283 research outputs found
Fast computation of Lyot-style coronagraph propagation
We present a new method for numerical propagation through Lyot-style
coronagraphs using finite occulting masks. Standard methods for coronagraphic
simulations involve Fast Fourier Transforms (FFT) of very large arrays, and
computing power is an issue for the design and tolerancing of coronagraphs on
segmented Extremely Large Telescopes (ELT) in order to handle both the speed
and memory requirements. Our method combines a semi-analytical approach with
non-FFT based Fourier transform algorithms. It enables both fast and
memory-efficient computations without introducing any additional
approximations. Typical speed improvements based on computation costs are of
about ten to fifty for propagations from pupil to Lyot plane, with thirty to
sixty times less memory needed. Our method makes it possible to perform
numerical coronagraphic studies even in the case of ELTs using a contemporary
commercial laptop computer, or any standard commercial workstation computer.Comment: 17 pages, 9 figures, accepted for publication in Optics Expres
Efficient Radio Frequency filters for space constrained cryogenic set-ups
Noise filtering is an essential part for measurement of quantum phenomena at
extremely low temperatures. Here, we present the design of a filter which can
be installed in space constrained cryogenic environment containing a large
number of signal carrying lines. Our filters have a -3db point of 65kHz and its
performance at GHz frequencies are comparable to the best available RF filters.Comment: 9 pages, 4 figures, The capacitor reference in the first version was
wrong and has been changed to the right on
Effects of Repetitive Transcranial Magnetic Stimulation on Spike Pattern and Topography in Patients with Focal Epilepsy
Repetitive Transcranial Magnetic Stimulation (rTMS) is a non-invasive method for brain stimulation. Group-studies applying rTMS in epilepsy patients aiming to decrease epileptic spike- or seizure-frequency have led to inconsistent results. Here we studied whether therapeutic trains of rTMS have detectable effects on individual spike pattern and/or frequency in patients suffering from focal epilepsy. Five patients with focal epilepsy underwent one session of rTMS online with EEG using a 6Hz prime/1Hz rTMS protocol (real and sham). The EEG was recorded continuously throughout the stimulation, and the epileptic spikes recorded immediately before (baseline) and after stimulation (sham and real) were subjected to further analysis. Number of spikes, spike-strength and spike-topography were examined. In two of the five patients, real TMS led to significant changes when compared to baseline and sham (decrease in spike-count in one patient, change in topography of the after-discharge in the other patient). Spike-count and topography remained unchanged the remaining patients. Overall, our results do not indicate a consistent effect of rTMS stimulation on interictal spike discharges, but speak in favor of a rather weak and individually variable immediate effect of rTMS on focal epileptic activity. The individuation of most effective stimulation patterns will be decisive for the future role of rTMS in epilepsies and needs to be determined in larger studie
Silica-supported Z-selective Ru olefin metathesis catalysts
Under embargo until: 2022-01-17Recently reported thiolate-coordinated ruthenium alkylidene complexes show promise in Z-selective and stereoretentive olefin metathesis reactions. Herein we describe the immobilization of three Ru complexes containing a bulky aryl thiolate on mesostructured silica via surface organometallic chemistry. The applied methodology gives isolated catalytic sites homogeneously distributed on the silica surface. The catalytic results with two model substrates show comparable Z-selectivities to those of the homogeneous counterparts.acceptedVersio
Speckle noise and dynamic range in coronagraphic images
This paper is concerned with the theoretical properties of high contrast
coronagraphic images in the context of exoplanet searches. We derive and
analyze the statistical properties of the residual starlight in coronagraphic
images, and describe the effect of a coronagraph on the speckle and photon
noise. Current observations with coronagraphic instruments have shown that the
main limitations to high contrast imaging are due to residual quasi-static
speckles. We tackle this problem in this paper, and propose a generalization of
our statistical model to include the description of static, quasi-static and
fast residual atmospheric speckles. The results provide insight into the
effects on the dynamic range of wavefront control, coronagraphy, active speckle
reduction, and differential speckle calibration. The study is focused on
ground-based imaging with extreme adaptive optics, but the approach is general
enough to be applicable to space, with different parameters.Comment: 31 pages, 18 figure
Constraining mass ratio and extinction in the FU Orionis binary system with infrared integral field spectroscopy
We report low resolution near infrared spectroscopic observations of the
eruptive star FU Orionis using the Integral Field Spectrograph Project 1640
installed at the Palomar Hale telescope. This work focuses on elucidating the
nature of the faint source, located 0.5" south of FU Ori, and identified in
2003 as FU Ori S. We first use our observations in conjunction with published
data to demonstrate that the two stars are indeed physically associated and
form a true binary pair. We then proceed to extract J and H band
spectro-photometry using the damped LOCI algorithm, a reduction method tailored
for high contrast science with IFS. This is the first communication reporting
the high accuracy of this technique, pioneered by the Project 1640 team, on a
faint astronomical source. We use our low resolution near infrared spectrum in
conjunction with 10.2 micron interferometric data to constrain the infrared
excess of FU Ori S. We then focus on estimating the bulk physical properties of
FU Ori S. Our models lead to estimates of an object heavily reddened, A_V
=8-12, with an effective temperature of ~ 4000-6500 K . Finally we put these
results in the context of the FU Ori N-S system and argue that our analysis
provides evidence that FU Ori S might be the more massive component of this
binary syste
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