An image rotator is available in front of the slit to compensate for
field rotation at the coudé focus. It is a fused silica Dove prism with
anti-reflection coated silica windows optically bonded to each surface.
The throughput of the 1 arcmin field is 
96%.
Since the beam rotator introduces an optical element
upstream of the slit, the night assistant must be kept informed if it
is being moved in and out while observing, as the telescope focus has to be
offset each time.
Information concerning the RA and Dec of the telescope is fed from the telescope control computer via CAMAC to the UCLES program on the VAX, and commands are available which cause the slit orientation to be maintained at a given position angle on the sky ( TRACKPA pa) or to be kept vertical to the horizon, i.e. at the parallactic angle ( VERTICAL). Communication via CAMAC requires that the UCLES software be run on AAT40A. The beam rotator is automatically driven out of the beam for UHRF.
The most common use of the beam rotator is to track the parallactic angle,
so that the UCLES slit is aligned with atmospherically dispersed images.
The value of this, especially for bluer wavelengths,
may be judged from Fig. 
which shows the difference
in refraction for several wavelengths compared to 5500 Å, near the peak in
sensitivity of the acquisition TV. For zenith distances exceeding 25
,
a stellar image at 4000 Å will lie on the edge of a 1 arcsec wide slit
unless it is orientated close to the parallactic angle.
Main commands: BR IN/OUT; TRACKPA pa; VERTICAL; CANCELPA; CANCELVERTICAL
Figure: Atmospheric dispersion