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We develop a high-sensitivity matrix radiometer which will be used on the large altazimuth telescope BTA of the Special Astrophysical Observatory (SAO) of the Russian Academy of Sciences ( Fig. 1) for performing astronomical observations in atmospheric transparency windows in the poorly explored frequency range from 0.13 to 0.38 THz (wavelengths from 2.3 to 0.8 mm). Mounting of the radiometer at the focal point of the 6-m BTA telescope will enhance the capability of the SAO in the transparency windows of the Earth's atmosphere from the UV range (wavelengths about 3000Å) up to the ultimate wavelength of the RATAN-600 telescope (about 0.5 m). This will make it possible to perform unique comprehensive multifrequency studies in one observatory. The radiometer is presumed to be used with other radio and optical telescopes, both in the above-mentioned and higher-frequency intervals of the terahertz range.

A helium 3 adsorption refrigerator cooled with a pulse tube cryocooler is developed. The refrig erator has two stages of adsorption pumping. The first stage is designed for the condensing and pumping of helium 4, whose cooling capacity is used for the condensation of helium 3. The second stage is designed for pumping helium 3. The final temperature reached by the refrigerator is 0.305 K. This temperature is retained for at least 24 h. The time required for helium recondensation and the obtainment of low temperature is 1.5 h.