A three-channel spectrometer for gas detection, comprising: a beamsplitter element that receives an incident radiation and transmits a first portion, a second portion, and a third portion of the incident radiation, a first slit that receives the first portion and transmits a first slit output radiation of a first wavelength; a second slit that receives the second portion and transmits a second slit output radiation of a second wavelength; a third slit that receives the third portion of the incident radiation and transmits a third slit output radiation of a third wavelength, wherein the first, the second and the third wavelength differ from each other; a dispersive element; and a focal plane array; characterised by a common optical form that receives and collimates the first, second, and third slit output radiation, wherein the optical form is a double-pass all-reflective optical form, and wherein: the dispersive element receives and reflects the collimated first, second, and third slit output radiation from the optical form, the product of a wavelength and a diffraction order is made into the substantially same value with respect to each of the said first, second and third wavelength, the common optical form images the reflected first, second and third slit output radiation onto the focal plane array; and the focal plane array receives and detects the reflected first, second, and third slit output radiation from the dispersive element. a beamsplitter element that receives an incident radiation and transmits a first portion, a second portion, and a third portion of the incident radiation, a first slit that receives the first portion and transmits a first slit output radiation of a first wavelength; a second slit that receives the second portion and transmits a second slit output radiation of a second wavelength; a third slit that receives the third portion of the incident radiation and transmits a third slit output radiation of a third wavelength, wherein the first, the second and the third wavelength differ from each other; a dispersive element; and a focal plane array; characterised by a common optical form that receives and collimates the first, second, and third slit output radiation, wherein the optical form is a double-pass all-reflective optical form, and wherein: the dispersive element receives and reflects the collimated first, second, and third slit output radiation from the optical form, the product of a wavelength and a diffraction order is made into the substantially same value with respect to each of the said first, second and third wavelength, the common optical form images the reflected first, second and third slit output radiation onto the focal plane array; and the focal plane array receives and detects the reflected first, second, and third slit output radiation from the dispersive element. the dispersive element receives and reflects the collimated first, second, and third slit output radiation from the optical form, the product of a wavelength and a diffraction order is made into the substantially same value with respect to each of the said first, second and third wavelength, the common optical form images the reflected first, second and third slit output radiation onto the focal plane array; and the focal plane array receives and detects the reflected first, second, and third slit output radiation from the dispersive element.