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Fibre System Performance

Geometrical losses, mainly caused by focal ratio degradation, give a mean efficiency of around 25% of the value when observing directly through the slit. Transmission losses in the 2.5 metre fibre length are small, except shortward of 4200 ; between 7200 and 7500 ; and longward of 8700 . However acquisition is critical, and much larger losses can be produced if the astrometry is not better than 0.5 arcsec for any reason. Some on-sky measurements, taken in 1.3 arcsec seeing, show that the total sensitivity of the system, is 1 photon/s/ at 4500 for a star of B=15 with the TEK1 CCD on the blue arm; and 1.7 photon/s/ at 7000 for a star of R=15 with the EEV3 CCD on the red arm. There is a fibre-to-fibre scatter of 30% in each of these values.

The efficiency is very sensitive to the acquisition procedure, a shift of 1 arcsec from the best position loses about 55% of the light, and a shift of 11 arcmin in the Cassegrain rotator position angle loses about 50% of the light, although this latter figure depends upon the radial distribution of objects.

Tue Aug 15 16:42:46 BST 1995