The SWS instrument has three different physical apertures. 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). There is also a virtual aperture 4, used by the long wavelength FP. This is physically the same as aperture 3, but with its nominal central position offset slightly from aperture 3. Aperture 4 was implemented because of a slight misalignment between the LW grating detectors and the FP detectors. Its introduction increased the amount of light falling onto the FP detectors and hence improved their efficiency (see Section 3.6).
Each aperture is used for two wavelength ranges, one for the short-wavelength section of the spectrometer and one for the long-wavelength section. Since those two sections are otherwise independent, two wavelength ranges can be observed simultaneously. While using the FP virtual aperture 4, however, data from the SW grating section refer to an offset position and are therefore of restricted use only. Table 2.1 indicates which aperture is used for each band.
Beam splitters, consisting of `reststrahlen'
crystal filters
(
, 
and 
), 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 (Figure 2.5), 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 reflected input, which has a
larger width, adapted to the larger diffraction image at these wavelengths.
The slits are in the focus of the telescope, in the plane where the sky is
imaged.
The edges of the apertures are oriented along the spacecraft y- and z-axis.
in
calibration documents), parallel to the
spacecraft y-axis, is the non-dispersion, or cross-dispersion, direction.
), parallel to the spacecraft
z-axis, is the dispersion direction.
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
for bands 1A to 4, and from 39
to 40
for the FP bands 5A to 6.
Along the z-axis, the aperture size is determined by entrance slit width (14
or 20
) 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
.
Table 2.1 gives the nominal size of the aperture for each band.