The next generation radiotelescopes
Comptes Rendus Physique 13 (2012)
by T. Le Bertre & A. Deschamps
Herschel and the TeraHertz sky
by L. Pagani, F. Herpin, M. Gérin, & P. Encrenaz, page 5
Abstract: Herschel is a spatial submillimetre observatory with spectroscopic
and imaging capabilities covering the range from 55 to 671 μm (0.44 to
5.5 THz) partly explored for the first time here. With a primary mirror of
3.5 m, it is presently the largest telescope launched. Its primary targets
are the cold dust, the light hydrides, with a special focus on H2O, and a few
species of high interest like C+ and
O2 in both our Galaxy and other galaxies.
Its main focus is on star formation in all its possible aspects including
cosmological metal enrichment evolution, statistics on prestellar cores or
chemistry of protostar outflow terminal shocks to name only a few.
We will describe the telescope and its three instruments, a selection
of general results and we will focus on one typical case in greater detail,
the observation of water in massive star forming regions. Herschel was
launched in May 2009 and should function 3.5 years.
High Dynamic-Range Radio-Interferometric Images at 327 MHz
by J. M. Uson, & W. D. Cotton, page 14
Abstract: Radio astronomical imaging using aperture synthesis telescopes
requires deconvolution of the point spread function as well as calibration
of the instrumental characteristics (primary beam) and foreground
(ionospheric/atmospheric) effects. These effects vary in time and also across
the field of view, resulting in directionally-dependent (DD), timevarying
gains. The primary beam will deviate from the theoretical estimate in real
cases at levels that will limit the dynamic range of images if left
uncorrected. Ionospheric electron density variations cause time and position
variable refraction of sources. At low frequencies and sufficiently high
dynamic range this will also defocus the images producing error patterns that
vary with position and also with frequency due to the chromatic aberration of
synthesis telescopes. Superposition of such residual sidelobes can lead to
spurious spectral signals. Field-based ionospheric calibration as well as
''peeling'' calibration of strong sources leads to images with higher dynamic
range and lower spurious signals but will be limited by sensitivity on the
necessary short-time scales. The results are improved images although
some artifacts remain.
Radioastronomy with LOFAR
by J.-M. Griessmeier, P. Zarka, & M. Tagger, page 23
Abstract: LOFAR is the first radiotelescope of a new generation, which can be
described as "software telescopes". Observing between 15 and 240 MHz, the main
complexity of LOFAR does not lie in the receivers (crossed, active dipoles),
but in the hierarchical organisation of a large number of antennae
(almost 50 000) and in the analysis of the incoming data in a large
computing facility. Rather than mechanically steering the telescope, pointing
occurs fully numerical, and all observations are pre-processed on the fly
to obtain a reasonable data volume. LOFAR will be 10 to 100 times more
sensitive than the current instruments in the same frequency range.
It will achieve sub-arcsecond resolution, which is 10 to 100 times
better than the resolution of existing low-frequency instruments.
It is also one of the most flexible instruments, making it interesting
for a large number of scientific fields.
LOFAR calibration and wide-field imaging
by C. Tasse, G. van Diepen, S. van der Tol, R. J. van Weeren,
J. E. van Zwieten, F. Batejat, S. Bhatnagar, I. van Bemmel, L.Bîrzan,
A. Bonafede, J. Conway, C. Ferrari, F. De Gasperin, K. Golape,
G. Heald, N. Jackson, G. Macario, J. McKean, N. Mohan, E. Orrù,
R. Pizzo, D. Rafferty, U. Rau, H. Röttgering, & A. Shulevski, page 28
Abstract: LOFAR is a revolutionary instrument, operating at low frequencies
(nu < 240 MHz). It will drive major breakthroughs in the area of observational
cosmology, but its use requires the development of challenging techniques and
algorithms. Since its field of view and sensitivity are increased by orders of
magnitude as compared to the older generation of instruments, new technical
problems have to be addressed. The LOFAR survey team is in charge of
commissioning the first LOFAR data produced in the imager mode as part of
building the imaging pipeline. We are developing algorithms to tackle the
problems associated with calibration (ionosphere, beam, etc.) and wide-field
imaging for the achievement of the deep extragalactic surveys. New types of
problems arise in that context, and notions such as algorithmic complexity
and parallelism become fundamental.
Antenna design and distribution for a LOFAR Super Station Super Station in Nancay
by J.N. Girard, P. Zarka, M. Tagger, L. Denis, D. Charrier, A.A. Konovalenko, & F. Boone, page 33
Abstract: The Nancay radio astronomy observatory and associated laboratories
are developing the concept of a "Super Station" for extending the LOFAR
station now installed and operational in Nancay. The LOFAR Super Station (LSS)
will increase the number of high sensitivity long baselines, provide short
baselines, act as an alternate core, and be a large standalone instrument.
It will operate in the low frequency band of LOFAR (15-80 MHz) and extend
this range to lower frequencies. Three key developments for the LSS are
described here: (i) the design of a specific antenna, and the distribution of
such antennas; (ii) at small-scale (analog-phased mini-array); and (iii)
at large-scale (the whole LSS).
Analysis of the RATAN-600 radiotelescope antenna with a multilevel Physical Optics algorithm
by C. Letrou, V. Khaikin, & A. Boag, page 38
Abstract: The RATAN-600 antenna is a flexible multireflector system composed
of reflectors of very large dimensions. An extended system, with improved
performance in the millimetric range, includes a focal receiver array.
Accurate electromagnetic analysis of such a system, and simulation of
three-dimensional (3D) patterns, represents a substantial computational
challenge. A fast Physical Optics method based on a multilevel subdivision
of the surfaces of integration is proposed to address this problem.
This method allows to perform Physical Optics integrals with a computational
complexity comparable to that of the Fast Fourier Transform. The algorithm
and initial numerical results of its application to the RATAN-600
antenna system are presented.
BAORadio: A digital pipeline for a radio interferometry and 21 cm mapping of large scale structures
by R. Ansari, J.-E. Campagne, P. Colom, C. Magneville, J.-M. Martin, M. Moniez, J. Rich, & C. Yèche, page 46
Abstract: 3D mapping of matter distribution in the Universe through the 21 cm
radio emission of atomic hydrogen (HI) is a complementary approach to optical
surveys for the study of the Large Scale Structures, in particular for
measuring the BAO (Baryon Acoustic Oscillation) scale up to redshifts
z < 3, and therefore constraining dark energy parameters. We propose a novel
method to map the HI mass distribution in three dimensions in radio,
without detecting or identifying individual compact sources. This method would
require an instrument with a large instantaneous bandwidth (> 100 MHz) and
high sensitivity, while a rather modest angular resolution (~10 arcmin)
should be sufficient. These requirements can be met by a dense interferometric
array or a phased array (FPA) in the focal plane of a large primary reflector,
representing a total collecting area of a few thousand square meters with few
hundred simultaneous beams covering a 20 to 100 square degrees field of view.
We describe the development and qualification of an electronic and data
processing system for digital radio interferometry and beam forming suitable
for such instruments with several hundred receiver elements.
Sub-millimetre Wave Radiometry for Cloud and Rain Characterization : from Simulation to Earth Observation Mission Concept
by E. Defer, C. Jimenez, & C. Prigent, page 54
Abstract: Different studies and instrument/mission proposals have demonstrated
the potential of the passive sub-millimetre/millimetre wave (SMMW) radiometry
to provide remote measurements for the characterization of clouds and rain.
At these wavelengths radiation received by a satellite over ice clouds is
dominantly scattered by the ice particles. Vertically integrated ice and water
contents, hydrometeor properties and profiles can then be retrieved from the
radiometric observations with the use of multi-frequency retrieval schemes
based on inversion databases simulating realistic brightness temperatures from
in situ or modeled cloud microphysics profiles. Current technological SMMW
developments offer new observational perspectives for the characterization
of the atmosphere.
Kinetic inductance detectors for millimeter and submillimeter astronomy
by N. Boudou, A. Benoit, O. Bourrion, M. Calvo, F.-X. Désert, J. Macias-Perez, A. Monfardini, & M. Roesch, page 62
Abstract: We present recent developments in Kinetic Inductance Detectors (KID)
for large arrays of detectors. The main application is ground-based millimeter
wave astronomy. We focus in particular, as a case study, on our own
experiment: NIKA (Néel IRAM KID Arrays).
NIKA is today the best in-the-field
experiment using KID-based instruments, and consists of a dual-band imaging
system designed for the IRAM 30 meter telescope at Pico Veleta. We describe
in this article, after a general context introduction, the KID working
principle and the readout electronics, crucial to take advantage of the
intrinsic KID multiplexability. We conclude with a small subset of the
astronomical sources observed simultaneously at 2 mm
and 1.4 mm by NIKA during the last run, held in October 2010.
Cyclostationary approaches for spatial RFI mitigation in radioastronomy
by G. Hellbourg, R. Weber, C. Capdessus, & A.-J. Boonstra, page 71
Abstract: Radio astronomical observations are increasingly corrupted by radio
frequency interferences (RFIs), and real time filtering algorithms are
becoming essential. In this article, it is shown how spatial processing
techniques can limit the impact of the incoming RFIs for phased array radio
telescopes. The proposed approaches are based on estimation of the RFI spatial
signature. It requires the diagonalization of either the classic correlation
matrix or the cyclic correlation matrix of the array. Different
diagonalization techniques are compared. Then, RFI detection and RFI filtering
techniques are illustrated through simulations on data acquired with the Low
Frequency Array Radio telescope, LOFAR. The originality of the study is
the use of the cyclostationarity property, in order to improve the spatial
separation between cosmic sources and RFIs.
Blind detection of giant pulses: GPU implementation
by D. Ait-Allal, R. Weber, C. Dumez-Viou, I. Cognard, & G. Theureau, page 80
Abstract: Radio astronomical pulsar observations require specific
instrumentation and dedicated signal processing to cope with the dispersion
caused by the interstellar medium. Moreover, the quality of observations can
be limited by radio frequency interference (RFI) generated by
Telecommunications activity. This article presents the innovative pulsar
instrumentation based on graphical processing units (GPU) which has been
designed at the Nancay Radio Astronomical Observatory. In addition, for giant
pulsar search, we propose a new approach which combines a hardware-efficient
search method and some RFI mitigation capabilities. Although this approach is
less sensitive than the classical approach, its advantage is that no a priori
information on the pulsar parameters is required. The validation of a GPU
implementation is under way.
Last modifications on : 14-dec-2011