SWS has three different apertures (aperture 4, used by the LW FP, is a virtual aperture offset slightly from aperture 3 to increase the amount of light falling onto the FPs and hence improve efficiency, see section 4.5). A shutter system allows the selection of one aperture, while closing off the other two (the spacecraft pointing has to be adjusted so that the target is imaged onto the selected aperture).
Each aperture is used for two wavelength ranges, one for the short-wavelength (SW) section of the spectrometer and one for the long-wavelength (LW) section. Since those two sections are otherwise independent, two wavelength ranges can be observed simultaneously.
Beamsplitters, consisting of Reststrahlen crystal filters (Al2O3, LiF and SrF2), are located behind the apertures. The beams transmitted by the first crystal enter the SW section; the reflected beams enter the LW section, after a second reflection against identical material. As is seen in the schematic (Fig. 3.3), the actual entrance slits are located behind the beam-splitting crystal. In this way, each of the 6 possible input beams has its own slit. All slits have been given the same width, except for the SrF2 reflected input, which has a larger width, adapted to the larger diffraction image at these wavelengths (see Table 3.2). The slits are in the focus of the telescope, in the plane where the sky is imaged.
In the direction perpendicular to the dispersion, the slits are oversized. There the fields-of-view are determined by the dimensions of the detectors. The cross-dispersion dimensions are different for almost all detector bands. Since the imaging of the slits onto the detectors (or vice versa) is imperfect due to aberrations and diffraction, the short sides of the fields of view (detector edges) are more fuzzy than the long sides (the slit jaws).
Small offsets of the fields of view perpendicular to the dispersion can be caused by alignment errors. The internal alignment specification adhered to amounts to 10% of the detector size alias spectrum width.
The monochromatic images of the grating detectors fill about 55% of the slit widths. This means that the spectral resolution for point sources is significantly higher than for extended sources, in a ratio that is affected, of course, by diffraction.
For the Fabry-Pérot section the situation is more complex. There the monochromatic detector images just fill the slit width. With the spectral channeling by the FPs, a subtle interplay arises between spectral and spatial properties. Effectively, the spatial resolution may increase due to the narrowness of the F resonance. Point sources have less leakage in unwanted FP orders than extended sources. Spatial extent does not influence the spectral resolution of the FP's.
The edges of the apertures are oriented along the spacecraft y- and z-axis.
Along the y-axis, the effective size of the aperture is determined by the projection of the detector array on the sky. This amounts to 20, 27 or 33 arcsec for bands 1A to 4, and from 39 to 40 arcsec for the FP bands 5A to 6.
Along the z-axis, the aperture size is determined by entrance slit width (14 or 20 arcsec) for bands 1A to 4. For the FP bands 5A to 6, the aperture is effectively as wide as a detector image on the sky, i.e. either 10 or 17 arcsec.
To derive the position angle of an aperture information on the spacecrafts position on the sky must be used. This can be found in the header keywords INSTRA, INSTDEC, INSTROLL found, for example, in the AAR. Specifically, INSTROLL is the angle, measured anticlockwise, between north and the spacecraft z-axis (ref. the ISDM for a description of these keywords). See also section 5.10.
Information on how the spacecraft pointing can affect observations of extended objects can be read in the document `Status of study of AOT02 line profiles and fluxes on NGC6543', by H. Feuchtgruber issued 2 June 1998. In this case the observed line fluxes varied by 10% over the course of several months. This was attributed to the spatial extent of NGC 6543, the SWS slit size projected on the sky and the change in roll angle between the observations. This is also discussed in section 5.10.
Beam profiles for all bands were derived during flight by raster observations of point sources (stars). An example of a band 1 beam profile is shown in figure 3.4. More information on the beam profiles can be found in the document `SWS Beam Profiles and ISO Pointing', by A. Salama, 9/9/98, which should be consulted by observers needing further information on the beam profile of the various bands.