  This paper describes the Wide Field Spectrograph (WiFeS) under construction
at the Research School of Astronomy and Astrophysics (RSAA) of the Australian
National University (ANU) for the ANU 2.3m telescope at the Siding Spring
Observatory. WiFeS is a powerful integral field, double-beam, concentric,
image-slicing spectrograph designed to deliver excellent thoughput, wavelength
stability, spectrophotometric performance and superb image quality along with
wide spectral coverage throughout the 320-950 nm wavelength region. It provides
a 25x38 arcsec. field with 0.5 arcsec. sampling along each of twenty five 38X1
arcsec slitlets. The output format is optimized to match the 4096x4096 pixel
CCD detectors in each of two cameras individually optimized for the blue and
the red ends of the spectrum, respectively. A process of "interleaved
nod-and-shuffle" will be applied to permit quantum noise-limited sky
subtraction. Using VPH gratings, spectral resolutions of 3000 and 7000 are
provided. The full spectral range is covered in a single exposure at R=3000,
and in two exposures in the R=7000 mode. The use of transmissive coated optics,
VPH gratings and optimized mirror coatings ensures a throughput (including
telescope atmosphere and detector) > 30% over a wide spectral range. The
concentric image-slicer design ensures an excellent and uniform image quality
across the full field. To maximize scientific return, the whole instrument is
configured for remote observing, pipeline data reduction, and the accumulation
of calibration image libraries.
