The invention relates to a spectrometer comprising a hollow main optical body having at least one light channel, a light source, a diffraction grating having a grating central point, a light inlet opening, and a detector unit, which are arranged in such a way that the focal curve of the spectrometer satisfies the back focus equation. In order to create a spectrometer having sufficient spectral resolution from a low-price, light, and easy-to-process material, which spectrometer is able to operate in a large temperature interval even without thermostatic control, according to the invention the light inlet opening is arranged on a compensation body, the compensation body is arranged in the light channel and fastened to the main optical body between the light source and the diffraction grating, and the compensation body is dimensioned in such a way that the compensation body changes the distance between the light inlet opening and the grating central point when the main optical body thermally expands.