Coronagraphic methods for detecting faint astronomical objects near bright ones: Application to the 2.5-m telescope of Sternberg Astronomical Institute

Possible methods for reducing the instrumental background of astronomical telescopes are discussed: the classical Lyot coronagraphic method, the multi-cascade coronagraphic method, the use of special masks in the plane of the entrance aperture, and the use of super-smooth and medium-smooth primary optics. For the first time, analytical expressions describing the intensity distribution at the exit pupil are presented for the case when a mask is placed in the primary focal plane in order to block the light from a bright object. An algorithm for computing the point-spread function (PSF) of a coronagraphic telescope employing super-smooth primary optics and the coronagraphic method is proposed. Computed PSFs for the 2.5-m telescope of the Caucasian Mountain Observatory of Sternberg Astronomical Institute are presented, without allowance for diffusive and turbulent scattering in the Earth’s atmosphere, and for operation both without coronagraphic elements and in the coronagraphic mode of the telescope.