A step sector is described which can be used with astigmatic spectrographs to give results in every way as good as those obtained with conventional sectors and stigmatic instruments. In its action it may be described as a cross between a focal plane shutter and a band saw, as it consists of a continuous band of opaque film with a succession of stepped openings cut from its center. This film is passed at high speed in front of the focal curve next to the sensitive emulsion, then around a return path which is not in the optical path of the instrument. Added versatility is achieved by the inclusion of adjustable vertical and horizontal masks.
RALPH E. THIERS, "A Step-Sector for the Astigmatic Spectrograph," J. Opt. Soc. Am. 40, 849-851 (1950)