Digitally Pressure-Scanned Fabry-Perot Interferometer for Studying Weak Spectral Lines

A system is described for digitally scanning a Fabry-Perot interferometer by changing the pressure of the gas in the interferometer chamber by small discrete steps. The system, intended for recording weak signals, i.e., of the order of a few counts per second, is very linear. Repetitive scans are reproduced within 1/640 of the spectral free range of the interferometer. An example of the depolarized Rayleigh line scattered by H(2) at 2.5 amagat is given.