ASTRONOMY: New petal designs improve planet-finding occulters

March 1, 2010
When I saw three satellites traverse the sky one evening in a triangular formation, I thought I had seen a UFO.

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 betweenyou guessed itmultiple 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."


  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,"˜dns/Theia/nastheiav14.pdf.
About the Author

Gail Overton | Senior Editor (2004-2020)

Gail has more than 30 years of engineering, marketing, product management, and editorial experience in the photonics and optical communications industry. Before joining the staff at Laser Focus World in 2004, she held many product management and product marketing roles in the fiber-optics industry, most notably at Hughes (El Segundo, CA), GTE Labs (Waltham, MA), Corning (Corning, NY), Photon Kinetics (Beaverton, OR), and Newport Corporation (Irvine, CA). During her marketing career, Gail published articles in WDM Solutions and Sensors magazine and traveled internationally to conduct product and sales training. Gail received her BS degree in physics, with an emphasis in optics, from San Diego State University in San Diego, CA in May 1986.

Sponsored Recommendations

Request a quote: Micro 3D Printed Part or microArch micro-precision 3D printers

April 11, 2024
See the results for yourself! We'll print a benchmark part so that you can assess our quality. Just send us your file and we'll get to work.

Request a free Micro 3D Printed sample part

April 11, 2024
The best way to understand the part quality we can achieve is by seeing it first-hand. Request a free 3D printed high-precision sample part.

How to Tune Servo Systems: The Basics

April 10, 2024
Learn how to tune a servo system using frequency-based tools to meet system specifications by watching our webinar!

Precision Motion Control for Sample Manipulation in Ultra-High Resolution Tomography

April 10, 2024
Learn the critical items that designers and engineers must consider when attempting to achieve reliable ultra-high resolution tomography results here!

Voice your opinion!

To join the conversation, and become an exclusive member of Laser Focus World, create an account today!