The Optical Angle Marker is a double mirror device, consisting of two first surface flats, and is used together with an autocollimator. The two mirrors are aligned with their faces nearly parallel. When the mirror assembly is rotated in azimuth relative to the autocollimator, a series of return images, spaced at equal angular intervals, can be viewed. In this way, the angular range of an autocollimator may be extended by a factor of six or more, depending on size and quality of the flat mirrors. Experimental data is described in this paper.