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The ISIS fibre system enables the ISIS spectrograph on the WHT to be fed
with fibre optic bundles. The bundles link the auxiliary focus,
located on the acquisition and guiding box, with the ISIS slit area (see
figure 9). Below this position, ISIS can be configured in the same way as
for the long-slit unit. Then, both arms and FOS can be used in the usual form.
However, several ISIS facilities, including autoguider and
comparison lamps, cannot
be used.
The ISIS fibres system is composed of the following elements:
- Auxiliary focus. As the Cassegrain focus is inaccessible
when ISIS is mounted on the telescope, the fibres will be placed at the
Auxiliary Port of the A&G box. This focus, with a 15 arcmin field, is fed
through the large mirror of the A&G box. FOCAP aperture plates are
used at this auxiliary port to plug the fibres.
- Fibre optic bundle. One bundle with 61 fibres allows up to 61 objects
to be observed simultaneously. The diameter of the fibre core is 400 microns
or 1.8 arcsec.
- Guiding system. One coherent fibre bundle and two semicoherent fibre
bundles allow autoguiding or manual guiding. These bundles
also enable the focussing and set up of the telescope.
- Calibration system.
Wavelength calibration is provided by a box which can be
mounted on the
opposite side of the A&G box from the Auxiliary port, in which any of
Argon, Neon or Mercury lamps can be located. The calibration lamps
provide a diffuse illumination over the entire Auxiliary port of the
A&G box; no attempt is made to mimic the pupil of the telescope.