227 research outputs found
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
A dual-mask coronagraph for observing faint companions to binary stars
Observations of binary stars for faint companions with conventional
coronagraphic methods are challenging, as both targets will be bright enough to
obscure any nearby faint companions if their scattered light is not suppressed.
We propose coronagraphic examination of binary stars using an apodized pupil
Lyot coronagraph and a pair of actively-controlled image plane masks to
suppress both stars simultaneously. The performance is compared to imaging with
a band-limited mask, a dual-mask Lyot coronagraph and with no coronagraph at
all. An imaging procedure and control system for the masks are also described.Comment: 17 pages, 6 figure
Optimized focal and pupil plane masks for vortex coronagraphs on telescopes with obstructed apertures
We present methods for optimizing pupil and focal plane optical elements that improve the performance of vortex coronagraphs on telescopes with obstructed or segmented apertures. Phase-only and complex masks are designed for the entrance pupil, focal plane, and the plane of the Lyot stop. Optimal masks are obtained using both analytical and numerical methods. The latter makes use of an iterative error reduction algorithm to calculate "correcting" optics that mitigate unwanted diffraction from aperture obstructions. We analyze the achieved performance in terms of starlight suppression, contrast, off-axis image quality, and chromatic dependence. Manufacturing considerations and sensitivity to aberrations are also discussed. This work provides a path to joint optimization of multiple coronagraph planes to maximize sensitivity to exoplanets and other faint companions.VORTE
Speckle noise reduction techniques for high-dynamic range imaging
High-dynamic range imaging from space in the visible, aiming in particular at
the detection of terrestrial exoplanets, necessitates not only the use of a
coronagraph, but also of adaptive optics to correct optical defects in real
time. Indeed, these defects scatter light and give birth to speckles in the
image plane. Speckles can be cancelled by driving a deformable mirror to
measure and compensate wavefront aberrations. In a first approach, targeted
speckle nulling, speckles are cancelled iteratively by starting with the
brightest ones. This first method has demonstrated a contrast better than 1e9
in laboratory. In a second approach, zonal speckle nulling, the total energy of
speckles is minimized in a given zone of the image plane. This second method
has the advantage to tackle simultaneously all speckles from the targeted zone,
but it still needs better experimental demonstration.Comment: 7 pages, 3 figures, in Optical techniques for direct imaging of
exoplanets (a special issue of Comptes Rendus de Physique
First On-Sky High Contrast Imaging with an Apodizing Phase Plate
We present the first astronomical observations obtained with an Apodizing
Phase Plate (APP). The plate is designed to suppress the stellar diffraction
pattern by 5 magnitudes from 2-9 lambda/D over a 180 degree region. Stellar
images were obtained in the M' band (4.85 microns) at the MMTO 6.5m telescope,
with adaptive wavefront correction made with a deformable secondary mirror
designed for low thermal background observations. The measured PSF shows a halo
intensity of 0.1% of the stellar peak at 2 lambda/D (0.36 arcsec), tapering off
as r^{-5/3} out to radius 9 lambda/D. Such a profile is consistent with
residual errors predicted for servo lag in the AO system.
We project a 5 sigma contrast limit, set by residual atmospheric
fluctuations, of 10.2 magnitudes at 0.36 arcsec separation for a one hour
exposure. This can be realised if static and quasi-static aberrations are
removed by differential imaging, and is close to the sensitivity level set by
thermal background photon noise for target stars with M'>3. The advantage of
using the phase plate is the removal of speckle noise caused by the residuals
in the diffraction pattern that remain after PSF subtraction. The APP gives
higher sensitivity over the range 2-5 lambda/D compared to direct imaging
techniques.Comment: 22 pages, 5 figures, 1 table, ApJ accepte
Accurate Astrometry and Photometry of Saturated and Coronagraphic Point Spread Functions
Accurate astrometry and photometry of saturated and coronagraphic point
spread functions (PSFs) are fundamental to both ground- and space-based high
contrast imaging projects. For ground-based adaptive optics imaging,
differential atmospheric refraction and flexure introduce a small drift of the
PSF with time, and seeing and sky transmission variations modify the PSF flux
distribution. For space-based imaging, vibrations, thermal fluctuations and
pointing jitters can modify the PSF core position and flux. These effects need
to be corrected to properly combine the images and obtain optimal
signal-to-noise ratios, accurate relative astrometry and photometry of detected
objects as well as precise detection limits. Usually, one can easily correct
for these effects by using the PSF core, but this is impossible when high
dynamic range observing techniques are used, like coronagrahy with a
non-transmissive occulting mask, or if the stellar PSF core is saturated. We
present a new technique that can solve these issues by using off-axis satellite
PSFs produced by a periodic amplitude or phase mask conjugated to a pupil
plane. It will be shown that these satellite PSFs track precisely the PSF
position, its Strehl ratio and its intensity and can thus be used to register
and to flux normalize the PSF. A laboratory experiment is also presented to
validate the theory. This approach can be easily implemented in existing
adaptive optics instruments and should be considered for future extreme
adaptive optics coronagraph instruments and in high-contrast imaging space
observatories.Comment: 25 pages, 6 figures, accepted for publication in Ap
Ground-Based Coronagraphy with High Order Adaptive Optics
We summarize the theory of coronagraphic optics, and identify a dimensionless
fine-tuning parameter, F, which we use to describe the Lyot stop size in the
natural units of the coronagraphic optical train and the observing wavelength.
We then present simulations of coronagraphs matched to adaptive optics (AO)
systems on the Calypso 1.2m, Palomar Hale 5m and Gemini 8m telescopes under
various atmospheric conditions, and identify useful parameter ranges for AO
coronagraphy on these telescopes. Our simulations employ a tapered, high-pass
filter in spatial frequency space to mimic the action of adaptive wavefront
correction. We test the validity of this representation of AO correction by
comparing our simulations with recent K-band data from the 241-channel Palomar
Hale AO system and its dedicated PHARO science camera in coronagraphic mode.Comment: To appear in ApJ, May 2001 (28 pages, 10 figs
First High Contrast Imaging Using a Gaussian Aperture Pupil Mask
Placing a pupil mask with a gaussian aperture into the optical train of
current telescopes represents a way to attain high contrast imaging that
potentially improves contrast by orders of magnitude compared to current
techniques. We present here the first observations ever using a gaussian
aperture pupil mask (GAPM) on the Penn State near-IR Imager and Spectrograph
(PIRIS) at the Mt. Wilson 100 telescope. Two nearby stars were
observed, Eridani and Her A. A faint companion was detected
around Her A, confirming it as a proper motion companion. Furthermore,
the observed H and K magnitudes of the companion were used to constrain its
nature. No companions or faint structure were observed for Eridani.
We found that our observations with the GAPM achieved contrast levels similar
to our coronographic images, without blocking light from the central star. The
mask's performance also nearly reached sensitivities reported for other ground
based adaptive optics coronographs and deep HST images, but did not reach
theoretically predicted contrast levels. We outline ways that could improve the
performance of the GAPM by an order of magnitude or more.Comment: 8 pages, 4 figures, accepted by ApJ letter
A Nulling Wide Field Imager for Exoplanets Detection and General Astrophysics
We present a solution to obtain a high-resolution image of a wide field with
the central source removed by destructive interference. The wide-field image is
created by aperture synthesis with a rotating sparse array of telescopes in
space. Nulling of the central source is achieved using a phase-mask
coronagraph. The full (u,v) plane coverage delivered by the 60m, six 3-meter
telescope array is particularly well-suited for the detection and
characterization of exoplanets in the infrared (DARWIN and Terrestrial Planet
Finder (TPF) missions) as well as for other generic science observations.
Detection (S/N=10) of an Earth-like planet is achieved in less than 10 hours
with a 1 micron bandwidth at 10 micron.Comment: 18 pages, 16 figures. Accepted for publication in A&
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