Patent ID: 8891082

Claim:
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 so that a focal curve of the spectrometer satisfies a back focus equation: cos 2 ⁢ α L A - cos ⁢ ⁢ α R + cos 2 ⁢ β L B - cos ⁢ ⁢ β R - n ⁢ ⁢ λ ⁢ ⁢ K = 0 where α is an angle of incidence, β is a diffraction angle, L A is an inlet back focal length, L B is an outlet back focal length, R is a grating radius indicative of a radius of curvature of a spherical grating surface, n is a diffraction order, λ is a wavelength at the diffraction angle β and K is a spherical grating constant, and further comprising: a compensation body, the light inlet opening disposed on the compensation body, the compensation body disposed in the light channel and fixed on the main optical body between the light source and the diffraction grating, the compensation body configured so that during thermal expansion of the main optical body the compensation body changes the distance between the light inlet opening and the grating central point such that a shift of the focal curve is corrected by overcompensating the thermal expansion of the main optical body by expansion of the compensation body.