1,752 research outputs found
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 5sigma 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*10^-7 confidence level detection threshold when averaging a
partially correlated non-Gaussian noise.Comment: 29 pages, 13 figures, accepted to Ap
Alien Registration- Macintosh, Vera B. (Presque Isle, Aroostook County)
https://digitalmaine.com/alien_docs/33386/thumbnail.jp
Effective genealogical history: Possibilities for critical accounting history research
This essay, following up on the recent Sy and Tinker [2005] and Tyson and Oldroyd [2007] debate, argues that accounting history research needs to present critiques of the present state of accounting\u27s authoritative concepts and principles, theory, and present-day practices. It proposes that accounting history research could benefit by adopting a genealogical, effective history approach. It outlines four fundamental strengths of traditional history investigate only the real with facts; the past is a permanent dimension of the present; history has much to say about the present; and the past, present, and future constitute a seamless continuum. It identifies Nietzsche\u27s major concerns with traditional history, contrasts it with his genealogical approach, and reviews Foucault\u27s [1977] follow up to Nietzsche\u27s approach. Two examples of genealogical historiography are presented Williams\u27 [1994] exposition of the major shift in British discourse regarding slavery and Macintosh et al.\u27s [2000] genealogy of the accounting sign of income from feudal times to the present. The paper critiques some of the early Foucauldian-based accounting research, as well as some more recent studies from this perspective. It concludes that adopting a genealogical historical approach would enable accounting history research to become effective history by presenting critiques of accounting\u27s present state
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
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
Astrometric Monitoring of the HR 8799 Planets: Orbit Constraints from Self-Consistent Measurements
We present new astrometric measurements from our ongoing monitoring campaign
of the HR 8799 directly imaged planetary system. These new data points were
obtained with NIRC2 on the W.M. Keck II 10 meter telescope between 2009 and
2014. In addition, we present updated astrometry from previously published
observations in 2007 and 2008. All data were reduced using the SOSIE algorithm,
which accounts for systematic biases present in previously published
observations. This allows us to construct a self-consistent data set derived
entirely from NIRC2 data alone. From this dataset, we detect acceleration for
two of the planets (HR 8799b and e) at 3. We also assess possible
orbital parameters for each of the four planets independently. We find no
statistically significant difference in the allowed inclinations of the
planets. Fitting the astrometry while forcing coplanarity also returns
consistent to within 1 of the best fit values, suggesting that if
inclination offsets of 20 are present, they are not detectable
with current data. Our orbital fits also favor low eccentricities, consistent
with predictions from dynamical modeling. We also find period distributions
consistent to within 1 with a 1:2:4:8 resonance between all planets.
This analysis demonstrates the importance of minimizing astrometric systematics
when fitting for solutions to highly undersampled orbits.Comment: 18 pages, 11 figures. Accepted for publication in A
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