Natural fluorescence is a very weak signal, which represents only a very small fraction of the light emanating from the surface. The only method to detect natural ground fluorescence is to observe in the Fraunhofer lines of the solar spectrum where the otherwise much stronger reflectance background is significantly reduced. Ideally, would a Fraunhofer line be completely dark, the fluorescence would introduce some light at the line position visible on a black background. The spectrometer is calculated for four Fraunhofer lines, any combination of which can be used for the measurements. The first concave pre - dispersing grating focuses the selected lines to the entrance slits. The second part of the device consists of two concave diffraction gratings, one for the blue channel, and one for the red channel. These gratings focus light to the same detector array. No other optical elements are necessary. Spectrometer shows diffraction limited image quality. All the gratings surfaces have spherical form. This spectrometer has small dimensions (about 350 x 250 x 70 mm) and can be attractive for the space applications. Several modifications of the spectrometer and some aspects of its diffraction gratings fabrication and their diffraction efficiency are discussed.