Optical vortex

A new optical device developed by engineers at the University of Arizona might allow astronomers to view extrasolar planets directly without the annoying glare of the parent star.

About ten 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.

But many astronomers would, however, like to view the planet directly - a difficult thing to do since 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.

Now an approach taken by Grover Swartzlander and his colleagues at the University of Arizona eliminates 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 its side.

The mask nulls the light from the star while leaving the light from the nearby planet unaffected.

 These laboratory images demonstrate how the optical vortex coronograph works. They were obtained with a green "star" and red "planet" (point light sources). (a) shows how the green light is "spun out," while the red light remains unaffected. Images of the point sources are shown when large (b) and small (c) apertures are used to limit the transmission of light from (a).

The idea of such an optical vortex mask 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.

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 a telescopic imaging system.