Aperture Masking Resolves Bright Star

Robert C. Pini

To study the secrets of a massive, bright blue star, astronomers at the University of California's Berkeley Space Sciences Laboratory dusted off an old technique -- aperture masking interferometry -- and applied it to one of the world's shiniest telescopes, the Keck 1 at Mauna Kea, Hawaii.

More than 100 years old, the technique masks a telescope's aperture to form a series of separate circular areas over the secondary mirror -- in effect, a series of separate telescopes. Although the method throws away most of the light the telescope gathers, the array of telescopes increases the number of images that can be simultaneously recorded, processed and combined into a single, detailed picture with the same resolution as the main mirror.

Peter G. Tuthill and Space Sciences colleagues John D. Monnier and William C. Danchi used Code V software from Optical Research Associates in Pasadena, Calif., to determine the geometry of their aluminum mask, which covered more than 90 percent of the mirror and created 36 areas corresponding to each of the telescope's segments. Each unmasked area on the 10-m mirror was only 35 cm across.

A new puzzle

Once the mask was bolted into place, the astronomers pointed the telescope to Wolf-Rayet 104, a massive star 4800 light-years from Earth and beyond the limits of conventional observation. They recorded light waves reaching the mirror with a near-infrared camera, and the 36 images were added together using Fourier techniques to make a single image that eliminated much of the distortion from the Earth's atmosphere.

The resulting image, the first clear resolution of a dusty spiral surrounding the brilliant star, surprised the astronomers. They expected the star's radiant ultraviolet energy would burn up dust as it formed. Instead, the dust remained and trailed out from the star, forming a pinwheel tail.