3,673 research outputs found
A New High Contrast Imaging Program at Palomar Observatory
We describe a new instrument that forms the core of a long-term high contrast
imaging program at the 200-inch Hale Telescope at Palomar Observatory. The
primary scientific thrust is to obtain images and low-resolution spectroscopy
of brown dwarfs and young Jovian mass exoplanets in the vicinity of stars
within 50 parsecs of the Sun. The instrument is a microlens-based integral
field spectrograph integrated with a diffraction limited, apodized-pupil Lyot
coronagraph, mounted behind the Palomar adaptive optics system. The
spectrograph obtains imaging in 23 channels across the J and H bands (1.06 -
1.78 microns). In addition to obtaining spectra, this wavelength resolution
allows suppression of the chromatically dependent speckle noise, which we
describe. We have recently installed a novel internal wave front calibration
system that will provide continuous updates to the AO system every 0.5 - 1.0
minutes by sensing the wave front within the coronagraph. The Palomar AO system
is undergoing an upgrade to a much higher-order AO system ("PALM-3000"): a
3388-actuator tweeter deformable mirror working together with the existing
241-actuator mirror. This system will allow correction with subapertures as
small as 8cm at the telescope pupil using natural guide stars. The coronagraph
alone has achieved an initial dynamic range in the H-band of 2 X 10^-4 at 1
arcsecond, without speckle noise suppression. We demonstrate that spectral
speckle suppression is providing a factor of 10-20 improvement over this
bringing our current contrast at an arcsecond to ~2 X 10^-5. This system is the
first of a new generation of apodized pupil coronagraphs combined with
high-order adaptive optics and integral field spectrographs (e.g. GPI, SPHERE,
HiCIAO), and we anticipate this instrument will make a lasting contribution to
high contrast imaging in the Northern Hemisphere for years.Comment: Accepted to PASP: 12 pages, 12 figure
Topology and Sizes of HII Regions during Cosmic Reionization
We use the results of large-scale simulations of reionization to explore methods for characterizing the topology and sizes of HII regions during reionization. We use four independent methods for characterizing the sizes of ionized regions. Three of them give us a full size distribution: the friends-of-friends (FOF) method, the spherical average method (SPA) and the power spectrum (PS) of the ionized fraction. These latter three methods are complementary: While the FOF method captures the size distribution of the small scale H~II regions, which contribute only a small amount to the total ionization fraction, the spherical average method provides a smoothed measure for the average size of the H~II regions constituting the main contribution to the ionized fraction, and the power spectrum does the same while retaining more details on the size distribution. Our fourth method for characterizing the sizes of the H II regions is the average size which results if we divide the total volume of the H II regions by their total surface area, (i.e. 3V/A), computed in terms of the ratio of the corresponding Minkowski functionals of the ionized fraction field. To characterize the topology of the ionized regions, we calculate the evolution of the Euler Characteristic. We find that the evolution of the topology during the first half of reionization is consistent with inside-out reionization of a Gaussian density field. We use these techniques to investigate the dependence of size and topology on some basic source properties, such as the halo mass-to-light ratio, susceptibility of haloes to negative feedback from reionization, and the minimum halo mass for sources to form. We find that suppression of ionizing sources within ionized regions slows the growth of H~II regions, and also changes their size distribution. Additionally, the topology of simulations including suppression is more complex. (abridged
Multi-scale initial conditions for cosmological simulations
We discuss a new algorithm to generate multi-scale initial conditions with
multiple levels of refinements for cosmological "zoom-in" simulations. The
method uses an adaptive convolution of Gaussian white noise with a real space
transfer function kernel together with an adaptive multi-grid Poisson solver to
generate displacements and velocities following first (1LPT) or second order
Lagrangian perturbation theory (2LPT). The new algorithm achieves RMS relative
errors of order 10^(-4) for displacements and velocities in the refinement
region and thus improves in terms of errors by about two orders of magnitude
over previous approaches. In addition, errors are localized at coarse-fine
boundaries and do not suffer from Fourier-space induced interference ringing.
An optional hybrid multi-grid and Fast Fourier Transform (FFT) based scheme is
introduced which has identical Fourier space behaviour as traditional
approaches. Using a suite of re-simulations of a galaxy cluster halo our real
space based approach is found to reproduce correlation functions, density
profiles, key halo properties and subhalo abundances with per cent level
accuracy. Finally, we generalize our approach for two-component baryon and
dark-matter simulations and demonstrate that the power spectrum evolution is in
excellent agreement with linear perturbation theory. For initial baryon density
fields, it is suggested to use the local Lagrangian approximation in order to
generate a density field for mesh based codes that is consistent with
Lagrangian perturbation theory instead of the current practice of using the
Eulerian linearly scaled densities.Comment: 22 pages, 24 figures. MNRAS in press. Updated affiliation
A new algorithm for point spread function subtraction in high-contrast imaging: a demonstration with angular differential imaging
Direct imaging of exoplanets is limited by bright quasi-static speckles in
the point spread function (PSF) of the central star. This limitation can be
reduced by subtraction of reference PSF images. We have developed an algorithm
to construct an optimized reference PSF image from a set of reference images.
This image is built as a linear combination of the reference images available
and the coefficients of the combination are optimized inside multiple
subsections of the image independently to minimize the residual noise within
each subsection. The algorithm developed can be used with many high-contrast
imaging observing strategies relying on PSF subtraction, such as angular
differential imaging (ADI), roll subtraction, spectral differential imaging,
reference star observations, etc. The performance of the algorithm is
demonstrated for ADI data. It is shown that for this type of data the new
algorithm provides a gain in sensitivity by up to a factor 3 at small
separation over the algorithm used in Marois et al. (2006).Comment: 7 pages, 11 figures, to appear in May 10, 2007 issue of Ap
Stellar Photometry of the Globular Cluster NGC 6229. I. Data Reduction and Morphology of the Brighter Part of the CMD
BV CCD photometry of the central (1.5 arcmin x 2.0 arcmin) part of the mildly
concentrated outer-halo globular cluster NGC 6229 is presented. The data
reduction in such a crowded field was based on a wavelet transform analysis.
Our larger dataset extends the previous results by Carney et al. (1991, AJ,
101, 1699) for the outer and less crowded fields of the cluster, and confirms
that NGC 6229 has a peculiar color-magnitude diagram for its position in the
Galaxy. In particular, NGC 6229's horizontal branch (HB) presents several
interesting features, among which stand out: a well populated and very extended
blue tail; a rather blue overall morphology, with (B-R)/(B+V+R) = 0.24+/-0.02;
a bimodal color distribution, resembling those found for NGC 1851 and NGC 2808;
and gaps on the blue HB. NGC 6229 is the first bimodal-HB cluster to be
identified in the Galactic outer halo. A low value of the R parameter is
confirmed, suggestive of a low helium abundance or of the presence of a quite
substantial population of extreme HB stars fainter than our photometric limit
(~ 2.5 mag below the RR Lyrae level in V). Twelve new possible variable stars
were found in the central part of the cluster. The morphology of the red giant
branch (RGB) also seems to be peculiar. In particular, the RGB luminosity
function ``bump'' is not a prominent feature and has only been tentatively
identified, on the basis of a comparison with a previously reported detection
for M3 (NGC 5272). Finally, we compare the properties of NGC 6229 with those
for other outer-halo globular clusters, and call attention to what appears to
be a bimodal HB distribution for the outer-halo cluster population, where
objects with very red or very blue HB types are much more frequently found than
clusters with intermediate HB types.Comment: 31 pages, LaTeX, uses AASTeX v4.0, 11 postscript figures and 7
postscript tables pasted into text. To appear in The Astronomical Journal
(Feb. 1997 issue
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
Illumination Correction on Biomedical Images
RF-Inhomogeneity Correction (aka bias) artifact is an important research field in Magnetic Resonance Imaging (MRI). Bias corrupts MR images altering their illumination even though they are acquired with the most recent scanners. Homomorphic Unsharp Masking (HUM) is a filtering technique aimed at correcting illumination inhomogeneity, but it produces a halo around the edges as a side effect. In this paper a novel correction scheme based on HUM is proposed to correct the artifact mentioned above without introducing the halo. A wide experimentation has been performed on MR images. The method has been tuned and evaluated using the simulated Brainweb image database. In this framework, the approach has been compared successfully against the Guillemaud filter and the SPM2 method. Moreover, the method has been successfully applied on several real MR images of the brain (0.18 T, 1.5 T and 7 T). The description of the overall technique is reported along with the experimental results that show its effectiveness in different anatomical regions and its ability to compensate both underexposed and overexposed areas. Our approach is also effective on non-radiological images, like retinal ones
Automatic solar feature detection using image processing and pattern recognition techniques
The objective of the research in this dissertation is to develop a software system to automatically detect and characterize solar flares, filaments and Corona Mass Ejections (CMEs), the core of so-called solar activity. These tools will assist us to predict space weather caused by violent solar activity. Image processing and pattern recognition techniques are applied to this system.
For automatic flare detection, the advanced pattern recognition techniques such as Multi-Layer Perceptron (MLP), Radial Basis Function (RBF), and Support Vector Machine (SVM) are used. By tracking the entire process of flares, the motion properties of two-ribbon flares are derived automatically. In the applications of the solar filament detection, the Stabilized Inverse Diffusion Equation (SIDE) is used to enhance and sharpen filaments; a new method for automatic threshold selection is proposed to extract filaments from background; an SVM classifier with nine input features is used to differentiate between sunspots and filaments. Once a filament is identified, morphological thinning, pruning, and adaptive edge linking methods are applied to determine filament properties. Furthermore, a filament matching method is proposed to detect filament disappearance. The automatic detection and characterization of flares and filaments have been successfully applied on Hα full-disk images that are continuously obtained at Big Bear Solar Observatory (BBSO). For automatically detecting and classifying CMEs, the image enhancement, segmentation, and pattern recognition techniques are applied to Large Angle Spectrometric Coronagraph (LASCO) C2 and C3 images.
The processed LASCO and BBSO images are saved to file archive, and the physical properties of detected solar features such as intensity and speed are recorded in our database. Researchers are able to access the solar feature database and analyze the solar data efficiently and effectively. The detection and characterization system greatly improves the ability to monitor the evolution of solar events and has potential to be used to predict the space weather
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