1,941 research outputs found
Calibration of quasi-static aberrations in exoplanet direct-imaging instruments with a Zernike phase-mask sensor
Context. Several exoplanet direct imaging instruments will soon be in
operation. They use an extreme adaptive optics (XAO) system to correct the
atmospheric turbulence and provide a highly-corrected beam to a near-infrared
(NIR) coronagraph for starlight suppression. The performance of the coronagraph
is however limited by the non-common path aberrations (NCPA) due to the
differential wavefront errors existing between the visible XAO sensing path and
the NIR science path, leading to residual speckles in the coronagraphic image.
Aims. Several approaches have been developed in the past few years to
accurately calibrate the NCPA, correct the quasi-static speckles and allow the
observation of exoplanets at least 1e6 fainter than their host star. We propose
an approach based on the Zernike phase-contrast method for the measurements of
the NCPA between the optical path seen by the visible XAO wavefront sensor and
that seen by the near-IR coronagraph. Methods. This approach uses a focal plane
phase mask of size {\lambda}/D, where {\lambda} and D denote the wavelength and
the telescope aperture diameter, respectively, to measure the quasi-static
aberrations in the upstream pupil plane by encoding them into intensity
variations in the downstream pupil image. We develop a rigorous formalism,
leading to highly accurate measurement of the NCPA, in a quasi-linear way
during the observation. Results. For a static phase map of standard deviation
44 nm rms at {\lambda} = 1.625 {\mu}m (0.026 {\lambda}), we estimate a possible
reduction of the chromatic NCPA by a factor ranging from 3 to 10 in the
presence of AO residuals compared with the expected performance of a typical
current-generation system. This would allow a reduction of the level of
quasi-static speckles in the detected images by a factor 10 to 100 hence,
correspondingly improving the capacity to observe exoplanets.Comment: 11 pages, 14 figures, A&A accepted, 2nd version after language-editor
correction
Atmospheric tomography with separate minimum variance laser and natural guide star mode control
This paper introduces a novel, computationally efficient, and practical atmospheric tomography wavefront control architecture with separate minimum variance laser and natural guide star mode estimation. The architecture is applicable to all laser tomography systems, including multi conjugate adaptive optics (MCAO), laser tomography adaptive optics (LTAO), and multi object adaptive optics (MOAO) systems. Monte Carlo simulation results for the Thirty Meter Telescope (TMT) MCAO system demonstrate its benefit over a previously introduced “ad hoc” split MCAO architecture, calling for further in-depth analysis and simulations over a representative ensemble of natural guide star (NGS) asterisms with optimized loop frame rates and modal gains
SLM-based Digital Adaptive Coronagraphy: Current Status and Capabilities
Active coronagraphy is deemed to play a key role for the next generation of
high-contrast instruments, notably in order to deal with large segmented
mirrors that might exhibit time-dependent pupil merit function, caused by
missing or defective segments. To this purpose, we recently introduced a new
technological framework called digital adaptive coronagraphy (DAC), making use
of liquid-crystal spatial light modulators (SLMs) display panels operating as
active focal-plane phase mask coronagraphs. Here, we first review the latest
contrast performance, measured in laboratory conditions with monochromatic
visible light, and describe a few potential pathways to improve SLM
coronagraphic nulling in the future. We then unveil a few unique capabilities
of SLM-based DAC that were recently, or are currently in the process of being,
demonstrated in our laboratory, including NCPA wavefront sensing,
aperture-matched adaptive phase masks, coronagraphic nulling of multiple star
systems, and coherent differential imaging (CDI).Comment: 14 pages, 9 figures, to appear in Proceedings of the SPIE, paper
10706-9
Atmospheric turbulence profiling with SLODAR using multiple adaptive optics wavefront sensors
The slope detection and ranging (SLODAR) method recovers atmospheric turbulence profiles from time averaged spatial cross correlations of wavefront slopes measured by Shack-Hartmann wavefront sensors. The Palomar multiple guide star unit (MGSU) was set up to test tomographic multiple guide star adaptive optics and provided an ideal test bed for SLODAR turbulence altitude profiling. We present the data reduction methods and SLODAR results from MGSU observations made in 2006. Wind profiling is also performed using delayed wavefront cross correlations along with SLODAR analysis. The wind profiling analysis is shown to improve the height resolution of the SLODAR method and in addition gives the wind velocities of the turbulent layers
The Vector-APP: a Broadband Apodizing Phase Plate that yields Complementary PSFs
The apodizing phase plate (APP) is a solid-state pupil optic that clears out
a D-shaped area next to the core of the ensuing PSF. To make the APP more
efficient for high-contrast imaging, its bandwidth should be as large as
possible, and the location of the D-shaped area should be easily swapped to the
other side of the PSF. We present the design of a broadband APP that yields two
PSFs that have the opposite sides cleared out. Both properties are enabled by a
half-wave liquid crystal layer, for which the local fast axis orientation over
the pupil is forced to follow the required phase structure. For each of the two
circular polarization states, the required phase apodization is thus obtained,
and, moreover, the PSFs after a quarter-wave plate and a polarizing
beam-splitter are complementary due to the antisymmetric nature of the phase
apodization. The device can be achromatized in the same way as half-wave plates
of the Pancharatnam type or by layering self-aligning twisted liquid crystals
to form a monolithic film called a multi-twist retarder. As the VAPP introduces
a known phase diversity between the two PSFs, they may be used directly for
wavefront sensing. By applying an additional quarter-wave plate in front, the
device also acts as a regular polarizing beam-splitter, which therefore
furnishes high-contrast polarimetric imaging. If the PSF core is not saturated,
the polarimetric dual-beam correction can also be applied to polarized
circumstellar structure. The prototype results show the viability of the
vector-APP concept.Comment: Proc. SPIE 8450-2
Turbulence characterisation for Astronomical Observatories
Atmospheric turbulence has two effects in astronomy; (i) the broadening of the point spread function due to phase fluctuations limiting the resolution of imaging and (ii) producing intensity fluctuations known as scintillation. Adaptive Optics (AO) can be installed on telescopes to correct for the effect of phase, and with the push to large telescopes more complex AO systems such as Multi Conjugate AO (MCAO) and Multi Object AO (MOAO) are desired. Operation of these systems requires a detailed profile of the turbulent atmosphere in real time.
In this thesis we consider two turbulence profilers, SLOpe Detection And Ranging (SLODAR) and SCIntillation Detection and Ranging (SCIDAR), two cross beam profilers that retrieve data using covariance of phase variations (SLODAR) and intensity variations (SCIDAR). We present a modification of SLODAR to allow an estimate for non resolved turbulence to be made by considering scintillation in the subapertures of a Shack Hartmann wavefront sensor. A new SCIDAR (Stereo--SCIDAR) is described, allowing dynamic re--conjugation to improve altitude resolution.
Practical considerations for the implementation of a SLODAR instrument are considered, including a discussion of potential false measurements of non Kolmogorov power spectra in the ground and surface layers of turbulence. Data is presented from SLODAR observing campaigns on La Palma, and at Paranal. Evidence is presented for orographic effects on measured turbulence, including those due to man made structures
A Wave Optics Propagation Code for Multi-Conjugate Adaptive Optics
We describe the purpose, theory, implementation and sample results of a wave optics propagation simulation developed to study multi-conjugate adaptive optics (MCAQ) for 4-10m class telescopes. This code was more specifically developed to assess the impact of diffraction effects and a variety of implementation error sources upon the performance of the Gemini-South MCAO system. These errors include: Hartmann sensing with extended and elongated laser guide stars, optical propagation effects through the optics and atmosphere, laser guide star (LGS) projection through the atmosphere, deformable mirror (DM) and wave front sensor (WFS) misregistration, and calibration for non-common path errors. The code may be run in either a wave optics or geometric propagation mode to allow the code to be anchored against linear analytical models and to explicitly evaluate the impact of diffraction effects. The code is written in MATLAB, and complete simulations of the Gemini-South MCAO design (including 3 deformable mirrors with 769 actuators, 5 LGS WFS with 1020 subapertures, 3 tip/tilt natural guide star (NGS) WFS, and 50 meter phase screens with 1/32nd meter resolution) are possible using a Pentium III but require 1 to 6 days. Sample results are presented for Gemini-South MCAO as well as simpler AO systems. Several possibilities for parallelizing the code for faster execution and the modeling of extremely large telescopes (ELT's) are discussed
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