12,740 research outputs found
Augmented Slepians: Bandlimited Functions that Counterbalance Energy in Selected Intervals
Slepian functions provide a solution to the optimization problem of joint
time-frequency localization. Here, this concept is extended by using a
generalized optimization criterion that favors energy concentration in one
interval while penalizing energy in another interval, leading to the
"augmented" Slepian functions. Mathematical foundations together with examples
are presented in order to illustrate the most interesting properties that these
generalized Slepian functions show. Also the relevance of this novel
energy-concentration criterion is discussed along with some of its
applications
Optimal-Dimensionality Sampling on the Sphere: Improvements and Variations
For the accurate representation and reconstruction of band-limited signals on
the sphere, an optimal-dimensionality sampling scheme has been recently
proposed which requires the optimal number of samples equal to the number of
degrees of freedom of the signal in the spectral (harmonic) domain. The
computation of the spherical harmonic transform (SHT) associated with the
optimal-dimensionality sampling requires the inversion of a series of linear
systems in an iterative manner. The stability of the inversion depends on the
placement of iso-latitude rings of samples along co-latitude. In this work, we
have developed a method to place these iso-latitude rings of samples with the
objective of improving the well-conditioning of the linear systems involved in
the computation of the SHT. We also propose a multi-pass SHT algorithm to
iteratively improve the accuracy of the SHT of band-limited signals.
Furthermore, we review the changes in the computational complexity and
improvement in accuracy of the SHT with the embedding of the proposed methods.
Through numerical experiments, we illustrate that the proposed variations and
improvements in the SHT algorithm corresponding to the optimal-dimensionality
sampling scheme significantly enhance the accuracy of the SHT.Comment: 5 Pages, 4 figure
Fast directional spatially localized spherical harmonic transform
We propose a transform for signals defined on the sphere that reveals their
localized directional content in the spatio-spectral domain when used in
conjunction with an asymmetric window function. We call this transform the
directional spatially localized spherical harmonic transform (directional
SLSHT) which extends the SLSHT from the literature whose usefulness is limited
to symmetric windows. We present an inversion relation to synthesize the
original signal from its directional-SLSHT distribution for an arbitrary window
function. As an example of an asymmetric window, the most concentrated
band-limited eigenfunction in an elliptical region on the sphere is proposed
for directional spatio-spectral analysis and its effectiveness is illustrated
on the synthetic and Mars topographic data-sets. Finally, since such typical
data-sets on the sphere are of considerable size and the directional SLSHT is
intrinsically computationally demanding depending on the band-limits of the
signal and window, a fast algorithm for the efficient computation of the
transform is developed. The floating point precision numerical accuracy of the
fast algorithm is demonstrated and a full numerical complexity analysis is
presented.Comment: 12 pages, 5 figure
Fast directional correlation on the sphere with steerable filters
A fast algorithm is developed for the directional correlation of scalar
band-limited signals and band-limited steerable filters on the sphere. The
asymptotic complexity associated to it through simple quadrature is of order
O(L^5), where 2L stands for the square-root of the number of sampling points on
the sphere, also setting a band limit L for the signals and filters considered.
The filter steerability allows to compute the directional correlation uniquely
in terms of direct and inverse scalar spherical harmonics transforms, which
drive the overall asymptotic complexity. The separation of variables technique
for the scalar spherical harmonics transform produces an O(L^3) algorithm
independently of the pixelization. On equi-angular pixelizations, a sampling
theorem introduced by Driscoll and Healy implies the exactness of the
algorithm. The equi-angular and HEALPix implementations are compared in terms
of memory requirements, computation times, and numerical stability. The
computation times for the scalar transform, and hence for the directional
correlation, of maps of several megapixels on the sphere (L~10^3) are reduced
from years to tens of seconds in both implementations on a single standard
computer. These generic results for the scale-space signal processing on the
sphere are specifically developed in the perspective of the wavelet analysis of
the cosmic microwave background (CMB) temperature (T) and polarization (E and
B) maps of the WMAP and Planck experiments. As an illustration, we consider the
computation of the wavelet coefficients of a simulated temperature map of
several megapixels with the second Gaussian derivative wavelet.Comment: Version accepted in APJ. 14 pages, 2 figures, Revtex4 (emulateapj).
Changes include (a) a presentation of the algorithm as directly built on
blocks of standard spherical harmonics transforms, (b) a comparison between
the HEALPix and equi-angular implementation
Photosensor Characterization for the Cherenkov Telescope Array: Silicon Photomultiplier versus Multi-Anode Photomultiplier Tube
Photomultiplier tube technology has been the photodetector of choice for the
technique of imaging atmospheric Cherenkov telescopes since its birth more than
50 years ago. Recently, new types of photosensors are being contemplated for
the next generation Cherenkov Telescope Array. It is envisioned that the array
will be partly composed of telescopes using a Schwarzschild-Couder two mirror
design never built before which has significantly improved optics. The camera
of this novel optical design has a small plate scale which enables the use of
compact photosensors. We present an extensive and detailed study of the two
most promising devices being considered for this telescope design: the silicon
photomultiplier and the multi-anode photomultiplier tube. We evaluated their
most critical performance characteristics for imaging gamma-ray showers, and we
present our results in a cohesive manner to clearly evaluate the advantages and
disadvantages that both types of device have to offer in the context of GeV-TeV
gamma-ray astronomy.Comment: submitted to SPIE Optics+Photonics proceeding
Measurement of the earthshine polarization in the B, V, R, and I band as function of phase
The characterization of the polarimetric properties of the planet Earth is
important for the interpretation of expected observations and the planning of
future instruments. We present a multi-wavelengths and multi-phase set of
benchmark values for the polarization signal of the integrated light from the
planet Earth derived from new polarimetric observations of the earthshine
back-scattered from the Moon's dark side. Using a new, specially designed wide
field polarimeter we measured the fractional polarization of the earthshine in
the B, V, R and I filters for Earth phase angles alpha between 30{\deg} and
110{\deg}. The phase dependence of the earthshine polarization is fitted by a
function p x sin(alpha)^2. To determine the polarization of the planet Earth we
correct our earthshine measurements by a polarization efficiency function for
the lunar surface derived from measurements of lunar samples from the
literature. The polarization of the earthshine decreases towards longer
wavelengths and is about a factor 1.3 lower for the higher albedo highlands.
For mare regions the measured maximum polarization is about 13 % at quadrature
in the B band. The resulting fractional polarizations for Earth are 24.6 % for
the B band, 19.1 % for the V band, 13.5 % for the R band, and 8.3 % for the I
band. Together with literature values for the spectral reflectivity of Earth we
obtain a contrast between the polarized flux of the Earth and the (total) flux
of the Sun with an uncertainty of less than 20 % and we find that the best
phase to detect an Earth twin is around an Earth phase alpha=65{\deg}. The
polarimetric models of Earth-like planets from Stam (2008) are in qualitative
agreement with our results but there are also significant differences which
might guide more detailed computations.Comment: 14 pages, 14 figures, accepted for publication in Astronomy &
Astrophysic
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