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Procedures for observing with FOS are in general similar to those used when observing with CCDs on ISIS (Section 9.1). FOS is a simpler spectrograph, and has no moving parts. A consequence of this is that it is not necessary to take wavelength calibration exposures so frequently, indeed for many observers one calibration exposure at the beginning of each night will suffice. Apart from this the calibration exposures required are the same as are required for ISIS CCD observing.
Data acquisition procedures are again the same as for ISIS observing, except that it is much more likely that the observer will seek to observe objects which cannot be seen on the slit viewing TV with FOS. Such objects should be acquired by the BLIND_OFFSET procedure os the Telescope Control Computer, outlined in The WHT Users' Manual.
The approximate positions of the first- and second-order spectra on the CCD are shown in Figure 8. There are two main observing modes: with 20-arcsec dekker, both orders; and long-slit, with the FCP-tray field lens inserted. The anamorphotic field lens improves illumination of the grating (without it, light spills over the edge of the grating, and illumination on the detector is reduced at each end of the slit). It also incorporates a GG495 (yellow) filter, which blots out the second order. The field lens is inserted with ICL command FIELD_LENS. During routine FOS observing, this and the dekker are the only components that need moving. Exposures longer than, say, half an hour, should be split to allow identification of spurious features due to cosmic rays.
The efficiency of the system (atmosphere at zenith + telescope + instrument + detector) is shown in Fig 16. In first order, it peaks at 17% at 7000Å.