Both the Cassegrain and Prime Focus A&G units are equiped with identical autoguiders. The autoguiders are controlled by a dedicated micro processor, which continuously analyses the image of a guide star at the centre of the guide probe and determines its position in the frame of the autoguider. Position errors, offsets from the probe centre due to tracking errors, are communicated to the Telescope Control software which can then make a pointing correction. Before the autoguider can lock onto a guide star, that star has to be positioned on the guide probe near the probe centre, by moving either the probe or the telescope. If the guide star is within the field of the probe, the autoguider will locate it in the `field' mode; once the star is detected, the probe (or telescope) can be moved to centre it, and the guider-telescope combination can lock onto the guide star in a closed guide loop. The tracking errors are sent to the Telescope Control computer (the PE 3220) in order to effectuate pointing or tracking corrections.
The control of the
probe position (for moving the probe around to acquire a guide star) is via
the Instrument Computer (the PE 3210, because that one controls the A&G Unit
functions). Therefore, one can centre a guide star on the probe in
two different ways: (1) by moving
the probe with the ADAM GDXY command (Instrument Computer) while the telescope
is tracking, or (2) by moving the telescope (handset or User Interface) with a
fixed probe position. Each possibility has its own advantages and disadvantages
which are discussed in section 
below. The GET GUIDE STAR
command of the Grinnell menu has the effect of moving the probe.
The standard way of acquiring guide stars is by locating them on the Finder
TV probe (at either focus). The telescope control software knows how to map
positions on the Finder TV onto the autoguider acquisition field provided
that (1) the finder zeropoint is determined correctly (see section
), and (2) the geometry parameters for the focus used are set
up correctly (see section ). Although guide stars can
be located on the finder TV, it is desirable to select guide stars in advance
of an observing run on survey plates and measure their coordinates, in
particular if there are restrictions on the rotator Position Angle (see
section ). If a guide star position is known, the coordinates
can be entered directly via the User Interface and the TCS will position the
guide probe straightaway. At Cassegrain, care must be taken to avoid
selecting a guide star so positioned that the autoguider probe obstructs
the spectrograph slit.