Abstract. Observations of stars with spectral resolution R=λ/δλ>200000 are reported to provide an opportunity to explore new effects and phenomena in stellar atmospheres, circumstellar shells and the interstellar medium. Several astrophysical problems requiring the observations with the spectral resolution of R≥200000 are reviewed briefly. Increasing spectral resolution necessitates collection of additional light, i.e., application of a telescope of large diameter. The value of the spectral resolution of a prism or a diffraction spectrograph is directly proportional to the diameter d of the collimated beam in the spectrograph and inversely proportional to the telescope diameter D. The dimensions of the ruled diffraction gratings with profiled grooves has reached their technological limits, so the potential for increasing d is exhausted. The use of a Fabry-Perot interferometer (FPI) in combination with a diffraction spectrograph is supposed to provide a perspective solution to the problem of registering stellar spectra with high and ultra-high-resolution. To increase the number of simultaneously registered spectral elements, a scheme including a spectrograph with crossed dispersion (an echelle spectrograph) used for the spatial separating orders of a FPI is considered. The approach is denoted as the method of double-crossed dispersion (DCD). Some problems of technical implementation and application of the DCD method at the 6-meter telescope BTA are discussed. Mathematical processing of the signal is also considered.

Keywords: large telescope, spectrograph, ultrahigh spectral resolution, Fabry-Perot interferometer, CCD matrix, stars, circumstellar envelopes, interstellar medium

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