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The efficiency of the spectrograph, excluding the telescope, image derotator and detector, is expected to be about 36 per cent. This assumes an efficiency for the cross-disperser of 86 per cent, for the echelle of 60 per cent, and for the camera of about 70 per cent.
The total throughput of the spectrograph also depends on the slit width. A wide slit will let in more light, but will degrade the wavelength resolution, and the optimum tradeoff between these considerations will vary from observation to observation.
For a one arcsecond slit, the limiting magnitude of the instrument, defined as the magnitude of a star from which 100 counts per resolution element is obtained at a wavelength of 5500 Å with an exposure time of 10,000 seconds, is estimated to be about m=17.5.
The throughput of UES+WHT is in fact measured to be very similar to that of UCLES+AAT. The extra light collected by the WHT is lost in the derotation optics. In general we therefore expect UES+WHT to go slightly deeper than UCLES+AAT, due to the better seeing conditions on La Palma.