The UES is normally used with the Nasmyth image derotator, so that the orientation of the slit on the sky remains constant while the telescope is tracking the source. If preferred, the derotator can be set to track the vertical direction on the sky, so as to align the wavelength spread produced by differential atmospheric refraction along the length of the slit. This procedure is recommended to avoid light losses at the slit when recording a wide wavelength range, since UES has no atmospheric dispersion compensator.
UES has a dedicated acquisition and guiding unit (Section , which uses a CCD autoguider. This allows (a) direct viewing of the field and of the slit, for target acquisition; (b) guiding off light reflected from the slit jaws (when bright objects are being observed); and (c) offset guiding from nearby stars.
There are drawbacks, however. First of all, the field of view passed by the image derotator, and therefore accessible to the autoguider, is limited to a diameter of 5 arcmin; secondly, only small portions of this field can be viewed by the autoguider at any one time, by means of optical fibres which can be moved in x and y.
Since October 1994, the limiting magnitude of the UES autoguider is around V=17 in good seeing in bright time. For most UES targets guiding from the slit image should be possible, if the programme object is fainter than this then an offset guide star must be used.