154,994 research outputs found
Revisiting the theory of interferometric wide-field synthesis
After several generations of interferometers in radioastronomy, wide-field
imaging at high angular resolution is today a major goal for trying to match
optical wide-field performances. All the radio-interferometric, wide-field
imaging methods currently belong to the mosaicking family. Based on a 30 years
old, original idea from Ekers & Rots, we aim at proposing an alternate
formalism. Starting from their ideal case, we successively evaluate the impact
of the standard ingredients of interferometric imaging. A comparison with
standard nonlinear mosaicking shows that both processing schemes are not
mathematically equivalent, though they both recover the sky brightness. In
particular, the weighting scheme is very different in both methods. Moreover,
the proposed scheme naturally processes the short spacings from both
single-dish antennas and heterogeneous arrays. Finally, the sky gridding of the
measured visibilities, required by the proposed scheme, may potentially save
large amounts of hard-disk space and cpu processing power over mosaicking when
handling data sets acquired with the on-the-fly observing mode. We propose to
call this promising family of imaging methods wide-field synthesis because it
explicitly synthesizes visibilities at a much finer spatial frequency
resolution than the one set by the diameter of the interferometer antennas.Comment: 22 pages, 6 PostScript figures. Accepted for publication in Astronomy
& Astrophysics. Uses aa LaTeX macros
Toward Depth Estimation Using Mask-Based Lensless Cameras
Recently, coded masks have been used to demonstrate a thin form-factor
lensless camera, FlatCam, in which a mask is placed immediately on top of a
bare image sensor. In this paper, we present an imaging model and algorithm to
jointly estimate depth and intensity information in the scene from a single or
multiple FlatCams. We use a light field representation to model the mapping of
3D scene onto the sensor in which light rays from different depths yield
different modulation patterns. We present a greedy depth pursuit algorithm to
search the 3D volume and estimate the depth and intensity of each pixel within
the camera field-of-view. We present simulation results to analyze the
performance of our proposed model and algorithm with different FlatCam
settings
Analysis techniques for complex-field radiation pattern measurements
Complex field measurements are increasingly becoming the standard for
state-of-the-art astronomical instrumentation. Complex field measurements have
been used to characterize a suite of ground, airborne, and space-based
heterodyne receiver missions [1], [2], [3], [4], [5], [6], and a description of
how to acquire coherent field maps for direct detector arrays was demonstrated
in Davis et. al. 2017. This technique has the ability to determine both
amplitude and phase radiation patterns from individual pixels on an array.
Phase information helps to better characterize the optical performance of the
array (as compared to total power radiation patterns) by constraining the fit
in an additional plane [4].
Here we discuss the mathematical framework used in an analysis pipeline
developed to process complex field radiation pattern measurements. This routine
determines and compensates misalignments of the instrument and scanning system.
We begin with an overview of Gaussian beam formalism and how it relates to
complex field pattern measurements. Next we discuss a scan strategy using an
offset in z along the optical axis that allows first-order optical standing
waves between the scanned source and optical system to be removed in
post-processing. Also discussed is a method by which the co- and
cross-polarization fields can be extracted individually for each pixel by
rotating the two orthogonal measurement planes until the signal is the
co-polarization map is maximized (and the signal in the cross-polarization
field is minimized). We detail a minimization function that can fit measurement
data to an arbitrary beam shape model. We conclude by discussing the angular
plane wave spectral (APWS) method for beam propagation, including the
near-field to far-field transformation
The ArgoNeuT Detector in the NuMI Low-Energy beam line at Fermilab
The ArgoNeuT liquid argon time projection chamber has collected thousands of
neutrino and antineutrino events during an extended run period in the NuMI
beam-line at Fermilab. This paper focuses on the main aspects of the detector
layout and related technical features, including the cryogenic equipment, time
projection chamber, read-out electronics, and off-line data treatment. The
detector commissioning phase, physics run, and first neutrino event displays
are also reported. The characterization of the main working parameters of the
detector during data-taking, the ionization electron drift velocity and
lifetime in liquid argon, as obtained from through-going muon data complete the
present report.Comment: 43 pages, 27 figures, 5 tables - update referenc
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