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A vacuum chamber is used for testing WFIRST and other coronagraphs. A star is simulated inside the chamber using light brought in by an optical fiber, and the light of this “star” is suppressed in the testbed by coronagraph masks and deformable mirrors.
Above: A vacuum chamber is used for testing WFIRST and other coronagraphs. A star is simulated inside the chamber using light brought in by an optical fiber, and the light of this “star” is suppressed in the testbed by coronagraph masks and deformable mirrors.

WFIRST

Exoplanetary First Light

Daniel Wilson
The vast majority of exoplanets have been detected indirectly by measuring the gravitational tug on their host stars, or by sensing the dip in brightness of that star as a planet transits across our line of sight. The Wide-Field Infrared Survey Telescope (WFIRST), is the first NASA space telescope designed to directly image and characterize exoplanets as well as debris disks in other star systems that are potential planetary birthplaces.

WFIRST would include the first high-contrast stellar coronagraph instrument ever flown, utilizing optical components created at JPL and Princeton University to directly image large exoplanets from reflected light. The telescope would also be capable of performing spectroscopy on exoplanet atmospheres, especially super Earths, planets much like our own except larger. This would aid in identifying whether these large terrestrial worlds might be capable of supporting life. WFIRST’s complex optical train uses two types of coronagraphs, a Hybrid-Lyot Coronagraph (HLC), designed and built at JPL, and a Shaped Pupil Coronagraph (SPC) designed by Princeton University. JPL’s MDL is fabricating the masks for both. MDL is one of the few facilities in the world capable of creating these ultra-accurate optical components, which require gray scale lithography with nanometer-scale accuracy. The SPC uses a pupil-plane reflective mask that has ultra-dark regions of plasma-etched “black silicon” light-absorbing optics, also fabricated and patterned using electron-beam lithography.

These masks work in conjunction with the rest of the coronagraph’s optics to suppress the light of a star by nine orders of magnitude (one billion times) to enable direct imaging of the star’s orbiting planets. Diffraction and optical error must be minimized at each step of the process, despite the numerous folds, bends and other optical manipulations in the telescope’s light path.

To fine-tune the final image, a deformable mirror, designed and built by Xinetics, a division of Northrop Grumman, is inserted in the light path. This optic uses ultra-thin reflective glass with tiny mechanical actuators behind it—more than 2000 of them for a single 48-square-millimeter mirror that is smaller than a postage stamp—to precisely control the wavefront. Minute errors detected in the star’s image are converted into signals for these actuators, which then flex and distort the mirror to create tiny corrections—a change of less than the size of a single atom. This correction, combined with the tight manufacturing tolerances of the optical train, suppresses the bright light of the host star to create a nearly perfect image. WFIRST will seek to answer questions about large exoplanets and their environments in unparalleled detail.
A black silicon mask suppresses illumination from the host star to reveal light from an orbiting exoplanet that is up to a billion times fainter.
A black silicon mask suppresses illumination from the host star to reveal light from an orbiting exoplanet that is up to a billion times fainter.
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