Number 110 (Story #2), January 15, 1993 by Phillip F. Schewe and Ben Stein STELLAR OBSERVATIONS WITH 2-MILLIARCSECOND RESOLUTION at optical wavelengths have been made using interferometry techniques. Richard Simon of the Naval Research Lab (NRL) reported at last week's meeting of the American Astronomical Society the measurement of the diameters of 10 giant stars with the Mark III Optical Interferometer at the Mt. Wilson Observatory. Mark III, which determines diameters but cannot make images, uses the Young's-double-slit technique: light from a source (including stars) passing through each of two slits (or, in this case, telescopes separated by as much as 31 m) interferes with itself, producing light and dark bands (fringes) at a detector. Changing the telescope configuration changes the fringes in a way that is characteristic of the source's size. One of Simon's observations showed that the star Gamma Cassiopeia is slightly ellipsoidal. The NRL is currently building the Big Optical Array at the Lowell Observatory. This six-element interferometer should actually produce images, with resolutions as good as 200 microarcseconds (100 times better than that of the Hubble Space Telescope). Shri Kulkarni of Caltech predicted that at the AAS meeting ten years hence perhaps as many as one third of all papers would draw on optical-interferometer measurements.