The ISO Short-Wavelength Spectrometer (SWS) provided medium and high spectral resolution in the wavelength region 2.4 ^m to 45.2 ^m. It consisted of two largely independent grating spectrometers operating in the SW range of 2.4 ^m to 12 ^m, and the LW range of 12 ^m to 45 ^m, and had a spectral resolving power of R = A/AA « 1,000-2,000. By inserting Fabry-Perot (FP) interferometers into the beam, one for the range 11-26 ^m and the other for the region 26-45 ^m, the spectral resolving power could be increased to R « 30,000.
The SW and LW parts of the spectrometer each had two sets, or blocks, of 12 detectors (photoconductive and photodiode), and by the switching in of different order-sorting filters each block recorded the spectrum in a "band" of wavelengths covering approximately half of the SW and LW spectral ranges, respectively, when the gratings were scanned. Hence, in grating mode the SW and LW spectra were each made up of 24 individual subspectra from each of the 24 detectors, which were then overlapped. In addition, the FP interferometers each had 2 double detectors (only one of each pair being used to gather valid data) giving a total of 52 detectors in all.
The aperture size of the spectrometer in grating mode was 14 x 20" for A < 12 ^m (detector blocks 1 and 2), 14 x 27" or 20 x 27" (depending on the order-sorting filter) for A > 12 in detector block 3, and 20 x 33" in detector block 4. In FP mode, the aperture size was 10 x 39" for A < 26 ^m, and 17 x 40" for A > 26 ^m.
ISO/SWS recorded many spectra of the giant planets, and averaged spectra are shown in Figure 7.25. For Uranus and Neptune, whose angular diameters are much less than the FOV, these are pure disk-averaged spectra. However, the spectra for Saturn and Jupiter are not quite disk-averaged since Saturn's apparent diameter was comparable with the FOV, and Jupiter completely filled it. Although there are some gaps in the spectra as we go from Jupiter to Neptune, the spectra that were measured can be seen to be in good agreement with the synthetic spectra shown in Figure 7.10 and in Chapter 6.
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