Short Wavelength Spectrometer (SWS)
The Short Wavelength Spectrometer (SWS) is one of the four instruments onboard of ESA's Infrared Space Observatory ISO. It is designed for spectroscopic observations with medium and high resolution in the 2.38-45.2 µm and the 11.4-44.5 µm range, respectively.

Although the satellite is not operational any more, due to the fact that it ran out of helium coolant, all its observations are now publicly available. The SWS observations are all in DIDAC. A large number of scientific projects are in progress or already finished.

The SWS wavelength range is of great scientific interest, not only because cool objects with temperatures of 1500-80 K radiate the bulk of their energy in this range, but also because of its rich variety of atomic, ionic, molecular and solid-state spectral features. These provide unique and excellent tools for studies of the physical and chemical processes in the universe, especially of those regions optically hidden by interstellar dust. The SWS spectral resolution allows probing of kinematic processes in a variety of objects ranging from nuclei of galaxies to planetary atmospheres. With the SWS sensitivity, line studies of extragalactic objects out to the distance of the Virgo cluster, and even beyond in the case of IRAS galaxies, can be carried out. Direct observation of ground state H2 in the interstellar medium is possible.


 


The SWS consists of two nearly independent grating spectrometers, one for the short wavelength range from 2.4 - 12 µm and one for the long wavelength range from 12 - 45 µm. In the long wavelength spectrometer Fabry-Pérot filters can be inserted for which the grating works as an order sorter. SWS has 17 AOT bands (11 for the grating and 5 for the FP), 3 apertures, 6 detector arrays (4 arrays of 12 detectors for the grating and 2 arrays of 2 detectors for the FP) and the instrument covers 4 orders. The scanning of the wavelength is achieved through a rotating mirror which can scan the total range in discrete steps. 

       band order  aperture    det.  wavelength   resolution L_AOT
                  no    area         range (µm)

SW-gr    1A   S4   1   14-20  InSb   2.38 - 2.61   1870-2110   756
SW-gr    1B   S3   1   14-20  InSb   2.60 - 3.03   1470-1750  1043
SW-gr    1D   S3   2   14-20  InSb   3.02 - 3.53   1750-2150  1282
SW-gr    1E   S2   2   14-20  InSb   3.52 - 4.06   1290-1540   867
SW-gr    2A   S2   2   14-20  Si:Ga  4.05 - 5.31   1540-2130  2115
SW-gr    2B   S1   2   14-20  Si:Ga  5.30 - 7.01    930-1250  1377
SW-gr    2C   S1   3   14-20  Si:Ga  7.00 - 12.1   1250-2450  4276

LW-gr    3A   L2   1   14-27  Si:As  12.0 - 16.6   1250-1760  2047
LW-gr    3C   L2   2   14-27  Si:As  16.5 - 19.6   1760-2380  1879
LW-gr    3D   L1   2   14-27  Si:As  19.5 - 27.6    980-1270  2524
LW-gr    3E   L1   2   14-27  Si:As  27.5 - 29.0    980-1270  2524
LW-gr    4    L1   3   20-33  Ge:Be  28.9 - 45.2   1020-1630  4324

LW-FP1   5A   L3   1   10-39  Si:Sb  11.4 - 12.2  20600-24000 
LW-FP1   5B   L2   1   10-39  Si:Sb  12.2 - 16.0  24000-32000 
LW-FP1   5C   L2   2   10-39  Si:Sb  16.0 - 19.0  32000-34500 
LW-FP1   5D   L1   2   10-39  Si:Sb  19.0 - 26.0  34500-35500 
LW-FP2   6    L1   3   17-40  Ge:Be  26.0 - 44.5  29000-31000
Notes
`Aperture area' refers to the dimensions of the SWS entrance apertures projected on the sky
SW = short-wavelength region
LW = long-wavelength region
L_AOT = total number of scan steps in AOT band
The SWS has three entrance apertures. The IR radiation is reflected into the SWS apertures by the ISO pyramidal mirror. The appropriate aperture is selected by specific pointing of the ISO satellite. The spacecraft has to be adjusted in a way, that the target is imaged onto the selected aperture. A four-position shutter mechanism permits the opening of any one of the three apertures or blocking of all.

Each of the entrance apertures is used for two wavelength ranges. Dichroic beamsplitters behind the apertures split the incoming radiation up in a short and a long wavelength part. The beams transmitted by the crystal enter the Short Wavelength (SW) section of the spectrometer. The reflected beams enter the Long Wavelength (LW) section, after a second reflection against identical material. Since the two sections are otherwise independent, the two wavelength ranges can be observed simultaneously.

The actual entrance slits of the SWS are located behind the beam-splitting crystals. In this way, each of the 6 possible input beams has its own slit. The slits are in the focus of the telescope, i.e. the plane where the sky is imaged. Backprojected onto the sky they show a field-of-view of 14"x20" to 20"x33" for the six grating spectrometer bands and of 10"x39" to 17"x40" for the two F-P bands.

After the entrance slits, the light path in both sections of the instrument is very similar. The incoming radiation passes the spectral order separation filters. Then its collimated and reflected onto the gratings by the scanner mirrors. Each grating has its own scanning mechanism, enabling the use of both parts of the spectrograph at the same time, albeit through one aperture. Wavelength scanning is achieved by rotation of the flat scanner mirrors close to each grating in discrete scan steps.

After reflection from the gratings, the light almost retraces its path and, by means of small-diameter re-imaging relay optics, the high resolution spectral image of each wavelength band is re-imaged onto the detector block. These relay optics have various functions:

By the use of Fabry-Pérot (F-P) filters, the resolution of the instrument can be increased by a factor of more than 100 compared to the resolution of the grating spectometers. To enter the Fabry-Pérot section the radiation returning from the LW grating is collimated again, transmitted through a tunable F-P interference filter and imaged onto separate detectors in the F-P detector block. The two F-P's are mounted on a single pair of parallel plates. Their separation and parallelism can be varied by changing the currents in three pull coils.
The principal investigator for the Short Wavelength Spectrograph is Th. de Graauw (SRON, Groningen, the Netherlands).
Do Kester, 24 April 2001.