A photoelectric stellar spectrophotometer, with 0.3-Å resolution, for use near Hα (6563 Å) is described. A Fabry-Perot etalon and an echelle grating are simultaneously pressure-scanned. The Fabry-Perot can be bypassed and the grating system used alone to obtain narrow-band (4.89-Å) Hα indices, in which case the pressure system allows the necessary radial velocity corrections to be made conveniently.
James Caplan, "Pressure-Scanned Echelle Grating plus Fabry-Perot Stellar Spectrophotometer," Appl. Opt. 11, 1978-1985 (1972)