new optical device might allow astronomers to view extrasolar
planets directly without the annoying glare of the parent star. It
would do this by "nulling" out the light of the parent star by
exploiting its wave nature, leaving the reflected light from the
nearby planet to be observed in space-based detectors.
years ago, the presence of planets around stars other than our sun
was first deduced by the very tiny wobble in the star's spectrum of
light imposed by the mutual tug between the star and its satellite.
Since then, more than 100 extrasolar planets have been detected in
this way. Also, in a few cases the slight diminution in the star's
radiation caused by the transit of the planet across in front of the
star has been observed.
Many astronomers would, however, like to view the planet directly, a
difficult thing to do.
Seeing the planet next to its bright star
has been compared to trying to discern, from a hundred meters away,
the light of a match held up next to the glare of an automobile's
headlight. The approach taken by Grover Swartzlander and his
colleagues at the University of Arizona is to eliminate the star's
light by sending it through a special helical-shaped mask, a sort of
lens whose geometry resembles that of a spiral staircase turned on
The process works in the following way: light passing
through the thicker and central part of the mask is slowed down.
Because of the graduated shape of the glass, an "optical vortex" is
created: the light coming along the axis of the mask is, in effect,
spun out of the image. It is nulled, as if an opaque mask had been
placed across the image of the star, but leaving the light from the
nearby planet unaffected.
The idea of an optical vortex has been around for many years, but it
has never been applied to astronomy before. In lab trials of the
optical vortex mask, light from mock stars has been reduced by
factors of 100 to 1000, while light from a nearby "planet" was
unaffected (see figure).
Attaching their device to a telescope on Mt. Lemon outside
Tucson, Arizona, the researchers took pictures of Saturn and its
nearby rings to demonstrate the ease of integrating the mask into
telescopic imaging system. This is, according to Swartzlander
(520-626-3723, firstname.lastname@example.org), a more practical
technique than merely attempting to cover the star's image, as is
done in coronagraphs, devices for observing our sun's corona by
masking out the disk of the sun. It could fully come into its own
on a project like the Terrestrial Planet Finder, or TPF, a proposed
orbiting telescope to be developed over the coming decade and
designed to image exoplanets.
Foo et al., Optics Letters, 15
Summary of articles related to optical vortex on
Swartzlander's Web page