A high resolution etalon-grating spectrometer. A preferred embodiment presents an 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 and the diffused light exiting the diffuser illuminates an etalon. 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 and the collimated light illuminates a grating positioned in an approximately Littrow configuration which disburses the light according to wavelength. A portion of the dispursed light representing the wavelength corresponding to the selected etalon fringe is passed through a second slit and monitored by a light detector. When the etalon and the grating are tuned to the same precise wavelength a slit function is defined which is extremely narrow such as about 0.034 pm (FWHM) and about 0.091 pm (95 percent integral). The etalon and the grating are placed in a leak-fight enclosure filled with a gas, such as nitrogen or helium. The wavelength scanning of the spectrometer is done by changing the gas pressure in the enclosure during the scan.