A detector system for spatially resolved detection of a gas substance in an area, comprising; - at least one detector (251, 252, 351, 352) comprising an image sensor; - a band filter arranged in an optical beam path before the detector for transferring a wavelength spectrum of interest including an absorption wavelength corresponding to the gas substance; - a telescope, a polarizing beam splitter, and at least one interferometric stage (I, II) comprising a retarder for creating a required optical path difference for measuring absorption dips due to the presence of the gas substance; - wherein the telescope is arranged to image the area onto the at least one detector; - wherein the polarizing beam splitter is arranged to produce polarized beams of said optical beam of mutually orthogonal polarization in respective branches (i, ii), said at least one detector provided in at least one of the branches; and - wherein said at least one interferometric stage is arranged to receive one of said polarized beams to produce an interferometric image on said at least one detector; - wherein the retarder comprises multiple birefringent media (20, 30) having mutually different birefringence and each having optical axes that are not aligned with the polarized beam in order to create polarized beam components traveling along the respective optical axes of the birefringent media having the required optical path difference; said multiple birefringent media arranged with the optical axes relative to each other so that at least one increases an optical path difference and at least one decreases said optical path difference between the polarized beam components, wherein the thicknesses of said birefringent media are tuned to minimize a focal shift between the polarized beam components. - at least one detector (251, 252, 351, 352) comprising an image sensor; - a band filter arranged in an optical beam path before the detector for transferring a wavelength spectrum of interest including an absorption wavelength corresponding to the gas substance; - a telescope, a polarizing beam splitter, and at least one interferometric stage (I, II) comprising a retarder for creating a required optical path difference for measuring absorption dips due to the presence of the gas substance; - wherein the telescope is arranged to image the area onto the at least one detector; - wherein the polarizing beam splitter is arranged to produce polarized beams of said optical beam of mutually orthogonal polarization in respective branches (i, ii), said at least one detector provided in at least one of the branches; and - wherein said at least one interferometric stage is arranged to receive one of said polarized beams to produce an interferometric image on said at least one detector; - wherein the retarder comprises multiple birefringent media (20, 30) having mutually different birefringence and each having optical axes that are not aligned with the polarized beam in order to create polarized beam components traveling along the respective optical axes of the birefringent media having the required optical path difference; said multiple birefringent media arranged with the optical axes relative to each other so that at least one increases an optical path difference and at least one decreases said optical path difference between the polarized beam components, wherein the thicknesses of said birefringent media are tuned to minimize a focal shift between the polarized beam components.