The telescope's position is determined by (Ferranti) Moire-fringe grating encoders mounted on each axis. There are three reading heads per axis, moving over a ruled stainless-steel tape. Each head produces a sinusoidal signal which is converted to a pulse train by a receiver in the CLIP Centre. These pulses are counted up (or down) by the so-called Grubb Parsons modules, whose values are transferred to a CAMAC register on receipt of a clock pulse. The registers are in turn read (at 0.1 s intervals) by the drive program. One encoder bit corresponds to 0.3 arcsec in RA and 1.0 arcsec in Dec, although systematic errors are currently somewhat larger. These errors have been partially calibrated, but still limit the absolute pointing of the telescope. The pointing accuracy of the telescope under computer control, using a regularly updated pointing model, is about 5 arcsec rms. Note that the offset accuracy is unreliable in declination at a level of 1-2 arcsec.
Because the Ferranti encoders are of the incremental
type, their zero points have to be determined. This is done by means
of two zeroset modules, one for each telescope axis.
The modules contain proximity sensors which detect
targets positioned at HA = 0 and Dec = Latitude (i.e. corresponding
to the telescope pointing towards zenith).
Once a target is detected, when moving the telescope for this purpose, the
``ZEROSET'' procedure (section 
),
a pulse is produced which persuades the hardware to reset the counters to zero.
A pair of powerful motors driving though spur gearing are used for the quick
motion, slew, of the telescope; they also provide the anti-backlash
preload. The maximum slewing speeds for hour angle and declination
are 0.5 s 
and 1.3 s , respectively.
The slow-motion drives on both telescope axes are provided by a pair of DC servo motors with integral tachogenerators. The RA axis is driven by a worm/wormwheel assembly and the Dec axis by a recirculating ball screw and nut. Because of its appearance the latter mechanism is known as the ``Banjo''. When the telescope is being driven by the slow-motion motors, e.g. when tracking, optical incremental (Baldwin) encoders, with resolutions of 0.01875 and 0.01 arcsec in HA and Dec, respectively, are used. These encoders are mounted on the slow-motion drive shafts, which are disconnected from the telescope during a slew in quick motion. Consequently, the Baldwin encoders are not used unless the slow-motion clamps have been engaged. The guided tracking accuracy (using the autoguider) is better than 0.3 arcsec. Unguided tracking is accurate to 1 arcsec over a time period of 3 minutes, and accurate to 2 arcsec over 10 minutes.
Table: Summary of Mechanical Performance