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 .
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 ).
Some important parameters of the two version of FOS are summarised
in Table
.
Table: Summary of FOS parameters