The CCD Autoguider is described in detail in:
WHT-AUTO-2: Autoguider User Manual by A.B. Gentles, and
is similar to the one at Prime Focus; see Section 1.5 for more
details.
The autoguider consists of a CCD detector head fed by a right-angled
prism and focal reducing optical system, with a field diameter of 1.8
arcmin. The autoguider utilises the off-axis field.
The centre of the autoguider field rotates about the centre of
the main field at a radius of 110 to 150 mm (8.2 to 11.2 arcmin)
and the entire probe assembly has a radial displacement of 40 mm.
The extreme edge of the autoguider field is partially vignetted,
but only by about 5%. The autoguider has an azimuthal scan of
180 degrees, so the total area scanned at a field scale of
4.51 arcsec mm 
equals 152 square arcmin or 0.04 square degrees.
This gives a good chance of finding a star brighter than 11th magnitude
at the galactic equator or 13th magnitude at the galactic pole
(C.W.Allen,Astrophysical Quantities, publ. Athlone Press, 1976).
The autoguider head is mounted inside the Cassegrain
A&G box, and contains a frame-transfer CCD having 385 by 288 active pixels,
with 22 
m pixels, with a peak quantum efficiency of 50% at 7500Å.
This is mounted on a copper block connected to a two stage Peltier
thermo-electric cooler. This cools the CCD to around -35 
C to reduce
the dark current. The CCD must be flushed with Nitrogen when the cooler is
in operation to prevent the formation of condensation and ice crystals on
the CCD surface. If this is not done the CCD could be badly damaged. There
is a flowmeter on the nitrogen outlet line from the CCD Autoguider, the user
should check that nitrogen is flowing through this as soon as the CCD begins
cooling.
The limiting magnitude of the autoguider, for guiding at 1 Hz, is about V=17.5 in good conditions (i.e. 1-arcsec seeing and a dark sky). The ICL GSSEARCH procedure may be used to automatically find guide stars (see Sections 1.5 and 4.6 for more details). The available filters for the Auxiliary Focus autoguider are the same (??) as for that at prime focus: CLEAR, EMPTY, OPAQUE, B, V, and I. The AUTOFOCUS command is used to set the Auxiliary Focus autoguider focus (see Section 6.2).