Autofib-2 (AF2) is a `classical' focal-plane pick-place fibre positioner, the third and most refined such instrument to be built by Parry's group at Durham. The (x,y) carriage supports the gripper head; it also carries a machine-vision system that allows the exact position of any fibre to be measured by centroiding on the back-illuminated input-face. Fibre placement can thus be made an iterative procedure, and there is a trade-off between placement accuracy and number of iterations ( i.e. cycle time).
Two imageguides feeding the autoguider CCD camera allow direct inspection of star images in the field of the telescope (with a magnitude limit of about 14). One is mounted on the gripper assembly to allow pseudo-astrometric mapping of star-fields, while the other `fixed sky probe' forms the off-axis autoguider. Ten 7-fibre fiducial bundles feed a TV viewing system; they are mainly used for field acquisition and visually monitoring the guiding.
The Vax-based Autofib Observing System allows the astronomer to communicate with the VME-based machine control system that interacts directly with the robot. Among the tasks available are CONFIGURE, which can be used off-line and allocates fibres to target objects, SET-UP, which sets up the fibres for observing, MAP, referred to above, and ACQUIRE, which maps and solves for the required offsets.