l. Introduction

At the request of the editors of the series Itogi nauki i Tekhniki (Soviet Scientific Reviews), we have attempted to tell how an extremely deep survey at centimeter wavelengths was carried out using the RATAN-600 radio telescope.

The entire project was called "Experiment Cold". Its goal was to test the limiting capabilities of the RATAN-600 radio telescope with the lowest noise possible at the present time in the telescope and radiometer system. The best radiometer (in terms of fluctuation sensitivity) known to us, based on a parametric amplifier with a closed-cycle helium cooling system, was used in the experiment; measures were taken to "cool" the radio telescope itself; and a method of controlling the numerous sources of interference was carefully thought out. All of these measures taken together allowed us to actually attain the fluctuation sensitivity of the radiometer on the sky and, after averaging the results over several months of observations, to reach a sensitivity close to the maximum possible for the RATAN-600 radio telescope. This limit turned out to be less than one millijansky in flux density and was set by faint background sources. Around 1980, when Experiment Cold was begun, the deepest surveys at centimeter wavelengths were being carried out at the Bonn 100-meter radio telescope with a limit of observation of roughly 14 millijanskys in flux density and a sensitivity of several millikelvins in brightness temperature (both limits determined by background sources).

Since we succeeded in obtaining a significantly higher sensitivity in both flux density and surface brightness in Experiment Cold, this experiment allowed new astrophysical results to be obtained in the fields of extragalactic and galactic radio astronomy. Here, we will describe results on the statistics of radio sources and the results of a search for anisotropy in the cosmic background radiation in detail, and briefly give the preliminary results of a study of the radio emission of the Galaxy. Several two-to-four-month cycles of observations in the "Cold" program, involving experiments in extremely diverse branches of radio astronomy, have already been carried out using the new radiometer, and it is impossible for us to describe only the first two cycles ("Experiment Cold-1" (1980) and "Experiment Cold-2" (1981)) here, and express our opinion on how the use of the new-generation radiometers on the RATAN-600 radio telescope will aOect prospects for the further development of radio astronomy and how this may significantly change the resources of radio astronomy for studying the most distant regions of the Universe.

August 24 1998, stokh@brown.nord.nw.ru