an improved raster scanner , and electrostatographic printing machines which use such scanners , in which facet tracking is achieved by incorporating a semiconductor laser having an electronically tunable wavelength and a wavelength dispersive element which directs the laser beam onto the facets of a rotating polygon . the wavelength dispersive element is positioned , and the wavelength output from the laser is adjusted , such that as the polygon rotates the laser beam produces a spot on a facet which tracks the facet .