WFIRST coronagraph: Imaging Giant Exoplanets Around Nearby Stars
 
Technology Development: The Wide-Field Infrared Survey Telescope (WFIRST) is
the highest-ranked recommendation for a large space mission in the NRC 2010
decadal survey, New Worlds, New Horizons (NWNH) in Astronomy and Astrophysics.
The WFIRST coronagraph instrument (CGI) will be the first high-contrast
stellar coronagraph in space. It will enable WFIRST to respond to the goals of
NWNH by directly imaging and spectrally characterizing giant exoplanets
similar to Neptune and Jupiter, and possibly even super-Earths (extrasolar
planets with a mass higher than Earth's but lower than our Solar System's ice
giants, Neptune and Uranus), around nearby stars. The WFIRST CGI includes both
a Shaped Pupil Coronagraph (SPC) and a Hybrid Lyot Coronagraph (HLC). All
three of WFIRST's CGI technology milestones for 2015 were passed successfully.
coronograph imaging
 
Measured milestone contrasts for the HLC (middle) and SPC (left) in a vacuum
testbed in2015, where the milestone target contrast of 10-8 average in the
dark hole (the annularand wedge-shaped regions, respectively) was achieved for
both coronagraphs, as plannedand on schedule.
 
First, the HLC demonstrated a raw contrast (speckle/star intensity ratio) of
10-8, using a 10% bandwidth filter in visible light (550 nm), in a static
environment. Second, the SPC achieved the same milestone under the same
conditions. For both the HLC and SPC, the figure above shows excellent average
contrast (blue-green) over most of the field of view, and slight turn-up (red)
at the inner and outer radii, as expected. The third milestone was
accomplished when the Low Order Wavefront Sensing and Control (LOWFS)
subsystem achieved its goal of providing sensing of pointing jitter and
control at the 0.4 milli-arc-second rootmean- square (RMS) level, which will
keep a target star sufficiently centered on the coronagraph star-blocking
mask, when the WFIRST telescope experiences pointing drift and jitter.
Masks fabricated for WFIRST coronograph
 
Pupil-plane reflective mask for the SPC, 24-mm diameter, black silicon on
mirror (left).Image-plane reflective mask for the back-up technology Phase
Induced AmplitudeApodization Complex Mask Coronagraph (PIAA-CMC) coronagraph,
155-æm diameter,raised elements on silicon (center). Image-plane transmitting
mask for HLC, 100-æmdiameter, raised dielectric and metal on glass (right).
All masks were fabricated in theMicro-Devices Lab (MDL) at the Jet Propulsion
Laboratory (JPL).
 
Impact: With achievement of these milestones, NASA is a major step closer to
being confident that WFIRST will be able to directly image planets and dust
disks around nearby stars. There are at least 15 radial-velocity exoplanets
that both coronagraphs will be able to image in their dark hole regions, in a
few hours integration time each. The WFIRST coronagraph will enable scientists
to see these exoplanets directly for the first time, and the images will be in
their true colors (using some of the other color filters in the CGI). A
simulation is shown in the figure on page 9, where the blocked star is hidden
inside the annulus; a planet is seen at about 5 o'clock, and the star is
assumed to have no zodiacal dust around it (left) or a strong dust cloud
(right).
 
Status and Future Plans: WFIRST successfully completed its Mission Concept
Review in December 2015, in preparation for its Phase-A start the following
January (which was also successful). The CGI is baselined as a technology
demonstration instrument on WFIRST; it does not drive mission requirements
beyond those needed for the Wide Field Instrument. However, with one year of
allocated observing time out of a six-year mission, NASA expects that it will
achieve breakthrough science, and will demonstrate key technology elements for
follow-up missions, the next of which could be aimed at finding habitable
Earth-like planets around nearby stars.
simulated CGI imagery of planet
 
Simulation of expected image with CGI on WFIRST of a planet (at about 5
o'clock) with nozodiacal dust cloud (left) and with a zodiacal dust cloud
(right).
 
Sponsoring Organization: This coronagraph technology is jointly funded by the
Astrophysics Division's SAT program, in partnership with the NASA Space
Technology Mission Directorate (STMD). NASA JPL currently leads the
coronagraph development effort, and key contributions of the coronagraph team
have been provided by three former SAT PIs: Jeremy Kasdin at Princeton
University, John Trauger at NASA JPL, and Olivier Guyon at the University of
Arizona.
 
 
Regards,
 
Roger

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