2.2 Instrument Design

Figure 2.1 gives an impression of the layout of the SWS, while Figure 2.2 shows an expanded view of the optical systems. Light from the telescope was reflected into the SWS by the ISO pyramidal mirror (not shown in Figures 2.1 or 2.2). The SWS had three entrance apertures, each with its own dichroic beam splitter feeding the Short Wavelength (SW) section and the Long Wavelength (LW) section. The entrance aperture appropriate to the wavelength range of the requested observation was automatically selected by specific pointing of the ISO satellite, with the relationship between aperture and wavelength ranges given in Table 2.1. A four-position selection mechanism permitted opening of any one of these apertures or blocking of all three - see Section 8.2.1 for further details.

Figure 2.2: Internal layout of the Short Wavelength Spectrometer (SWS). On the left is the light-path for grating observations, while on the right an abbreviated light-path for Fabry-Pérot observations is shown.

Figure 2.3: A qualification model of one of the SWS grating scanners.

Each grating had its own scanning mechanism, enabling the use of both parts of the spectrograph at the same time, albeit through the same aperture. Changing the wavelength of light falling on the detectors was achieved by rotating the flat mirrors close to each grating in discrete scan steps. A qualification model of one of the scanners is shown in Figure 2.3. (Their location is shown in Figures 2.1 and 2.2).

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

• They relayed the high resolution image to a more easily-accessible location on the detector block.
• They changed the relative opening of the beam to a value that was optimised to the detector dimensions.
• They enabled efficient straylight rejection.