Abstract—Two modifications of the Fabry–Perot interferometer were developed that use oblique incidence planar fronts from the source: Fabry–Perot interferometer with an open beam input and Fabry–Perot interferometer with an open beam output. The modifications differ only in the direction of the rays. The first modification has already been used as a basic element of the solar interference spectrograph layout (Kulagin, 1980) and the stellar interference spectrograph layout (Kulagin and Panchuk, 2017). The second modification has no analogues. Estimates of the increase in the illumination of interference patterns in the modifications are given. The same formula describes the dependence of the intensity of the outgoing radiation on the interference phase in both devices. However, the properties of the modifications differ dramatically. Possible applications of the two modifications combined with a diffraction grating are described.