IFU Unit in SCORPIO-2 Focal Reducer for Integral-Field Spectroscopy on
  the 6-m Telescope of the SAO RAS
  We describe the scheme and design features of the new IFU unit (Integral
Field Unit) meant to perform integral-field spectroscopy as a part of SCORPIO-2
focal reducer, which is mounted in the prime focus of the 6-m telescope of the
Special Astrophysical Observatory of the Russian Academy of Sciences. The
design of the unit is based on the principle of the formation of array spectra
using a lens raster combined with optical fibers. The unit uses a rectangular
raster consisting of 22x22 square 2-mm diameter lenses. The image of the object
is transferred by an optical system with a 23x magnification from the focal
plane of the telescope to the plane of the lens raster. The image scale is
0.75"/lens and the field of view of the instrument has the size of 16.5"x16.5".
The raster also contains two extra 2x7 lens arrays to acquire the night-sky
spectra whose images are offset by $\pm$3' from the center. Optical fibers are
used to transform micropupil images into two pseudoslits located at the IFU
collimator entrance. When operating in the IFU mode a set of volume phase
holographic gratings (VPHG) provides a spectral range of 4600-7300 \AA\ and a
resolution $\lambda/\delta\lambda$ of 1040 to 2800. The quantum efficiency of
SCORPIO-2 field spectroscopy is 6-13% depending on the grating employed. We
describe the technique of data acquisition and reduction using IFU unit and
report the results of test observations of the Seyfert galaxy Mrk 78 performed
on the 6-m telescope of the Special Astrophysical Observatory of the Russian
Academy of Sciences.
