21 research outputs found
High Contrast Imaging and Wavefront Control with a PIAA Coronagraph: Laboratory System Validation
The Phase-Induced Amplitude Apodization (PIAA) coronagraph is a high
performance coronagraph concept able to work at small angular separation with
little loss in throughput. We present results obtained with a laboratory PIAA
system including active wavefront control. The system has a 94.3% throughput
(excluding coating losses) and operates in air with monochromatic light.
Our testbed achieved a 2.27e-7 raw contrast between 1.65 lambda/D (inner
working angle of the coronagraph configuration tested) and 4.4 lambda/D (outer
working angle). Through careful calibration, we were able to separate this
residual light into a dynamic coherent component (turbulence, vibrations) at
4.5e-8 contrast and a static incoherent component (ghosts and/or polarization
missmatch) at 1.6e-7 contrast. Pointing errors are controlled at the 1e-3
lambda/D level using a dedicated low order wavefront sensor.
While not sufficient for direct imaging of Earth-like planets from space, the
2.27e-7 raw contrast achieved already exceeds requirements for a ground-based
Extreme Adaptive Optics system aimed at direct detection of more massive
exoplanets. We show that over a 4hr long period, averaged wavefront errors have
been controlled to the 3.5e-9 contrast level. This result is particularly
encouraging for ground based Extreme-AO systems relying on long term stability
and absence of static wavefront errors to recover planets much fainter than the
fast boiling speckle halo.Comment: 18 pages, 12 figures. Accepted for publication in PASP. The pointing
control scheme for this system is described in a separate paper
(Coronagraphic Low-Order Wave-Front Sensor: Principle and Application to a
Phase-Induced Amplitude Coronagraph, The Astrophysical Journal, Volume 693,
Issue 1, pp. 75-84 (2009)
Performance of an Achromatic Focal Plane Mask for Exoplanet Imaging Coronagraphy
Coronagraph technology combined with wavefront control is close to achieving the contrast and inner working angle requirements in the lab necessary to observe the faint signal of an Earth-like exoplanet in monochromatic light. An important remaining technological challenge is to achieve high contrast in broadband light. Coronagraph bandwidth is largely limited by chromaticity of the focal plane mask, which is responsible for blocking the stellar PSF. The size of a stellar PSF scales linearly with wavelength; ideally, the size of the focal plane mask would also scale with wavelength. A conventional hard-edge focal plane mask has a fixed size, normally sized for the longest wavelength in the observational band to avoid starlight leakage. The conventional mask is oversized for shorter wavelengths and blocks useful discovery space. Recently we presented a solution to the size chromaticity challenge with a focal plane mask designed to scale its effective size with wavelength. In this paper, we analyze performance of the achromatic size-scaling focal plane mask within a Phase Induced Amplitude Apodization (PIAA) coronagraph. We present results from wavefront control around the achromatic focal plane mask, and demonstrate the size-scaling effect of the mask with wavelength. The edge of the dark zone, and therefore the inner working angle of the coronagraph, scale with wavelength. The achromatic mask enables operation in a wider band of wavelengths compared with a conventional hard-edge occulter
Experimental study of a low-order wavefront sensor for the high-contrast coronagraphic imager EXCEDE
The mission EXCEDE (EXoplanetary Circumstellar Environments and Disk
Explorer), selected by NASA for technology development, is designed to study
the formation, evolution and architectures of exoplanetary systems and
characterize circumstellar environments into stellar habitable zones. It is
composed of a 0.7 m telescope equipped with a Phase-Induced Amplitude
Apodization Coronagraph (PIAA-C) and a 2000-element MEMS deformable mirror,
capable of raw contrasts of 1e-6 at 1.2 lambda/D and 1e-7 above 2 lambda/D. One
of the key challenges to achieve those contrasts is to remove low-order
aberrations, using a Low-Order WaveFront Sensor (LOWFS). An experiment
simulating the starlight suppression system is currently developed at NASA Ames
Research Center, and includes a LOWFS controlling tip/tilt modes in real time
at 500 Hz. The LOWFS allowed us to reduce the tip/tilt disturbances to 1e-3
lambda/D rms, enhancing the previous contrast by a decade, to 8e-7 between 1.2
and 2 lambda/D. A Linear Quadratic Gaussian (LQG) controller is currently
implemented to improve even more that result by reducing residual vibrations.
This testbed shows that a good knowledge of the low-order disturbances is a key
asset for high contrast imaging, whether for real-time control or for post
processing.Comment: 12 pages, 20 figures, proceeding of the SPIE conference
Optics+Photonics, San Diego 201
Exoplanets imaging with a Phase-Induced Amplitude Apodization Coronagraph - I. Principle
Using 2 aspheric mirrors, it is possible to apodize a telescope beam without
losing light or angular resolution: the output beam is produced by
``remapping'' the entrance beam to produce the desired light intensity
distribution in a new pupil. We present the Phase-Induced Amplitude Apodization
Coronagraph (PIAAC) concept, which uses this technique, and we show that it
allows efficient direct imaging of extrasolar terrestrial planets with a
small-size telescope in space. The suitability of the PIAAC for exoplanet
imaging is due to a unique combination of achromaticity, small inner working
angle (about 1.5 ), high throughput, high angular resolution and
large field of view. 3D geometrical raytracing is used to investigate the
off-axis aberrations of PIAAC configurations, and show that a field of view of
more than 100 in radius is available thanks to the correcting
optics of the PIAAC. Angular diameter of the star and tip-tilt errors can be
compensated for by slightly increasing the size of the occulting mask in the
focal plane, with minimal impact on the system performance. Earth-size planets
at 10 pc can be detected in less than 30s with a 4m telescope. Wavefront
quality requirements are similar to classical techniques.Comment: 35 pages, 16 figures, Accepted for publication in Ap
EXCEDE Technology Development III: First Vacuum Tests
This paper is the third in the series on the technology development for the
EXCEDE (EXoplanetary Circumstellar Environments and Disk Explorer) mission
concept, which in 2011 was selected by NASA's Explorer program for technology
development (Category III). EXCEDE is a 0.7m space telescope concept designed
to achieve raw contrasts of 1e6 at an inner working angle of 1.2 l/D and 1e7 at
2 l/D and beyond. This will allow it to directly detect and spatially resolve
low surface brightness circumstellar debris disks as well as image giant
planets as close as in the habitable zones of their host stars. In addition to
doing fundamental science on debris disks, EXCEDE will also serve as a
technological and scientific precursor for any future exo-Earth imaging
mission. EXCEDE uses a Starlight Suppression System (SSS) based on the PIAA
coronagraph, enabling aggressive performance.
We report on our continuing progress of developing the SSS for EXCEDE, and in
particular (a) the reconfiguration of our system into a more flight-like
layout, with an upstream deformable mirror and an inverse PIAA system, as well
as a LOWFS, and (b) testing this system in a vacuum chamber, including IWA,
contrast, and stability performance. The results achieved so far are 2.9e-7
contrast between 1.2-2.0 l/D and 9.7e-8 contrast between 2.0-6.0 l/D in
monochromatic light; as well as 1.4e-6 between 2.0-6.0 l/D in a 10% band, all
with a PIAA coronagraph operating at an inner working angle of 1.2 l/D. This
constitutes better contrast than EXCEDE requirements (in those regions) in
monochromatic light, and progress towards requirements in broadband light. Even
though this technology development is primarily targeted towards EXCEDE, it is
also germane to any exoplanet direct imaging space-based telescopes because of
the many challenges common to different coronagraph architectures and mission
requirements.Comment: 12 pages, 12 figures, to be published in proceedings of SPIE
Astronomical Telescopes + Instrumentation (2014
Telescope to Observe Planetary Systems (TOPS): a high throughput 1.2-m visible telescope with a small inner working angle
The Telescope to Observe Planetary Systems (TOPS) is a proposed space mission
to image in the visible (0.4-0.9 micron) planetary systems of nearby stars
simultaneously in 16 spectral bands (resolution R~20). For the ~10 most
favorable stars, it will have the sensitivity to discover 2 R_E rocky planets
within habitable zones and characterize their surfaces or atmospheres through
spectrophotometry. Many more massive planets and debris discs will be imaged
and characterized for the first time. With a 1.2m visible telescope, the
proposed mission achieves its power by exploiting the most efficient and robust
coronagraphic and wavefront control techniques. The Phase-Induced Amplitude
Apodization (PIAA) coronagraph used by TOPS allows planet detection at 2
lambda/d with nearly 100% throughput and preserves the telescope angular
resolution. An efficient focal plane wavefront sensing scheme accurately
measures wavefront aberrations which are fed back to the telescope active
primary mirror. Fine wavefront control is also performed independently in each
of 4 spectral channels, resulting in a system that is robust to wavefront
chromaticity.Comment: 12 pages, SPIE conference proceeding, May 2006, Orlando, Florid