A high resolution etalon-grating spectrometer or monochromator. A preferred embodiment presents as extremely narrow slit function in the ultraviolet range and is very useful for measuring bandwidth of narrow band excimer lasers used for integrated circuit lithography. Light from the laser is focused into a diffuser (D) and the diffused light exiting the diffuser illuminates an etalon (ET). A portion of its light exiting the etalon is collected and directed into a slit positioned at a fringe pattern of the etalon. Light passing through the slit is collimated (L3) and the collimated light illuminates a grating (GR1) positioned in an approximately Littrow configuration which disperses the light according to wavelength. A portion of the dispersed light representing the wavelength corresponding to the selected etalon fringe is passed through a second slit (2) and monitored by a light detector (PMT). When the etalon and the grating are tuned to the same precise wavelength a slit function is defined which is extremely narrow as about 0.034pm "FWHM" and about 0.091pm "95 percent integral". The etalon and the grating are placed in a leak-tight containment (50) filled with a gas, such a nitrogen or helium.