  The 2dF (Two-degree Field) facility at the prime focus of the
Anglo-Australian Telescope provides multiple object spectroscopy over a 2
degree field of view. Up to 400 target fibres can be independently positioned
by a complex robot. Two spectrographs provide spectra with resolutions of
between 500 and 2000, over wavelength ranges of 440nm and 110nm respectively.
The 2dF facility began routine observations in 1997.
  2dF was designed primarily for galaxy redshift surveys and has a number of
innovative features. The large corrector lens incorporates an atmospheric
dispersion compensator, essential for wide wavelength coverage with small
diameter fibres. The instrument has two full sets of fibres on separate field
plates, so that re-configuring can be done in parallel with observing. The
robot positioner places one fibre every 6 seconds, to a precision of 0.3 arcsec
(20micron) over the full field. All components of 2dF, including the
spectrographs, are mounted on a 5-m diameter telescope top-end ring for ease of
handling and to keep the optical fibres short in order to maximise UV
throughput . There is a pipeline data reduction system which allows each data
set to be fully analysed while the next field is being observed.
  In this paper we provide the historical background to the 2dF facility, the
design philosophy, a full technical description and a summary of the
performance of the instrument. We also briefly review its scientific
applications and possible future developments.
