In this mode the Westinghouse SEC TV camera views the Cassegrain image directly after a 90reflection by a plane mirror. Simultaneously, the spectrograph slit can be illuminated by the comparison arc source. In principle, an object can be acquired whilst the arc is exposed. However, arc exposures should be made with the telescope and spectrograph oriented close to the celestial exposure as is usual practice. Arc exposures are of the order of seconds. Cu-Ar and Cu-Ne hollow cathode lamps are available.