  The SPHERE-2 balloon-borne detector designed for extensive air shower (EAS)
observations using EAS optical Vavilov-Cherenkov radiation (``Cherenkov
light''), reflected from the snow-covered surface of Lake Baikal is described.
We briefly discuss the concept behind the reflected Cherenkov light method,
characterize the conditions at the experimental site and overview the
construction of the tethered balloon used to lift the SPHERE-2 telescope above
the surface. This paper is mainly dedicated to a detailed technical description
of the detector, including its optical system, sensitive elements, electronics,
and data acquisition system (DAQ). The results of some laboratory and field
tests of the optical system are presented.
