A small grazing incidence monochromator was flown on the NASA OSO-4 satellite to monitor variations in the absolute flux of the solar He II Lyman-α line at 303.8 Å. The design of the instrument is described in some detail, and the procedures for photometric calibration are discussed. Examples are shown of the results obtained during the two-year period of operation in orbit.
P. J. Bowen, J. A. Bowles, W. M. Glencross, R. J. Speer, A. F. Timothy, J. G. Timothy, and A. P. Willmore, "A Grazing Incidence Monochromator for Satellite Studies of the Solar He II Lyman-α Line at 303.8 Å," Appl. Opt. 10, 28-33 (1971)