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More on Screening Out Starshine to Find Planets

Centauri Dreams has written before about Grover Swartzlander (University of Arizona), who is developing new ways to screen out the light of a star to make it possible for astronomers to study the planets around it. At the heart of Swartzlander’s effort is something called an optical vortex mask, which is said to be ‘a thin, tiny, transparent glass chip that is etched with a series of steps in a pattern similar to a spiral staircase.’

And here’s how this chip does its job: incoming light slows down more in the thicker parts of the chip than in the thinner ones, with the result that some waves of light eventually becomes 180 degrees out of phase with others. Reaching the ‘eye’ of the vortex, the waves that are 180 degrees out of phase with one another cancel each other out, so that a dark central core remains. Swartzlander says the effect is like light following the threads of a bolt; the distance between adjacent threads is crucial to the outcome.

So could this technology be used to see a planet that is ten billion times fainter than its parent star? A coronograph using such a mask might pull it off. A standard coronograph, which uses an opaque disk to block the star’s light, wouldn’t suffice because of diffraction around the disk, which as we recently saw, is a problem that has also been under intense investigation by Webster Cash at the University of Colorado, Boulder, not to mention the Jet Propulsion Laboratory’s own efforts.

Swartzlander has tested his ideas at Steward Observatory’s 60-inch Mount Lemmon telescope in Arizona, and thinks that he too can cancel out that diffraction problem. “Any small amount of diffracted light from the star is still going to overwhelm the signal from the planet,” Swartzlander explained. “But if the spiral of the vortex mask coincides exactly with the center of the star, the mask creates a black hole where there is no scattered light, and you’d see any planet off to the side.”

Centauri Dreams‘ take: The Mount Lemmon work was limited by the fact that the telescope there is not equipped with adaptive optics that could zero out atmospheric turbulence. Swartzlander is now applying for grants to improve the quality of the mask, and working on numerical simulations to make it more effective. All of which is promising in a variety of areas — there is talk of using the technology in microscopy as well — but in terms of planet hunting, the occulter spacecraft Webster Cash is proposing to NASA this spring uses technologies that seem to be much further along and therefore likely to pay off sooner for the purposes Terrestrial Planet Finder was originally designed to accomplish. Cash’s consortium — Ball Aerospace, Goddard Space Flight Center, Northrop Grumman and Princeton University — is ready to design a practical spacecraft around its ideas.