Optical Vortex May Enable Imaging of Extrasolar Planets

Hank Hogan

Despite the nursery rhyme, that star twinkling in the night sky is far from little. Moreover, any planets that orbit the star are exceedingly dim and tiny in comparison, making it difficult for astronomers to spot such extrasolar planets.

Researchers at the University of Arizona in Tucson may have come up with a solution to this problem in the form of an optical vortex coronagraph that blots out a star’s light while leaving a planet’s image untouched. Similar techniques have been proposed before, but optical science associate professor Grover A. Swartzlander Jr. said that his team’s approach offers some advantages.

“Our vortex phase mask is simpler to fabricate and has fewer parameters to control,” he said. “Our mask can be made achromatic.”

Spotting a planet-size object close to a star that is typically 10 million times as bright as the body is a challenge. As a result, the dozens of extrasolar planets discovered to date have been detected by measuring slight wobbles in the star’s motion or minute changes in its apparent brightness.

Large planets in strange orbits have been the easiest to find, but there has been no direct confirmation of their existence. There also has been no way to find planets that are more like the ones in our solar system. What is needed is a way to get rid of the starlight without reducing the light from any extrasolar planets.

The optical vortex coronagraph can be integrated into existing telescope systems, as demonstrated in this image of Saturn, taken using the apparatus. Atmospheric turbulence prevented the researchers from using the mask in coronagraph mode. Courtesy of Grover A. Swartzlander Jr.

The optical vortex coronagraph accomplishes this by using three lenses, a spatial filtering mask and a Lyot stop. The filtering mask, which is placed at the focus of the first lens, eliminates light from a source on the optical axis while allowing light from an off-axis source to pass. The combination of the second and third lenses, along with the Lyot stop, results in a system that, in theory, can extinguish starlight without attenuating the signal from a resolvable planet.

In experimental tests, the concept works, with lab trials demonstrating that the light from artificial stars could be reduced by up to a factor of 1000. The goal is to build a system suitable for use on a space telescope. For that to happen, more research and development is needed.

“We need to make and test an achromatic vortex mask that achieves a star-planet contrast of about 10 orders of magnitude,” Swartzlander explained.

The achromatic aspect, he noted, requires overcoming wavelength-dependent effects in the lenses and other components. This could be done with dispersion-compensating techniques, and the group is working on such an approach to solve this problem.