73 research outputs found
Double stage Lyot coronagraph with the apodized reticulated stop for extremely large telescope
One of the science drivers for the extremely large telescope (ELT) is imaging
and spectroscopy of exo-solar planets located as close as 20mas to their parent
star. The application requires a well thought-out design of the high contrast
imaging instrumentation. Several working coronagraphic concepts have already
been developed for the monolithic telescope with the diameter up to 8 meter.
Nevertheless the conclusions made about the performance of these systems cannot
be applied directly to the telescope of the diameter 30-100m. The existing
schemes are needed to be reconsidered taking into account the specific
characteristics of a segmented surface. We start this work with the classical
system ? Lyot coronagraph. We show that while the increase in telescope
diameter is an advantage for the high contrast range science, the segmentation
sets a limit on the performance of the coronagraph. Diffraction from
intersegment gaps sets a floor to the achievable extinction of the starlight.
Masking out the bright segment gaps in the Lyot plane although helps increasing
the contrast, does not solve completely the problem: the high spatial frequency
component of the diffractive light remains. We suggest using the Lyot stop
which acts on the light within gaps in order to produce the uniform
illumination in the Lyot plane. We show that for the diffraction limit regime
and a perfect phasing this type of coronagraph achieves a sufficient star light
extinction.Comment: 11 pages, 12 figures, SPIE conference 5905, 200
Adaptive Optics Correction of the Wavefront Distortions Induced by Segments Misalignment in Extremely Large Telescope
The capability of the adaptive optics to correct for the segmentation error
is analyzed in terms of the residual wavefront RMS and the power spectral
density of the phase. The analytical model and the end-to-end simulation give
qualitatively equal results justifying the significance of the geometrical
matching between segmentation geometry and the actuators/subaperture
distribution of the adaptive optics. We also show that the design of the
wavefront sensor is rather critical.Comment: 12 pages, 17 figure
Astrometry and Photometry with Coronagraphs
We propose a solution to the problem of astrometric and photometric
calibration of coronagraphic images with a simple optical device which, in
theory, is easy to use. Our design uses the Fraunhofer approximation of Fourier
optics. Placing a periodic grid of wires (we use a square grid) with known
width and spacing in a pupil plane in front of the occulting coronagraphic
focal plane mask produces fiducial images of the obscured star at known
locations relative to the star. We also derive the intensity of these fiducial
images in the coronagraphic image. These calibrator images can be used for
precise relative astrometry, to establish companionship of other objects in the
field of view through measurement of common proper motion or common parallax,
to determine orbits, and to observe disk structure around the star
quantitatively. The calibrator spots also have known brightness, selectable by
the coronagraph designer, permitting accurate relative photometry in the
coronagraphic image. This technique, which enables precision exoplanetary
science, is relevant to future coronagraphic instruments, and is particularly
useful for `extreme' adaptive optics and space-based coronagraphy.Comment: To appear in ApJ August 2006, 27 preprint style pages 4 figure
Probability density function and detection threshold in high contrast imaging with partially polarized light
We obtain an expression for the probability density function (PDF) of
partially developed speckles formed by light with an arbitrary degree of
polarization. From the probability density we calculate the detection threshold
corresponding to the 5sigma confidence level of a normal distribution. We show
that unpolarized light has an advantage in high contrast imaging for low ratios
of the deterministic part of the point spread function (DL PSF) to the halo,
typical in coronagraphy.Comment: 8 pages, 3 figure
On-sky multi-wavelength phasing of segmented telescopes with the Zernike phase contrast sensor
Future Extremely Large Telescopes will adopt segmented primary mirrors with
several hundreds of segments. Cophasing of the segments together is essential
to reach high wavefront quality. The phasing sensor must be able to maintain
very high phasing accuracy during the observations, while being able to phase
segments dephased by several micrometers. The Zernike phase contrast sensor has
been demonstrated on-sky at the Very Large Telescope. We present the
multi-wavelength scheme that has been implemented to extend the capture range
from \pmlambda/2 on the wavefront to many micrometers, demonstrating that it is
successful at phasing mirrors with piston errors up to \pm4.0 micron on the
wavefront. We discuss the results at different levels and conclude with a
phasing strategy for a future Extremely Large Telescope.Comment: 17 pages, 8 figures, 2 tables. Accepted for publication in Applied
Optics; he final publised version is available on the OSA website:
http://www.opticsinfobase.org/abstract.cfm?msid=13671
Lyot Coronagraphy on Giant Segmented-Mirror Telescopes
We present a study of Lyot style (i.e., classical, band-limited, and Gaussian
occulter) coronagraphy on extremely large, highly-segmented telescopes. We show
that while increased telescope diameter is always an advantage for high dynamic
range science (assuming wavefront errors have been corrected sufficiently
well), segmentation itself sets a limit on the performance of Lyot
coronagraphs. Diffraction from inter-segment gaps sets a floor to the
achievable extinction of on-axis starlight with Lyot coronagraphy. We derive an
analytical expression for the manner in which coronagraphic suppression of an
on-axis source decreases with increasing gap size when the segments are placed
in a spatially periodic array over the telescope aperture, regardless of the
details of the arrangement. A simple Lyot stop masking out pupil edges produces
good extinction of the central peak in the point-spread function (PSF), but
leaves satellite images caused by inter-segment gaps essentially unaffected.
Masking out the bright segment gaps in the Lyot plane with a reticulated mask
reduces the satellite images'intensity to a contrast of 5x10^{-9} on a 30 m
telescope with 10 mm gaps, at the expense of an increase in the brightness of
the central peak. The morphology of interesting targets will dictate which Lyot
stop geometry is preferable: the reticulated Lyot stop produces a conveniently
uni-modal PSF, whereas a simple Lyot stop produces an extended array of
satellite spots. A cryogenic reticulate Lyot stop will also benefit both direct
and coronagraphic mid-IR imaging.Comment: 4 pages, 2 figure
Improved High Contrast Imaging with On-Axis Telescopes using a Multi-Stage Vortex Coronagraph
The vortex coronagraph is one of the most promising coronagraphs for high
contrast imaging because of its simplicity, small inner working angle, high
throughput, and clear off-axis discovery space. However, as with most
coronagraphs, centrally-obscured on-axis telescopes degrade contrast. Based on
the remarkable ability of vortex coronagraphs to move light between the
interior and exterior of pupils, we propose a method, based on multiple
vortices, that, without sacrificing throughput, reduces the residual light
leakage to (a/A)^n, with n >=4, and a and A being the radii of the central
obscuration and primary mirror, respectively. This method thus enables high
contrasts to be reached even with an on-axis telescope.Comment: 3 pages, 2 figure
Comparison of coronagraphs for high contrast imaging in the context of Extremely Large Telescopes
We compare coronagraph concepts and investigate their behavior and
suitability for planet finder projects with Extremely Large Telescopes (ELTs,
30-42 meters class telescopes). For this task, we analyze the impact of major
error sources that occur in a coronagraphic telescope (central obscuration,
secondary support, low-order segment aberrations, segment reflectivity
variations, pointing errors) for phase, amplitude and interferometric type
coronagraphs. This analysis is performed at two different levels of the
detection process: under residual phase left uncorrected by an eXtreme Adaptive
Optics system (XAO) for a large range of Strehl ratio and after a general and
simple model of speckle calibration, assuming common phase aberrations between
the XAO and the coronagraph (static phase aberrations of the instrument) and
non-common phase aberrations downstream of the coronagraph (differential
aberrations provided by the calibration unit). We derive critical parameters
that each concept will have to cope with by order of importance. We evidence
three coronagraph categories as function of the accessible angular separation
and proposed optimal one in each case. Most of the time amplitude concepts
appear more favorable and specifically, the Apodized Pupil Lyot Coronagraph
gathers the adequate characteristics to be a baseline design for ELTs.Comment: 12 pages, 6 figures, Accepted for publication in A&
Design, analysis and test of a microdots apodizer for the Apodized Pupil Lyot Coronagraph
Coronagraphic techniques are required to detect exoplanets with future
Extremely Large Telescopes. One concept, the Apodized Pupil Lyot Coronagraph
(APLC), is combining an apodizer in the entrance aperture and a Lyot opaque
mask in the focal plane. This paper presents the manufacturing and tests of a
microdots apodizer optimized for the near IR.
The intent of this work is to demonstrate the feasibility and performance of
binary apodizers for the APLC. This study is also relevant for any coronagraph
using amplitude pupil apodization.
A binary apodizer has been designed using a halftone dot process, where the
binary array of pixels with either 0% or 100% transmission is calculated to fit
the required continuous transmission, i.e. local transmission control is
obtained by varying the relative density of the opaque and transparent pixels.
An error diffusion algorithm was used to optimize the distribution of pixels
that best approximates the required field transmission. The prototype was
tested with a coronagraphic setup in the near IR.
The transmission profile of the prototype agrees with the theoretical shape
within 3% and is achromatic. The observed apodized and coronagraphic images are
consistent with theory. However, binary apodizers introduce high frequency
noise that is a function of the pixel size. Numerical simulations were used to
specify pixel size in order to minimize this effect, and validated by
experiment.
This paper demonstrates that binary apodizers are well suited for being used
in high contrast imaging coronagraphs. The correct choice of pixel size is
important and must be adressed considering the scientific field of view.Comment: A&A accepted, 8 page
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