877 research outputs found
Angular Differential Imaging: a Powerful High-Contrast Imaging Technique
Angular differential imaging is a high-contrast imaging technique that
reduces quasi-static speckle noise and facilitates the detection of nearby
companions. A sequence of images is acquired with an altitude/azimuth telescope
while the instrument field derotator is switched off. This keeps the instrument
and telescope optics aligned and allows the field of view to rotate with
respect to the instrument. For each image, a reference PSF is constructed from
other appropriately-selected images of the same sequence and subtracted to
remove quasi-static PSF structure. All residual images are then rotated to
align the field and are combined. Observed performances are reported for Gemini
North data. It is shown that quasi-static PSF noise can be reduced by a factor
\~5 for each image subtraction. The combination of all residuals then provides
an additional gain of the order of the square root of the total number of
acquired images. A total speckle noise attenuation of 20-50 is obtained for
one-hour long observing sequences compared to a single 30s exposure. A PSF
noise attenuation of 100 was achieved for two-hour long sequences of images of
Vega, reaching a 5-sigma contrast of 20 magnitudes for separations greater than
8". For a 30-minute long sequence, ADI achieves 30 times better signal-to-noise
than a classical observation technique. The ADI technique can be used with
currently available instruments to search for ~1MJup exoplanets with orbits of
radii between 50 and 300 AU around nearby young stars. The possibility of
combining the technique with other high-contrast imaging methods is briefly
discussed.Comment: 27 pages, 7 figures, accepted for publication in Ap
Differential Imaging with a Multicolor Detector Assembly: A New ExoPlanet Finder Concept
Simultaneous spectral differential imaging is a high contrast technique by
which subtraction of simultaneous images reduces noise from atmospheric
speckles and optical aberrations. Small non-common wave front errors between
channels can seriously degrade its performance. We present a new concept, a
multicolor detector assembly (MCDA), which can eliminate this problem. The
device consists of an infrared detector and a microlens array onto the flat
side of which a checkerboard pattern of narrow-band micro-filters is deposited,
each micro-filter coinciding with a microlens. Practical considerations for
successful implementation of the technique are mentioned. Numerical simulations
predict a noise attenuation of 10^-3 at 0.5" for a 10^5 seconds integration on
a mH=5 star of Strehl ratio 0.9 taken with an 8-m telescope. This reaches a
contrast of 10^-7 at an angular distance of 0.5" from the center of the star
image.Comment: 13 pages, 5 figures, accepted APJ
Reference-less detection, astrometry, and photometry of faint companions with adaptive optics
We propose a complete framework for the detection, astrometry, and photometry
of faint companions from a sequence of adaptive optics corrected short
exposures. The algorithms exploit the difference in statistics between the
on-axis and off-axis intensity. Using moderate-Strehl ratio data obtained with
the natural guide star adaptive optics system on the Lick Observatory's 3-m
Shane Telescope, we compare these methods to the standard approach of PSF
fitting. We give detection limits for the Lick system, as well as a first guide
to expected accuracy of differential photometry and astrometry with the new
techniques. The proposed approach to detection offers a new way of determining
dynamic range, while the new algorithms for differential photometry and
astrometry yield accurate results for very faint and close-in companions where
PSF fitting fails. All three proposed algorithms are self-calibrating, i.e.
they do not require observation of a calibration star thus improving the
observing efficiency.Comment: Astrophysical Journal 698 (2009) 28-4
Effects of Quasi-Static Aberrations in Faint Companion Searches
We present the first results obtained at CFHT with the TRIDENT infrared
camera, dedicated to the detection of faint companions close to bright nearby
stars. The camera's main feature is the acquisition of three simultaneous
images in three wavelengths (simultaneous differential imaging) across the
methane absorption bandhead at 1.6 micron, that enables a precise subtraction
of the primary star PSF while keeping the companion signal. The main limitation
is non-common path aberrations between the three optical paths that slightly
decorrelate the PSFs. Two types of PSF calibrations are combined with the
differential simultaneous imaging technique to further attenuate the PSF:
reference star subtraction and instrument rotation to smooth aberrations. It is
shown that a faint companion with a DeltaH of 10 magnitudes would be detected
at 0.5 arcsec from the primary.Comment: 12 pages, 10 figures, to appear in Astronomy with High Contrast
Imaging, EAS Publications Serie
ASSESSING IMPACTS OF ENVIRONMENTAL CHANGE ON ABORIGINAL PEOPLE: AN ECONOMIC EXAMINATION OF SUBSISTENCE RESOURCE USE AND VALUE
The report describes the research design, data collection and preliminary analysis of an economic assessment of non-timber resource use by Aboriginal People in Northwest Saskatchewan. The project is designed to develop methods of valuing resource use by Aboriginal People so that these values can be incorporated into forest resource management decisions and to evaluate the impact of forest management actions on the economic well-being of Aboriginal People living in the region. Data on non-timber resource use are collected and spatially explicit economic models are developed in order to construct estimates of behavioral change and value associated with changes in the environment and landscape (through forestry, access, or other landscape changes).Environmental Economics and Policy, Resource /Energy Economics and Policy,
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Confidence Level and Sensitivity Limits in High Contrast Imaging
In long adaptive optics corrected exposures, exoplanet detections are currently limited by speckle noise originating from the telescope and instrument optics, and it is expected that such noise will also limit future high-contrast imaging instruments for both ground and space-based telescopes. Previous theoretical analysis have shown that the time intensity variations of a single speckle follows a modified Rician. It is first demonstrated here that for a circular pupil this temporal intensity distribution also represents the speckle spatial intensity distribution at a fix separation from the point spread function center; this fact is demonstrated using numerical simulations for coronagraphic and non-coronagraphic data. The real statistical distribution of the noise needs to be taken into account explicitly when selecting a detection threshold appropriate for some desired confidence level. In this paper, a technique is described to obtain the pixel intensity distribution of an image and its corresponding confidence level as a function of the detection threshold. Using numerical simulations, it is shown that in the presence of speckles noise, a detection threshold up to three times higher is required to obtain a confidence level equivalent to that at 5{sigma} for Gaussian noise. The technique is then tested using TRIDENT CFHT and angular differential imaging NIRI Gemini adaptive optics data. It is found that the angular differential imaging technique produces quasi-Gaussian residuals, a remarkable result compared to classical adaptive optic imaging. A power-law is finally derived to predict the 1-3 x 10{sup -7} confidence level detection threshold when averaging a partially correlated non-Gaussian noise
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