD camera that is co-mounted with MIRSI. The cold dichroic that was added to MIRSI reflects the IR beam into MIRSI, while the visible light passes through to MOC mounted below MIRSI. It uses the same model Andor EMCCD camera as MORIS, and thus its use and sensitivity will be very similar. Like MORIS, it will have a 1 arcminute field of view.
Simultaneous visible light and mid-IR observations of a given target. This combination is especially useful to determine the albeidos of asteroids.
Visible light guiding on the science target or a field star. This will help to put a target onto MIRSI's slit, and to keep it there. For imaging faint targets, we expect that observers will be able to use the MOC images to determine how to align their MIRSI images. This will allow 'blind' stacking of MIRSI images; the target would not need to be visible in each chop/nod quad and thus fainter targets could be observed.
MOC's Estimated Sensitivity (50 sigma in 1 minute)
Before MIRSI came off of the telescope, the entrance window was made from KRS-5. The two windows were badly damaged by exposure to moisture, and the effort to repair them was not successful. While KRS-5 is transmissive across a wide spectral range, it is highly vulnerable to moisture damage, and this is almost inevitable in the dome environment. KRS-5 also has about 30% emissivity across the mid-IR, significantly adding to the sky background.
For these reasons, we chose to make new zinc selenide entrance windows. The advantages are that ZnSe is transparent in more of the visible (helpful for MOC), is easier to polish, has higher transmission (and lower emissivity) at N-band, and is much more weather resistant. The disadvantage is that its transmission drops at 18 microns, and is very low at 20 microns, and essentially opaque beyond 20 microns. This strongly affects observing in the Q-band, but we felt that the trade off was worth it. As such, we expect that the sensitivity of MIRSI in the Q-band will be worse than stated in Table 5. The sensitivity of the narrow 18.5 micron filter may not be strongly affected, but the other Q-band filters and grism would be.
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Last modified Sept 1, 2021
Questions to Mike Connelley firstname.lastname@example.org