The Anglo-Australian Observatory 2dF facility

The 2dF (Two-degree Field) facility at the prime focus of the Anglo-Australian Telescope provides multiple-object spectroscopy over a 2° 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 440 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 6s, to a precision of 0.3arcsec (20μm) 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 maximize 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. 2dF has achieved its initial astronomical goals. The redshift surveys obtain spectra at the rate of 2500 galaxies per night, yielding a total of about 200000 objects in the first four years. Typically a B=19 galaxy gives a spectrum with a signal-to-noise ratio of better than 10 per pixel in less than an hour; redshifts are derived for about 95 per cent of all galaxies, with 99 per cent reliability or better. Total system throughput is about 5 per cent. The failure rate of the positioner and fibre system is about 1:10000 moves or once every few nights, and recovery time is usually short. 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.