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Faint Object Spectrograph

Two versions of the Faint Object Spectrograph (FOS) exist on La Palma, one on the WHT and one on the INT, both built as a result of a collaboration between the RGO and the Department of Physics of Durham University. The FOS is a highly efficient fixed format CCD based spectrograph, designed for low resolution (15-20Å) spectrophotometry, over a wide spectral range. The optical design, described by Charles Wynne ( Optica Acta, 1982, 29, 1557) is based on a Schmidt camera working without a collimator in the diverging beam from the Cassegrain focus. The major components are shown in figure gif. The dispersion is provided by a transmission grating and a cross dispersing prism (grism), which together give a multi-order format covering the wavelength range 3500Å to 10500Å for FOS-1 on the INT, and 3500Å to 9700Å for FOS-2 on the WHT. As a result of locating the CCD within the camera, there are a small number of optical surfaces and minimum vignetting, which produces a system which is more efficient than most conventional spectrographs. In addition, the fixed spectral format of the device makes it relatively straightforward to carry out on-line data reduction and analysis operations. These include wavelength and flux calibration, as well as redshift determination.

FOS-1 is mounted below the IDS, and FOS-2 is located below ISIS, sharing the same slit and calibration facilities. This location offers the advantage that it is straightforward to change between FOS and IDS/ISIS during the night. For FOS-1 this involves removing a single folding prism, a manual operation normally carried out by support staff which takes about half an hour. For FOS-2, the folding mirror can be removed remotely under computer control. Alternatively, it is possible to use a dichroic to observe simultaneously with the blue arm of ISIS and the FOS (see Section gif). Some important parameters of the two version of FOS are summarised in Table gif.

  
Table: Summary of FOS parameters





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Next: FOS-1 Up: Spectroscopic instruments Previous: Overview




Tue Aug 15 16:42:46 BST 1995