Spectroastrometry: a new approach to astronomy on small spatial scales

The technique of spectroastrometry (measuring the wavelength dependence of the position of an object) provides a means of studying the spatial structure of astronomical sources on scales much smaller than the seeing disk size or the diffraction limit. Despite successful demonstrations in the past, the method does not seem to be widely known. This paper describes techniques used at the Anglo-Australian Telescope to obtain such observations with standard instrumentation, and presents some examples of astronomical results on sources including binary stars and active galactic nuclei. The potential for combining spectroastrometry with interferometry to observe structure on microarcsecond scales is described.