IRS provides Sp/Zzer with low-resolution and moderate-resolution spectroscopy from 5.3 ^m to 40 ^m. The IRS is composed of four separate modules, each of which is built around a 128 x 128 pixel-detecting array and each module has its own entrance slit in the focal plane. The IRS instrument has no moving parts and is designed to achieve high sensitivity through the use of cooled detectors.
Low-resolution (^ = A/AA = 60-124), long-slit spectra can be obtained from 5.3 ^m to 14 ^m with a FOV of 3.6 x 54.6", and from 14 ^m to 40 ^m with a FOV of 9.7 x 151.3". The pixel sizes of these two modes are 1.8" and 4.8", respectively. In this mode, both spectral and one-dimensional spatial information are acquired simultaneously on the same detector array. Two small, imaging subarrays (peak-up arrays) in the shortwavelength, low-resolution module (SL) also allow objects to be placed accurately into any of the IRS entrance slits. Moderate-resolution spectra (A/AA = 600) may also be obtained in Echelle mode from 10 ^m to 19.5 ^m with a FOV of 5.3 x 11.8" and from 19 ^m to 37 ^m with a FOV of 11.1 x 22.4". The pixel sizes for these two high-resolution modes are 2.4" and 4.8", respectively.
Although spectral resolution is less than that of the ISO SWS, spatial resolution and sensitivity are significantly better and thus this instrument has proved useful in measuring the abundance and mapping the spatial distribution of gases in the giant planet atmospheres, especially those of Uranus and Neptune. Initial observations of Uranus by Spitzer IRS (Burgdorf et al., 2006) were reported in Chapter 4. However, on a practical note, Spitzer is primarily designed to look at extremely cold targets, which explains its very low operating temperature. Hence, to Spitzer the larger giant planets, in particular Jupiter, appear very warm, and thus any observations have to be strictly limited to avoid excessive heating of the telescope and thus loss of precious cryogen.
Multiband Imaging Photometer for Spitzer (MIPS)
MIPS provides imaging and photometry in broad spectral bands centered nominally at 24 ^m, 70 ^m, and 160 ^m, together with low-resolution spectroscopy between 52 ^m and 99 ^m. The nominal 24 ^m channel uses a 128 x 128 pixel SiAs array giving a pixel size of 2.45", while the nominal 70 ^m channel uses a 32 x 32" GeGa array with a pixel size of either 4.9" or 9.9". The nominal 160 ^m channel uses a 2 x 20 pixel stressed GeGa array giving a pixel size of 16". All three arrays view different areas of the sky simultaneously, and thus multiband imaging is provided through telescope pointing. This instrument is of limited use for the study of the giant planets of our own solar system.
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