The Visible Nulling Coronagraph (VNC) combines an interferometer with a coronagraph to image and characterize Jovian-size planets. In this photo, Goddard scientists Rick Lyon (foreground) and Udayan Mallik (left), who are joined by Pete Petrone (right), an employee of Sigma Space Corporation in Lanham, Md., are monitoring the progress of wavefront control using the VNC, which is operating inside a vacuum tank. Credit: NASA/Chris Gunn |
NASA’s
next flagship mission—the James Webb Space Telescope—will carry the
largest primary mirror ever deployed. This segmented behemoth will
unfold to 21.3 feet in diameter once the observatory reaches its orbit
in 2018.
A
team of scientists at NASA’s Goddard Space Flight Center in Greenbelt,
Md., now is developing an instrument that would image and characterize
planets beyond the solar system possibly from a high-altitude balloon
has borrowed a page from the Webb telescope’s playbook. It has created
an infinitely smaller segmented mirror that currently measures less than
a half-inch in diameter and promises to revolutionize space-based
telescopes in the future.
The
multiple mirror array (MMA), now being developed by the Berkeley,
Calif.-based Iris AO, Inc., under a NASA Small Business Innovative
Research grant, is one of the enabling technologies on the Visible
Nulling Coronagraph (VNC), a hybrid instrument combining an
interferometer with a coronagraph—in itself a first. In laboratory
tests, the VNC has proven that it can detect, image, and characterize
likely targets. “Nearly all the technologies are completed or are on
track,” said Principal Investigator Rick Lyon of NASA Goddard, who, with
his colleague, Mark Clampin, began working on the VNC more than three
years ago.
As
a result of that progress, the team hopes to apply the technology to a
balloon-borne instrument called the Big Balloon Exoplanet Nulling
Interferometer (BigBENI), which Lyon believes could be ready for
operations as early as 2016. Carried on a gondola attached to a
high-altitude balloon, the VNC-equipped BigBENI would be able to
suppress starlight and increase the contrast of Jupiter- and ultimately
Earth-sized planets.
The
science that BigBENI could perform is compelling, Lyon added. At
135,000 feet—the altitude at which NASA balloons fly—Lyon estimates he
could detect and image at least eight science targets in less than five
hours and an additional six in about 20 hours. “BigBENI offers a
nearer-term way to image planets” and search for specific chemicals that
might indicate the presence of life—a long-sought science goal.
Mirror array central to capability
Such
a capability is due in large part to the tiny mirrors, Lyon said. “MMA
is a legacy of the Webb telescope,” he added. “Segmented mirrors are the
future, not only for traditional observing missions like Webb, but also
for non-traditional uses, like the one we’ve developed for planet
finding. No other coronagraph has segmented mirrors.”
Under
the VNC/BigBENI concept—whose development NASA currently supports
through several technology-development programs—starlight collected by a
primary mirror or telescope travels down the instrument’s optical path
to the first of two beamsplitters within each arm of the VNC
interferometer. The MMA is located in only one arm. A second
beamsplitter recombines the beams into two output paths known as the
“bright” and “dark” channels. The starlight passes to the bright and the
planet light to the dark.
Because
MMA is a mirror image of the telescope, it can see wavefront and
amplitude errors caused by vibration, dust, and turbulence that prevent
the light from being perfectly focused as it’s collected. The MMA not
only senses those errors, but also corrects them.
Algorithms
that Lyon developed calculate errors in the telescope’s wavefront and
instruct MMA’s 169 tiny nano-size segments—each measuring the width of
three human hairs and perched atop tiny finger-like devices—to piston,
tip, and tilt up to thousands of times per second to precisely correct
the distortions and then cancel the starlight in the dark channel. A
second technology, the spatial filter array, passively acts in concert
with the MMA to further correct both amplitude and wavefront errors.
Combined,
these technologies allow the mirror array to create an internal
coronagraph to suppress starlight and increase the contrast of the
circumstellar region surrounding a star, thereby allowing scientists to
detect planets and dust disks. BigBENI’s mirror array would contain 313
segments, Lyon said.
Applications abound
While
unique in its application as a coronagraph, MMA and its associated
wavefront-sensing-and-control technologies, hold great promise for other
applications, including medical imaging, LASIK eye surgery, and even
military gun sights, Lyon said. But for NASA, the benefit lies in being
able to fly less expensive telescopes.
“Ultimately
with this technology, you can get away with a low-cost, low-risk
primary mirror,” Lyon said. In contrast, Webb’s much larger segmented
mirror was expensive to build. Technicians carefully constructed the
mirror segments to an exact optical prescription and then mounted them
on a mechanism that positions each to perfect alignment, much like the
tiny fingers on MMA.
To
assure a perfect focus, however, the Webb telescope will first image a
target. After ground controllers have analyzed the image with multiple
algorithms, they then can send commands to tweak the mirrors’ alignment.
This compares with MMA’s ability to perform up to thousands of
wavefront calculations per second, position the mirror segments, and
then maintain a tight alignment—all from onboard the instrument.
“The
idea is can we come up with something that is up to hundreds of times
more precise than the Webb telescope’s wavefront control? I think we
can. We’re doing it now in a standard lab. If you can do wavefront
sensing and control fast enough, which we’ve proven, you can get away
with a not-so-great telescope,” Lyon said.
