A method aimed to minimize the impact of spectral aberrations in a monochromator is proposed in which the spectrum of the source of radiation under study is scanned by the rectilinear translation of a plane chirped grating. The chirped grating, which has a spatially variable groove spacing, is used to diffract and to spectrally focus the radiation. Imaging properties of the chirped grating were analyzed in order to develop the expression of the aberration coefficients of the system and the expression of the width of the instrument line shape due to aberrations. The optimal rectilinear trajectory required to operate the monochromator without significant spectral aberrations in measurements has been obtained numerically and tested in the laboratory. Experimental measurements of the emission spectrum of a seven-wavelength helium-neon laser are presented, as well as the sensitivity of the monochromator performance to different geometrical parameters.