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The ISIS Multi-object fibre optic bundle consists of 61 Polymicro Technologies FHP400/475/510 fibres of 2.6m length each. Figure 10 shows their attenuation curves. On the auxiliary focus, the 400 micron diameter fibre core covers an area of 1.8 arcsec in diameter on the sky.
The focal plane end of the fibre is inserted into a connector which serves to protect and accurately align the fibre on the aperture plate. Figure 11 shows this connector, where A is the focal plane terminal ' B is a stainless steel microtube with diameters 685/1100 (inner/outer) and C is a silica microtube, TPS530/660 of Polymicro Technologies. B holds the fibre to the terminal, while C ensures that the fibre is concentric. Epoxy resin Epotek354 was used only between the fibre and C, and between C and B. In this way, stresses caused to the fibre end are reduced.
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For the mounting corresponding to the slit 61 holes have been made in an aluminium alloy piece. The hole diameter is 680 microns and the separation between hole centres is 860 microns. Then, the separation between the centres of two adjacent fibres is 2.15 times the fibre nucleus diameter. This is enough to avoid optical cross talk, except for extreme conditions. A TSP530/660 silica microtube has been used to ensure the concentricity between the fibres and the holes . Keeping in mind the 3.3 ISIS reduction factor, the fibre nucleus diameter is 5.5 pixels on the detector and the separation between the centres of two adjacent fibres is 11.8 pixels.
All the fibres were numbered at the focal plane fibre end. Each fibre number is fixed by its position at the slit.
Both bundle ends were polished using diamond abrasives from 25 to 0.1 microns of particle size. The bundle was tested in the laboratory in order to determine the focal ratio degradation of each fibre.
Figure 12 shows the results of this test. The efficiencies correspond to output beams with the same focal ratio of the input beam (f/11). Neither the fibre absorption or reflection losses at the fibre end are considered.
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