A structure of a sounding beam of optical radiation passing through the refraction channel at an angle of 90° to the channel optical axis is treated theoretically in the paper. An analytical solution has been obtained for the second - order mutual - coherence function of the sounding radiation field based on the Huygen-Kirchhoff method. The conditions have been determined when the maximum sensitivity of characteristics of a sounding optical beam to the parameters of refraction channel realizes. The wave-front bend of the sounding beam is shown to be large enough to ensure an acceptable accuracy when measuring the parameters of the refraction channel. The conditions of small aberration distortions of a sounding beam are given.