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The Cassegrain focal plane is inaccessible when the ISIS spectrograph is mounted on the telescope. An auxiliary focus is needed when fibre systems are used. This auxiliary focus is available at a side of the Cassegrain acquisition and guiding box. It is accessed via a large flat mirror which is inserted at a 45 angle to the main telescope beam. The large flat mirror provides a full 15 arcmin field.
The optical characteristics of the Cassegrain focus are well established theoretically and they would be expected to be the same on the auxiliary focus. In order to corroborate this, an experimental scale has be obtained for the auxiliary focal plane within this work. For multi-object spectroscopy with fibre optics, an accurate focal plane scale determination is necessary because the fibre positions (X and Y) on the focal plane are calculated from the object R.A. and DEC coordinates through the scale. An error of 0.2% in the scale results in a loss of 50% of the star's light on a fibre at the edge of the field.
To obtain the focal plane scale several photographic plates of good astrometric fields have been taken. With the star positions on the plates and their astrometry, apparent celestial coordinates have been found. Finally, effects of annual aberration, diurnal aberration, light deflection, atmospheric refraction have been taken into account to find the angular separation between each star pair. The average of the ratios between the separation of each star pair in plate and celestial coordinates yields the focal plane scale. The value of the auxiliary focal plane scale found in this way is:
The mounting for the fibre system at the auxiliary focus consists of two main structures; an interface for the auxiliary port and a plate holder. The fibres are placed at the auxiliary port using aperture plates, FOCAP system (see section 2.12.4). These aperture plates are held in position by four flanges on the plate holder. Three pilot pins determine the aperture plate position. Finally, the plate holder is mounted on the A&G box by means of the interface, using kinematic seats in order to be relocated accurately.
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