The SWS instrument consists of two nearly independent grating spectrometers,
together covering the wavelength range 2.38-45.2 
m with an overall
spectral resolution of 
1,000-2,000 (i.e., 
300-150
km s 
). By inserting Fabry-Pérot (F-P) filters, one for the range
15-26 m (with capability down to 11.4 m at reduced
resolution) and the other for the region
26-35 m (with capability to 44.5 m at reduced sensitivity), the
resolution can be increased to 30,000 (i.e., 10
km s ) .
Fig. 2.1 gives an impression of the lay-out of the SWS, an optical block diagram is shown in Fig. 2.2, and an optical schematic is shown in Fig. 2.3. The IR radiation is reflected into the SWS by the ISO pyramidal mirror. The SWS has three entrance apertures , each with its own dichroic beamsplitter feeding the Short Wavelength section and the Long Wavelength section. The appropriate entrance aperture will be selected by specific pointing of the ISO satellite. A four-position selection mechanism permits opening of any one of these apertures or blocking of all three.
Tab. 2.1 shows how the SWS grating wavelength region is subdivided in 17 wavelength bands , instrumentally dictated by aperture , spectral order, filter material and detector type.
band order aperture area detector wavelength resolution 
filter ( 
) type ( m) ( 
) SW-gr 1A 4 1 t 
14-20 InSb 2.38 - 2.60 1870 - 2110 756
SW-gr 1B 3 1 t 14-20 InSb 2.60 - 3.02 1470 - 1750 1043
SW-gr 1D 3 2 t LiF 14-20 InSb 3.02 - 3.52 1750 - 2150 1282
SW-gr 1E 2 2 t LiF 14-20 InSb 3.52 - 4.08 1290 - 1540 867
SW-gr 2A 2 2 t LiF 14-20 Si:Ga 4.08 - 5.30 1540 - 2130 2115
SW-gr 2B 1 2 t LiF 14-20 Si:Ga 5.30 - 7.00 930 - 1250 1377
SW-gr 2C 1 3 t 
14-20 Si:Ga 7.00 - 12.0 1250 - 2450 4276 LW-gr 3A 2 1 r 14-27 Si:As 12.0 - 16.5 1250 - 1760 2047
LW-gr 3C 2 2 r LiF 14-27 Si:As 16.5 - 19.5 1760 - 2380 1879
LW-gr 3D 1 2 r LiF 14-27 Si:As 19.5 - 27.5 980 - 1270 2524
LW-gr 3E 1 3 r 20-27 Si:As 27.5 - 29.0 1300 500
LW-gr 4 1 3 r 20-33 Ge:Be 29.0 - 45.2 1020 - 1630 4324 LW-F-P1 5A 3 1 r 10-39 Si:Sb 11.4 - 12.2 20600 - 24000
LW-F-P1 5B 2 1 r 10-39 Si:Sb 12.2 - 16.0 24000 - 32000
LW-F-P1 5C 2 2 r LiF 10-39 Si:Sb 16.0 - 19.0 32000 - 34500
LW-F-P1 5D 1 2 r LiF 10-39 Si:Sb 19.0 - 26.0 34500 - 35500
LW-F-P2 6 1 4 r 17-40 Ge:Be 26.0 - 44.5 29000 - 31000
Notes to Table 2.1:
`Aperture area' refers to the dimensions of the SWS entrance apertures
projected on the sky
SW = short-wavelength region
LW = long-wavelength region
t = transmission
r = reflection
= total number of scan steps in AOT band
AOTs SWS06 and SWS07 (SW grating) use finite (but small) band overlaps
Band 2C ends at 13.16 m for AOT SWS07.
