2,913 research outputs found
Multi-stage four-quadrant phase mask: achromatic coronagraph for space-based and ground-based telescopes
Less than 3% of the known exoplanets were directly imaged for two main
reasons. They are angularly very close to their parent star, which is several
magnitudes brighter. Direct imaging of exoplanets thus requires a dedicated
instrumentation with large telescopes and accurate wavefront control devices
for high-angular resolution and coronagraphs for attenuating the stellar light.
Coronagraphs are usually chromatic and they cannot perform high-contrast
imaging over a wide spectral bandwidth. That chromaticity will be critical for
future instruments. Enlarging the coronagraph spectral range is a challenge for
future exoplanet imaging instruments on both space-based and ground-based
telescopes. We propose the multi-stage four-quadrant phase mask that associates
several monochromatic four-quadrant phase mask coronagraphs in series.
Monochromatic device performance has already been demonstrated and the
manufacturing procedures are well-under control since their development for
previous instruments on VLT and JWST. The multi-stage implementation simplicity
is thus appealing. We present the instrument principle and we describe the
laboratory performance for large spectral bandwidths and for both pupil shapes
for space- (off-axis telescope) and ground-based (E-ELT) telescopes. The
multi-stage four-quadrant phase mask reduces the stellar flux over a wide
spectral range (30%) and it is a very good candidate to be associated with a
spectrometer for future exoplanet imaging instruments in ground- and
space-based observatories.Comment: 7 pages, 11 figures, 4 tables, accepted in A&
Review of high-contrast imaging systems for current and future ground- and space-based telescopes I. Coronagraph design methods and optical performance metrics
The Optimal Optical Coronagraph (OOC) Workshop at the Lorentz Center in
September 2017 in Leiden, the Netherlands gathered a diverse group of 25
researchers working on exoplanet instrumentation to stimulate the emergence and
sharing of new ideas. In this first installment of a series of three papers
summarizing the outcomes of the OOC workshop, we present an overview of design
methods and optical performance metrics developed for coronagraph instruments.
The design and optimization of coronagraphs for future telescopes has
progressed rapidly over the past several years in the context of space mission
studies for Exo-C, WFIRST, HabEx, and LUVOIR as well as ground-based
telescopes. Design tools have been developed at several institutions to
optimize a variety of coronagraph mask types. We aim to give a broad overview
of the approaches used, examples of their utility, and provide the optimization
tools to the community. Though it is clear that the basic function of
coronagraphs is to suppress starlight while maintaining light from off-axis
sources, our community lacks a general set of standard performance metrics that
apply to both detecting and characterizing exoplanets. The attendees of the OOC
workshop agreed that it would benefit our community to clearly define
quantities for comparing the performance of coronagraph designs and systems.
Therefore, we also present a set of metrics that may be applied to theoretical
designs, testbeds, and deployed instruments. We show how these quantities may
be used to easily relate the basic properties of the optical instrument to the
detection significance of the given point source in the presence of realistic
noise.Comment: To appear in Proceedings of the SPIE, vol. 1069
Self-coherent camera as a focal plane wavefront sensor: simulations
Direct detection of exoplanets requires high dynamic range imaging.
Coronagraphs could be the solution, but their performance in space is limited
by wavefront errors (manufacturing errors on optics, temperature variations,
etc.), which create quasi-static stellar speckles in the final image. Several
solutions have been suggested for tackling this speckle noise. Differential
imaging techniques substract a reference image to the coronagraphic residue in
a post-processing imaging. Other techniques attempt to actively correct
wavefront errors using a deformable mirror. In that case, wavefront aberrations
have to be measured in the science image to extremely high accuracy. We propose
the self-coherent camera sequentially used as a focal-plane wavefront sensor
for active correction and differential imaging. For both uses, stellar speckles
are spatially encoded in the science image so that differential aberrations are
strongly minimized. The encoding is based on the principle of light incoherence
between the hosting star and its environment. In this paper, we first discuss
one intrinsic limitation of deformable mirrors. Then, several parameters of the
self-coherent camera are studied in detail. We also propose an easy and robust
design to associate the self-coherent camera with a coronagraph that uses a
Lyot stop. Finally, we discuss the case of the association with a four-quadrant
phase mask and numerically demonstrate that such a device enables the detection
of Earth-like planets under realistic conditions. The parametric study of the
technique lets us believe it can be implemented quite easily in future
instruments dedicated to direct imaging of exoplanets.Comment: 15 pages, 14 figures, accepted in A&A (here is the final version
Focal plane wavefront sensor achromatization : The multireference self-coherent camera
High contrast imaging and spectroscopy provide unique constraints for
exoplanet formation models as well as for planetary atmosphere models. But this
can be challenging because of the planet-to-star small angular separation and
high flux ratio. Recently, optimized instruments like SPHERE and GPI were
installed on 8m-class telescopes. These will probe young gazeous exoplanets at
large separations (~1au) but, because of uncalibrated aberrations that induce
speckles in the coronagraphic images, they are not able to detect older and
fainter planets. There are always aberrations that are slowly evolving in time.
They create quasi-static speckles that cannot be calibrated a posteriori with
sufficient accuracy. An active correction of these speckles is thus needed to
reach very high contrast levels (>1e7). This requires a focal plane wavefront
sensor. Our team proposed the SCC, the performance of which was demonstrated in
the laboratory. As for all focal plane wavefront sensors, these are sensitive
to chromatism and we propose an upgrade that mitigates the chromatism effects.
First, we recall the principle of the SCC and we explain its limitations in
polychromatic light. Then, we present and numerically study two upgrades to
mitigate chromatism effects: the optical path difference method and the
multireference self-coherent camera. Finally, we present laboratory tests of
the latter solution.
We demonstrate in the laboratory that the MRSCC camera can be used as a focal
plane wavefront sensor in polychromatic light using an 80 nm bandwidth at 640
nm. We reach a performance that is close to the chromatic limitations of our
bench: contrast of 4.5e-8 between 5 and 17 lambda/D.
The performance of the MRSCC is promising for future high-contrast imaging
instruments that aim to actively minimize the speckle intensity so as to detect
and spectrally characterize faint old or light gaseous planets.Comment: 14 pages, 20 figure
Laboratory validation of the dual-zone phase mask coronagraph in broadband light at the high-contrast imaging THD-testbed
Specific high contrast imaging instruments are mandatory to characterize
circumstellar disks and exoplanets around nearby stars. Coronagraphs are
commonly used in these facilities to reject the diffracted light of an observed
star and enable the direct imaging and spectroscopy of its circumstellar
environment. One important property of the coronagraph is to be able to work in
broadband light.
Among several proposed coronagraphs, the dual-zone phase mask coronagraph is
a promising solution for starlight rejection in broadband light. In this paper,
we perform the first validation of this concept in laboratory.
First, we recall the principle of the dual-zone phase mask coronagraph. Then,
we describe the high-contrast imaging THD testbed, the manufacturing of the
components and the quality-control procedures. Finally, we study the
sensitivity of our coronagraph to low-order aberrations (inner working angle
and defocus) and estimate its contrast performance. Our experimental broadband
light results are compared with numerical simulations to check agreement with
the performance predictions.
With the manufactured prototype and using a dark hole technique based on the
self-coherent camera, we obtain contrast levels down to between 5
and 17 in monochromatic light (640 nm). We also reach contrast
levels of between 7 and 17 in broadband
( nm, nm and %), which demonstrates the excellent chromatic performance of the dual-zone
phase mask coronagraph.
The performance reached by the dual-zone phase mask coronagraph is promising
for future high-contrast imaging instruments that aim at detecting and
spectrally characterizing old or light gaseous planets.Comment: 9 pages, 16 figure
Optimized tracking of RF carriers with phase noise, including Pioneer 10 results
The ability to track very weak signals from distant spacecraft is limited by the phase instabilities of the received signal and of the local oscillator employed by the receiver. These instabilities ultimately limit the minimum loop bandwidth that can be used in a phase-coherent receiver, and hence limit the ratio of received carrier power to noise spectral density which can be tracked phase coherently. A method is presented for near real time estimation of the received carrier phase and additive noise spectrum, and optimization of the phase locked loop bandwidth. The method was used with the breadboard Deep Space Network (DSN) Advanced Receiver to optimize tracking of very weak signals from the Pioneer 10 spacecraft, which is now more distant that the edge of the solar system. Tracking with bandwidths of 0.1 Hz to 1.0 Hz reduces tracking signal threshold and increases carrier loop signal to noise ratio (SNR) by 5 dB to 15 dB compared to the 3 Hz bandwidth of the receivers now used operationally in the DSN. This will enable the DSN to track Pioneer 10 until its power sources fails near the end of the century
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