Vacuum spectroscopy, the spectroscopy of the short-wavelength UV region and of soft X-radiation (from 2*102 to 0.4—0.6 nm). In this region of the spectrum, the so-called vacuum region, air absorbs strongly, and to study spectra in it vacuum spectral instruments are used, whose optical parts and detector are placed in a chamber that is evacuated (to 10-5 mm Hg) or filled with an inert gas. The spectra observed in vacuum spectroscopy are due to electronic quantum transitions in singly and multiply ionised atoms, and also in some molecules. Vacuum spectroscopy studies emission and absorption spectra in order to obtain information about the energy levels of ions and molecules and for the systematics of spectra. The methods of vacuum spectroscopy are used to study processes in high-temperature plasma. The study of multiply charged ions by the methods of vacuum spectroscopy is of great importance for deciphering the spectra of stars, nebulae and other cosmic objects. The spectral instruments and methods of vacuum spectroscopy have a number of specific features. There are no optical materials transparent throughout the vacuum region, and so, for example, instruments with prisms and lenses made of LiF and CaF2 crystals are used only down to wavelengths of 1.1*102 and 1.25*102 nm. In the shorter-wavelength region diffraction gratings (including crystals, for example mica) are used as the optical elements. To photograph spectra in vacuum spectroscopy, so-called Schumann plates are used, with a high content of silver bromide and a very low content of gelatin (the gelatin of the photographic emulsion of ordinary plates absorbs strongly in the vacuum region). Sensitised plates are also used. Counters of ionising radiation are used as detectors in vacuum spectroscopy as well. The source of radiation in vacuum spectroscopy is usually a high-voltage vacuum, or “hot”, spark, produced at a voltage above 5*104 V across a spark gap of about 1 mm.