ASTRONOMY: New petal designs improve planet-finding occulters

When I saw three satellites traverse the sky one evening in a triangular formation, I thought I had seen a UFO. But when I learned about photon thrusters that can maintain nanometer precision between—you guessed it—multiple satellite elements that are part of a satellite array, I was not surprised to learn about another intriguing astronomical application of photonics: occulters that take the shape of a flower with numerous petals and fly in formation with a space-based telescope to block the light from a star so that orbiting planets can be found.

While circular-shaped occulters for solar coronagraphy were actually flown on the SOHO and STEREO solar missions and were attached to the telescope on booms, planet-finding occulters must be precisely shaped and located tens of thousands of kilometers from the telescope to provide very small angular size and deep suppression of starlight. For the flower-shaped occulters, the petals represent a form of diffractive optical element that is an approximation to radial apodization functions. Diffraction from the petals can be modeled as a propagation integral for a smooth apodization, suppressing the light from a star being studied for the existence of orbiting planets. To reduce the fragility of deployment in space, reduction in the number of petals for the occulter is desirable and was the impetus for research at Princeton University (Princeton, NJ) and the Jet Propulsion Laboratory (JPL; Pasadena, CA) in developing new petal designs that reduce the number of petals without affecting starlight suppression (see figure).¹

Asymmetric petals

By using Babinet's principle, in which the electric field at the pupil of a telescope is approximated by the Fresnel transform of the complementary hole (in this case, the flower-shaped occulter) subtracted from a propagated plane wave, the researchers can calculate just how the petal shape of the flowers impacts the shadow across the aperture of the telescope. Mathematical analysis, including nonlinear optimization methods, reveals that introduction of an asymmetry in the petal shape actually allows the number of petals in the occulter to be reduced without decreasing the amount of starlight suppressed at the aperture of the telescope.

For the Telescope for Habitable Exoplanets and Intergalactic/Galactic Astronomy (THEIA) occulter design, which is approximately 40 m in diameter with 20 petals (and designed to make observations from 250 to 700 nm at a distance of 55,000 km from the telescope and from 700 to 1000 nm at a distance of 35,000 km), the design has an unusual asymmetry in the edges of the petals that allows the occulter to be reduced to only 12 petals.²

Other petal shapes

In addition to asymmetric petals with unusual edge shapes, the researchers modeled several other designs that functioned as well as a 20-petal design, including adding connecting elements between the petals as well as including open spaces at the tips of the petals. Many of these designs also allowed the length of the petals to be shortened, again improving the chances for robust deployment of the occulter elements in space. They also allow engineering considerations, such as stability and configuration, to be explicitly included in the optical design. Stuart Shaklan, a researcher at JPL and one of the authors of the paper, says, "In addition to shortening the petals, this approach enables the creation of more engineering-friendly shapes, including minimum gap widths and convenient hinge locations."

Occulter missions such as THEIA are strong candidates for finding small, rocky planets around nearby stars. David Spergel of Princeton University, the principal investigator for THEIA, says, "The great technical challenge for astronomy for the coming years will be to directly image and eventually characterize an Earth-like planet around a nearby star. Planet imaging and detection is currently an important aspect of NASA long-term plans. The occulter technology outlined in the paper should enable us to detect an Earth around a nearby star with a modest-sized telescope and potentially enable us to discover whether the planet is habitable and to detect signs of life."

REFERENCES

1. E. Cady et al., Opt. Express 18, 2, 523–543 (Jan. 18, 2010).
2. N.J. Kasdin and the THEIA collaboration, "THEIA: Telescope for habitable exoplanets and interstellar/intergalactic astronomy," http://www.astro.princeton.edu/˜dns/Theia/nastheiav14.pdf.