At monitoring from flying apparatus of impurity gases in atmosphere an opportunity of lidars use are limited by virtue of low of an energy level of a scattered laser signal. The certain advantages, in this sense, are kept for spectrometers working in solar light. For example, in a window 1400 - 1700 nm, spectral density of radiation from a surface of ground is 200 W/m2/mkm. At such measurements the level of radiation falling on the photoreceiver does not depend on height of flight, and is determined by product of squares of the linear and angular apertures on a spectrometer input. At the same time it is known, that, at the given requirement under the resolution, the spectrometer work effectiveness is reduced in accordance with increase of the angular aperture. In the article Fabry-Perot interferometer behavior, with conjugate convex-concave mirrors, is considered. For this purpose by the method of Green's functions, in spherical coordinates, it is solved the non-uniform wave equation for a source uniform -- distributed in the coordinates origin. Is received, that the field on a surface of interferometer mirrors is increased only at increase of the cross sizes of a source up to 2 - 3 lengths of radiation waves. From a kind of the decision follows, that introduction in source area of an amplitude or phase mask of determined frequency consisting of rings results in increase of a signal on the interferometer mirror in a wide increasing range of the source sizes. From results of the decision of this problem follows, that for a return course of beams with the large angular aperture, from interferometer to center of curvature there is the system of concentric rings with shifts on a phase on 180 degrees. Introduction of a phase or amplitude mask allows to compensate or to remove appropriate antiphase components and by that essentially to increase a signal power level, and, accordingly, sensitivity of the device.