A neutron source for boron neutron capture therapy based on a vacuum-insulated tandem accelerator has been developed and operates at Budker Institute of Nuclear Physics. Conducting a ~10 mm proton beam with a power of up to 20 kW through a system of accelerating electrodes and 16 mm argon stripping tube is not an easy task. Any mistake made by operator or a malfunction of the equipment responsible for the correction of the beam position in the ion beam line can lead to permanent damage to the accelerator when a powerful beam hits the surface of the stripping tube or diaphragms of the electrodes. To determine the position of the proton beam inside the argon stripping tube, optical diagnostics have been developed based on the Celestron Ultima 80-45 telescope and a cooled mirror located at an angle of 45° to the beam axis in the straight-through channel of the bending magnet. The cooled mirror, in addition to the optical function, also performs the function of measuring the neutral current due to the electrical isolation of the mirror and the extraction of secondary electrons from its surface. The luminescence of a beam in the optical range, observed with the help of the developed diagnostics, made it possible for the first time to determine beam size and position inside the stripping tube with an accuracy of 1 mm. The light sensitivity of applied optical elements is sufficient for using a shutter speed from 2 to 20 ms to obtain a color image of the beam in real time. This makes it possible to realize a fast interlock on the event of a sudden displacement of the beam.