The ISIS Fibre System

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 ). 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. The ISIS Multi-object fibre optic bundle consists of 61 Polymicro Technologies FHP400/475/510 fibres of 2.6m length each. Figure shows their attenuation curves. On the auxiliary focus, the 400 m diameter fibre core covers an area of 1.8 arcsec in diameter on the sky.

• 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.

  
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Figure: The ISIS Spectrograph and the A&G box with the fibre system

  
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Figure: Attenuation curves for Polymicro high OF (FH series) optical fibre

• Guiding system.
• Aperture plates
• Observing coordinate determination.
• Fibre System Performance