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An attempt is made to evaluate the influence of shot noise from discrete sources (often known as the "confusion effect") on the sensitivity of a radio telescope, when its real power beam pattern is taken into account. The paper considers the influence of many discrete sources, which are in the radio telescope field of view, on the upper limit of instrument sensitivity. All discrete sources, which are located in the sky both in the telescope main beam region and in the side lobes of the pattern, are included in the calculations from the point of view of a stochastic process. It is shown that when the sensitivity and angular resolution of a radio telescope are increased notably, then a strong limitation should be made on its side lobe level and scattering. Instrument shot noise, due to sources that are blended from a scattering field, evokes an increase in beam number per weak detected source, which is observed close to a sensitivity threshold.

radio telescope; survay; confusion; sensitivity