15,909 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
The far-IR spectrum of Sagittarius B2 region: Extended molecular absorption, photodissociation and photoionization
We present large scale 9'x 27'(25 pc x 70 pc) far-IR observations around Sgr
B2 using the Long-wavelength spectrometer (LWS) on board the Infrared Space
Observatory (ISO). The spectra are dominated by the strong continuum emission
of dust, the widespread molecular absorption of light hydrides (OH, CH and H2O)
and the fine structure lines of [NII], [NIII], [OIII], [CII] and [OI]. The
molecular richness in the outer layers of Sgr B2 is probed by the ISO-LWS
Fabry-Perot (35 km s^-1) detections towards Sgr B2(M), where more that 70 lines
from 15 molecular and atomic species are observed at high signal to noise
ratio.Comment: 46 pages, 10 figures, 5 tables, accepted in ApJ part I. (Figs. 1, 2,
3, 9 and 10 have been bitmapped to low resolution
Entropy production in the early-cosmology pionic phase
We point out that in the early universe, for temperatures in the approximate
interval 175-80 MeV (after the quark-gluon plasma), pions carried a large share
of the entropy and supported the largest inhomogeneities. Thus, we examine the
production of entropy in a pion gas, particularizing to inhomogeneities of the
temperature, for which we benefit from the known thermal conductivity. We
finally put that entropy produced in relaxing such thermal inhomogeneities in
the broad context of this relatively unexplored phase of early-universe
cosmology.Comment: 10 pages, 10 figures
High–Speed Data Transmission Subsystem of the SEOSAR/PAZ Satellite
This paper analyzes a digital interface and bus system modeling and optimization of the SEOSAR/PAZ Earth Observation satellite. The important part of the satellite is an X–band Synthetic Aperture Radar instrument that integrates 384 Transmit/Receive Modules located in 12 antenna panels 7.5 m away from the central processor and controlled by a synchronous 10 Mbps bidirectional serial protocol. This type of mid–range point–to–multipoint transmission is affected by bit errors due to crosstalk, transmission line attenuation and impedance mismatches. The high–speed data communication network has been designed to optimize the transmission by using a simulation model of the data distribution system which takes into account the worst–case scenario and by developing a lab–scaled prototype which exhibits BER of 10-11 for an interfering signal of 10 Vpp. The result is a point–to–multipoint bidirectional transmission network optimized in both directions with optimal values of loads and equalization resistors. This high–speed data transmission subsystem provides a compact design through a simple solution
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