Expected efficiency of the Eurasian Submillimeter Telescopes (ESMT) at a Cassegrain focus in the range of 1-3 mm

The paper evaluates the atmospheric limitations and aberration losses of ESMT efficiency at the Cassegrain focus in the range of 1-3 mm. It is shown that the radio brightness temperatures of dry and humid atmospheres can differ by an order of magnitude and amount to Tb = 42 K / 69 K at a wavelength of 3 mm / 2 mm at PWV = 5 mm, which becomes critical for the sensitivity of cryo-radiometers. All ESMT aberration losses at a wavelength of 1–3 mm in the field of view FOV = ±15' do not exceed 2% except for the curvature of the image field (KPI). Expected losses from the KPI at a wave of 3 mm / 2 mm / 1 mm for a flat matrix with a counter-reflector diameter of d = 2.5 m are 10% / 20% / 90%, and the KPI itself can reach 17 mm at the edge of the image field. To compensate for the KPI, the phase centers of the horns of the matrix need to be located along the Petzval surface, which has a deviation from the plane of 2 mm at the edge of the image field. Single and multi-beam ESMT radiation patterns in the range of 2 mm and 3 mm for a two-dimensional and low-cost two-row matrix of receiving elements in the Cassegrain focus were calculated by the method of direct integration of the aperture. With their help, realistic expected images of the disk of the Sun with compact sources and active regions were obtained.