Optical vortex coronagraph demonstrated

One difficulty with finding extrasolar planets is that a planet’s light is overwhelmed by that of its parent star. To block that light, astronomers typically occult the star with a disk in an instrument called a coronagraph. At the University of Arizona, however, Grover Swartzlander’s group has developed another method: They focus the primary star onto the very center of a so-called optical vortex lens, which acts like a helical phase mask, and the on-axis source of light is removed from the optical path while the off-axis source of light passes through. Shown here is the central region of a 2-mm square OVL. The instrumentalists put the OVL into a coronagraph, incorporated some adaptive optics to eliminate the twinkling caused by atmospheric turbulence, and mounted the entire package on an 8-inch telescope that they pointed at the binary star system Cor Caroli in the constellation Canes Venatici. The false-color image on the left is what they obtained without the optical vortex coronagraph: Only the primary star, labeled a2, with its 12-fold more light flux than the secondary, can be seen. With the OVL in place, the secondary star, a1, became visible. The primary’s light was suppressed by 97%, but not over its entire disk because the optics were not optimally aligned. Next on the researchers’ agenda is to fabricate higher-quality OVLs and more advanced adaptive-optics and optomechanical alignment systems. (G. A. Swartzlander et al., Opt. Express 16, 10200, 2008.) — Stephen G. Benka