SCORPIO-2 guiding and calibration system in the prime focus of the 6-m telescope

We describe a device (adapter) for off-axis guiding and photometric calibration of wide-angle spectrographs operating in the prime focus of the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. To compensate coma in off-axis star images an achromatic lens corrector is used, which ensures maintaining image quality (FWHM) at a level of about 1aEuro(3) within 15' from the optical axis. The device has two 54aEuro(3)-diameter movable guiding fields, which can move in 10' x 4.'5 rectangular areas. The device can perform automatic search for guiding stars, use them to control the variations of atmospheric transmittance, and focus the telescope during exposure. The limiting magnitude of potential guiding stars is m (R) similar to 17(m). The calibration path whose optical arrangement meets the telecentrism condition allows the spectrograph to be illuminated both by a source of line spectrum (a He-Ne-Ar filled lamp) and by a source of continuum spectrum. The latter is usually represented either by a halogen lamp or a set of light-emitting diodes, which provide illumination of approximately uniform intensity over the wavelength interval from 350 to 900 nm. The adapter is used for observations with SCORPIO-2 multimode focal reducer.