MIRSI has a selection of broad-band and narrow-band filters, as well as a CVF. It is the only facility instrument that is cooled by liquid Helium, and the only instrument that uses the chopping mode of the secondary mirror. MIRSI was built by Boston University and is now based at the IRTF. MIRSI is a 2.2 to 25 µm thermal infrared imaging camera with grism spectrographic capability. As of 2019, iSHELL was the second-most heavily used instrument at IRTF (after SpeX). iSHELL also has an IR imaging mode and IR guiding camera, which covers a 42" diameter field. Five slits from 0.375" to 4.0" are available for use. Each pixel is 0.125" on the sky and the spectroscopic dispersion is 75,000 when used with a 0.375" slit. Due to the silicon grating, iShell will not be sensitive to light shorter than 1 µm. There will be two immersion gratings, one optimized for K band and one optimized for L band. Thus despite having much higher spectral resolution than SpeX, iShell will be slightly smaller. iShell uses a silicion immersion grating to achieve a high dispersion with a relatively small grating, which in turn allows the optics and the whole instrument to be much smaller than if it used a conventional grating. It replaced CSHELL by using a larger array and a cross disperser, iShell has much larger wavelength coverage per setting than did CSHELL. ISHELL is a 1 - 5.3 µm high resolution cross-dispersed echelle spectrograph which uses a 2048x2048 Hawaii-2RG infrared detector array. SpeX will be taken off of the telescope for about 6 months to upgrade its arrays starting in August 2012. SpeX is used for a wide array of planetary and astrophysical research programs, and is the most requested instrument on IRTF. The infrared slit viewer can also be used for imaging or photometry. A Raytheon Aladdin 2 512x512 InSb array in the infrared slit-viewer. SpeX also contains an infrared slit-viewer/guider covering a 60圆0arcsec field-of-view at 0.12arcsec/pixel. A Raytheon Aladdin 3 1024x1024 InSb array is used in the spectrograph. A high throughput prism mode is a provided for 0.8-2.5 µm spectroscopy at R~100 for solid state features and SEDs. Single order long slit (60 arcsec) modes are also available. This requirement has resulted in an instrument which provides spectral resolutions of R~1000-2000 across 0.8-2.4 µm, 2.0-4.1 µm, and 2.3-5.5 µm, using prism cross-dispersers (15 arcsec-long slits). The primary scientific driver of SpeX was to provide maximum simultaneous wavelength coverage at a spectral resolving power which is well-matched to many planetary, stellar and galactic features, and at resolving power which adequately separates sky emission lines and disperses sky continuum. SpeX is a medium-resolution 0.8-5.4 µm spectrograph built at the Institute for Astronomy (IfA), for the NASA Infrared Telescope Facility (IRTF) on Mauna Kea. IRTF also hosts a number of visiting instruments. The IRTF hosts four facility instruments: SpeX, NSFCam2, iSHELL, and MIRSI. Since the telescope is on a heavy mounting, it is relatively immune from vibration or wind shake. Since the telescope was primarily intended for planetary science, this restriction was considered to be acceptable. The yoke mount prevents the telescope from pointing north of +69 degrees declination. Since the telescope is on an equatorial mount, the telescope can observe targets through the zenith without concern for field rotation. The mount is very stiff, reducing flexure and allowing for accurate pointing of the telescope. The IRTF is mounted on a large English yoke equatorial mount. The secondary mirror is mounted on a chopping mechanism to rapidly switch the pointing of the telescope from target to sky at up to 4 Hz. The emissivity of the telescope is usually below 4%. The mirror coatings are chosen to have minimal thermal emission. The f/ratio is long to have a small secondary mirror, again to minimize the telescope's thermal emission. A small mirror in the center of the secondary mirror prevents the instrument from seeing its own thermal emission. The primary mirror itself is 126" in diameter, but only the center 118" is used. The secondary mirror is undersized to prevent the instrument from seeing the thermal emission from the telescope structure around the primary mirror. Several aspects of the design of IRTF are optimized for IR observations. The Cassegrain focus f/ratio is f/38 and the primary mirror f/ratio is 2.5. The IRTF is a 3.0 m (118" effective aperture) classical Cassegrain telescope. Asteroid collisions with Earth as visualized by the space Artist Don Davis
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