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The Intermediate Dispersion Spectrograph (IDS) is the principal common-user instrument at the Cassegrain focus of the INT. The IDS design is based on the principle of a folded-input, flat-bed instrument. The ``straight through'' position is occupied by the Faint Object Spectrograph (FOS-1; see the Observer's Guide). Figure 3.1 shows the overall optical layout; selection of either IDS or FOS-1 operation is determined by manual movement of a single optical element, the folding prism, so that both spectrographs, and indeed both cameras of the IDS, albeit with different detectors, can be used consecutively during the same night. Facilities for acquisition and guiding, such as calibration and comparison lamps, above-slit neutral-density and colour filters, the autoguider and a TV camera, are all located in a separate A&G unit to which the spectrograph is attached (Section 3.2). Dekker masks, slit assembly and below-slit filters are within the main body of the spectrograph itself.

Wed Apr 6 22:56:39 BST 1994