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Guiding system.

The autoguider system in the Cassegrain A&G box cannot be employed when the fibre system is used with ISIS, due to the size of the LARGEFEED mirror.

The guiding system consists of three fibre bundles - two semicoherent and one coherent - to carry the image of three stars to a CCD camera. The image of the guide star, obtained with the coherent bundle, can be use by the TV software to generate error signals for the telescope control computer, in order to improve the tracking of the telescope. This image and the two semi-coherent ones can also be used for the set up of the telescope rotator (see section 9.6.2) and for the acquisition of the field (see section 9.6.3).

The guiding system includes:

• A coherent bundle manufactured by ORIEL; it covers a rectangular area of 1.89 X 2.38 mm (8.5 X 10.7 arcsec on the sky). One half has a better packing quality than the other half and, therefore, it is used to monitor the object. The focal plane bundle end is connected to the aperture plate via a connector. This connector has a pin to fix the orientation (North) of the bundle. In order to group together the three guiding bundles, the other end of the coherent bundle was shaped to accommodate the semi-coherent bundles.

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• Two semi-coherent bundles 2.35 m long, each containing 15 fibres; the central fibre and 6 ones in the first ring are of type FHP100/110/125 while the 8 fibres in the second ring are of type FHP200/240/270. The 7 central fibres were introduced in a polymide microtube of type PPC406/444. This group covers an area of 1.6 arcsec on the sky. The second ring of fibres was glued surrounding this microtube. In order to protect it, the whole structure was inserted in a stainless steel tube of type 1100/1470. Each of the semi-coherent bundles built this way covers an area of 4 arcsec on the sky (Figure 14). The orientation (North) of each bundle is marked by a pin.

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• A piece to group the 3 guiding bundle ends when connected at the CCD camera. This can be rotated and must be mounted at a certain orientation which is clearly marked.

• A Westinghouse ISEC TV camera.

• A lens relay system to produce the image of the guiding bundles on the detector.

• A Melles Griot mechanical system for the manual focussing of the guiding bundles