Princeton's 80 cm diameter telescope with an ultraviolet spectrometer will be the prime experiment on board NASA's third Orbiting Astronomical Observatory (OAO-C) which is scheduled for launch in late 1971. Two scanning carriage arms within the spectrometer will each have one photomultiplier sensitive to a 1600–3200 Å first order spectrum and another photomultiplier which will register from 800 to 1600 Å in the second order. Since the primary purpose of the instrument will be to study interstellar absorption lines, the design philosophy has emphasized the role of carefully examining the detail of specific lines, rather than surveying a star's entire spectrum. Thus the carriage with the narrowest exit slits, providing 0.1 Å resolution in first order and 0.05 Å in second order, has been programmed to repetitively scan and retrace over any selected 0.7 Å interval in first order (or 0.35 Å in second order). Stepping motion in one direction every 16 s will occur for the second carriage, which will offer 0.4 and 0.2 Å resolutions for first and second orders, respectively. Approximately half of the starlight which is focussed on the entrance slit jaws is reflected into a fine guidance system which will enable the spacecraft to stabilize to within 0.1 arc sec on a star as faint as 7th magnitude.