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Richards, A. M. S., Allen, M. D., Garrington, S. T., Harrison, P. A., Lamb, P., Muxlow, T. W. B., Power, R., Reynolds, C., Stirling, A., Thomasson, P., Venturi, T., & Winstanley, N. 2003, in ASP Conf. Ser., Vol. 314 Astronomical Data
Analysis Software and Systems XIII, eds. F. Ochsenbein, M. Allen, & D. Egret (San Francisco: ASP), 539
VO Access to Complex Data - MERLIN and Other Interferometry Archives
A. M. S. Richards1, S. T. Garrington, P. A. Harrison2,
T. W. B. Muxlow, A. M. Stirling, P. Thomasson, N. Winstanley3
Jodrell Bank Observatory, University of Manchester, SK11 9DL, UK.
M. G. Allen4
CDS, Observatoire de Strasbourg, UMR 7550, France.
P. Lamb, R. Power
CSIRO, ANU, GPO Box 664, Canberra ACT 2601, Australia
C. Reynolds
JIVE, Postbus 2, 7990 AA, Dwingeloo, The Netherlands.
T. Venturi
IRA, CNR, Via Gobetti 101, Bologna 40129, Italy
Abstract:
Radio interferometry data should be as accessible as any other part of
the electromagnetic spectrum in the form of images, spectra or
whatever the astronomer requires, without laborious massive dataset
transport or esoteric software at the user end. Many existing
facilities are developing on-line access to archive and current data,
incorporating VO compatibility. The next generation of
interferometers will have data access for non-experts designed into
their archives.
Until recently, many astronomers regarded reducing radio astronomy
data as an unnatural art. Yet pipelines and other user-friendly tools
are now common. Most astronomers make some use of radio data such as
in identifying galaxy types from the spectral energy distribution or
probing obscured star-forming regions. Public access data are at last becoming accessible, albeit
usually via the individual observatory web sites. The next step for
VOs and data providers is to supply final data products
from any registered archive via a single interface.
The responses to our questionnaire to interferometry observatories show that
pipelined data reduction up to the production of images is possible
for fixed-element cm-wave telescopes such as
- ATCA
(Australia Telescope Compact Array);
- MERLIN
(Multi-Element-Radio-Linked-Interferometer-Network, UK);
- VLA
(Very Large Array, USA);
and is becoming possible, for the calibration stages at least, of VLBI
networks like the EVN
(European VLBI Network) and
the US VLBA.
Calibration pipelines and on-line access are also in use at mm wavelengths although data acess is more restricted e.g.
In the next decade more arrays will come on line, first the
wide-bandwidth upgrades e-MERLIN
and eVLA, then the LOFAR
low frequency
array and ALMA
(Atacama Large MM Array) and finally the SKA
(Square
Kilometre Array).
The data models and tools required are being developed in the
context of setting international standards for VOs, to provide a basis
for the next generation of interferometers. The IVOA hosts the
radiovo@ivoa.net mailing list (open to anyone interested) and
the radiovo
archive.
Raw interferometry data consists of a series of complex visibilties
which need to be calibrated and Fourier transformed to produce an
image. A single observation
can produce a range of resolutions (obtained by weighting the data or
combining data from different arrays); minimum beam size and maximum
sensitivity to extended emission are mutually incompatible. Moreover,
the potential field of view is typically or
pixels. The best way to meet user requirements is to extract
tailor-made products on demand from calibrated visibility data. Even
so, the data volume and visualisation can be daunting, for example
multi-epoch 3D monitoring of SiO masers in 512 spectral channels, two
transitions and full polarization (Diamond & Kemball 2003). The product may not even be a image, but extracted
spectra, a radio light
curve of an X-ray binary, or the more specialised time
series required for pulsar astrophysics.
Calibrated visibility data is the prime product for some
applications such as gravitational lens modelling. It may be the only
product for interferometers with a
small number of elements such as in the optical and IR (see e.g. Monnier 2003).
The questionnaire identified a
range of user requirements. Astronomers want a full data processing
history but only a small minority want to do it themselves (however
this possibility should always be open). Most
astronomers want a final product, commonly but not exclusively an
image. On-the-fly imaging will allow
the full extent of archives to be used such as the MERLIN prototype
(Fig. 1).
Interferometry is by definition a high-resolution science but two
surveys covering substantial fractions of the sky, WENSS
and NVSS
are already available via their host observatories and Aladin. The
first substantial multi-wavelength spectral data-cube survey to become
accessible via Aladin will be the CGPS.
The complementary and more traditional approach is to provide
catalogues of pointings and source properties. Almost all open-access
interferometers now provide on-line lists of observations. However
there are two significant obstacles:
- Archives often list observations by proposal code or
non-standard source name. The named source
may not be in the centre of the field. Considerable cross-referencing
can be required to
establish the spatial, spectral and temporal extent and the
resolution and sensitivity
of an observation.
- Catalogue entries are often non-trivial to identify with SIMBAD
sources. For
example, how do you identify radio lobes tens of arcsec apart with the
optical core of a QSO? Or distinguish between masers arising from an
even larger star-forming region and those associated with a Mira
variable along the same line of sight?
The first problem will be solved as observatories beome more aware of
the benefits from serendipitous use of their data and as data access
and VO use even in calibration
is developed from the planning stage of new
instruments, e.g. LOFAR: Smirnov (2004), ALMA: Schwarz (2004),
CARMA: Scott (2004).
The second is
being tackled at CDS using iteratively refined astronomical
knowledge (e.g. spectral index properties) to find counterparts to
catalogue radio sources. In the long term more sophisticated methods
will be needed to locate data related to complex objects using
astrophysical templates and pattern recognition, investigating
techniques already developed by planetary scientists.
At present, the MERLIN archive can be accessed by three routes; the MERLIN web page, via Vizier/Aladin
and via the prototype AstroGrid
registry. VO searches and automatic updating (harvesting) will be
implimented. This relies on accurate metadata consistent with global
standards. An IVOA
working group is developing a
radio interferometry data model. It is likely that most data
providers will need a specialised data model for internal use. This
should provide parameters needed by a generalised model (such as IDHA
or the
SIAP
and SSA
protocols) for providing data for users. For example, the factors
determining the field of view (integration time, channel width,
primary beam etc.) may have observatory-specific names and algorithms
but the VO just needs to know the availability and quality of data in
a given region.
Virtual observatories provide a great opportunity to open up all types
of observation. Tools as well as data are needed to enable this in the
case of complex interferometry data. Current experience suggests that observatories (real and virtual) should
prioritise providing FITS images, calibrated visibility data and other
products, in that order.
Archive access is being retro-fitted to most existing radio
observatories which is vital not only to provide a service to the
astronomers of today but to act as testbeds to ensure that the next generation of
telescopes meet the demands of all potential users.
Figure 1:
The top images (HST overlaid with MERLIN contours) show
a radio supernova (B) in NGC 7469 discovered by Colina (2001). The MERLIN
archive revealed that HI absorption against the core A had been
observed in 1993 and on-the-fly imaging of the calibrated
visibilities showed that the RSN was not then detectable (lower left).
|
References
Colina, L., Alberdi, A., Torrelles, J. M., Panagia, N., & Wilson, A. S. 2001, ApJ, 553, L19
Diamond, P. J. & Kemball, A. J. 2003, ApJ, 599, 1372
Monnier, J. D. 2003, Reports of Progress in Physics, 66, 789
Richards A. M. S., Garrington S. T. G., Reynolds C. & Allen
M. G. 2003 in The Scientific Promise of the SKA, Oxford, 2002, ed. Kramer M. & Rawlings S. http://www.jb.man.ac.uk/ska/oxfordfull.pdf
Schwarz, J. 2004, this volume, 643
Scott, S. L. 2004, this volume, 768
Smirnov, O. 2004, this volume, 18
Footnotes
- ... Richards1
- The Astrophysical Virtual Observatory
- ... Harrison2
- AstroGrid
- ... Winstanley3
- AstroGrid
- ... Allen4
- The Astrophysical Virtual Observatory
© Copyright 2004 Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, California 94112, USA
Next: Generalized Linear Multi-Frequency Imaging in VLBI
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