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«Management of Astronomical Data Archives and their Interoperability through the Virtual Observatory Fabio Pasian Chair, International Virtual ...»

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Management of Astronomical Data Archives and their

Interoperability through the Virtual Observatory

Fabio Pasian

Chair, International Virtual Observatory Alliance

Head, INAF Information Systems Unit

c/o INAF-OATS, Via G.B.Tiepolo 11 - 34143 Trieste, Italy

Abstract

Astronomy has changed and grown considerably in the past decade,

and the new instrumentation is going to further enhance this growth.

As a consequence, the data preservation issue has been tackled by

means of data archives, which have become an absolute must to cope with the incoming data avalanche. The effort to integrate and make the various data archives interoperable has been defined as the Virtual Observatory (VObs). This paper shortly describes VObs concepts and features.

1 Introduction Astronomy in recent years has expanded drastically its observing and modelling capabilities, increasing proportionally its demands for computational power and data access efficiency.

To understand completely the implications of this statement, one should remember that Astronomy is mostly an observational science, where any “experiment” is a snapshot of the fraction of the Universe being observed, and cannot be repeated as such. Astronomy measures I(λ), light intensity as a function of wavelength (or frequency, or energy). But, since most phenomena are in fact variable, the measured intensity is I(λ, t) which is convolved with the transfer function of the complex system (telescope, instrument, sky) actually involved in the observation. It is therefore easy to understand why every single observation needs to be kept, obviously creating a preservation issue.

The increasing complexity of instrumentation and the need to optimize observing procedures (e.g. the meteo conditions may be unacceptable for one type of observation while suitable for others) lead to a change in concept on modern observatories. Classical observing (the scientist located at the telescope is given the whole night to perform his/her observations) is replaced by flexible scheduling and service observing, i.e. the observations and the basic data handling are performed by observatory staff. In this case, the scientist owning the observations extracts the data from disk storage, written in the FITS standard data format, which has been used by the community since 1977. Data might have been already partially processed (i.e. calibrated, with removal of the instrument signature).

2 Archives The scenario described above has a pretty obvious extension, from data storages to full-fledged archives. Observatory policies have been defined so as to allow the data to become public typically after one year from the date of observation. Data taken for a specific scientific purpose can therefore be re-used for different purposes.

This approach,progressively emerged in the past fifteen-twenty years, has allowed the possibility of developing new science. Examples are monitoring the time variability of phenomena, comparing phenomena in different energy bands (the so-called multi-wavelength astronomy), performing statistical analysis or data mining on large quantities of data. From the technical point of view, the possibility of re-processing the raw data when a better know-ledge of instrumental effects is achieved.

An important effect of archives, and of the consequent re-use of data, is increasing the return for the investment: since a second of observation on modern facilities is worth roughly 1 US$, and the over-subscription factor is on the average a factor of 5, the possibility of re-using top-class data to do science, or for educational and outreach purposes has also important effects at the level of the society.

But archiving is an activity that is essential to cope with the data avalanche that is hitting the astrophysics community. As a matter of fact, the cumulative photon collecting area of telescopes world-wide has increased by a factor of 8 in the past 25 years, and the detector technology has allowed the production of panoramic chips which have multiplied the data flow. To give an example, the archive of the European Southern Observatory (ESO), the leading European institution, has grown in size a factor of 10 (from 40 to 400 TB) in three years, from 2005 to 2008.

And of course, there are world-wide many dozens of telescopes ground- and space-based, covering the full electromagnetic spectrum, and equipped with instruments providing data which are telescope and band-dependent.

3 The Virtual Observatory

3.1 Data and information sources The sources of data and information for the Astrophysical community are many and can be

summarized as follows:

• Telescopes (ground- and space-based, covering the full electromagnetic spectrum) are managed by Observatories

• Instruments (which are telescope and band dependent) are managed by Observatories and/or Consortia

• Data analysis software is usually instrument-dependent: it is managed by Observatories, Consortia and individual Researchers

• Computing is supported by dedicated service Centers, or Organizations that can often be Observatories

• Archives are managed by Observatories and/ or Agencies

• Publications are managed by Journals; there is however a single point of search and access, the ADS system

• Data curation, i.e. management of metadata (i.e. the description of data), tables and catalogues) is performed by Data curators





• Public Outreach is managed by Observatories and/or Agencies.

3.2 The Virtual Observatory concept The Virtual Observatory (VObs) is an innovative, still evolving, system to take advantage of astronomical data explosion (e.g., use statistical identification to perform multi-wavelength, multiparameter analysis, to allow astronomers to interrogate multiple data centers in a seamless and transparent way and to utilize at best astronomical data, to permit remote computing and data analysis and to foster new science. The VObs aims at achieving the goal of having all astronomical databases inside the scientist’s PC just as in the Web all documents are easily retrievable inside one’s own PC.

The situation depicted in the above section is pretty complex, and the desire has been felt to unify the sources of information within a federated framework. Achieving the ambitious goal of allowing the different sources of information to interoperate is not trivial: as a matter of fact, the current situation shows both positive and negative aspects.

On the positive side, one can consider that observational data are freely available on-line in general 1 year after observation; both data and catalogues are normally stored in astronomical archives. Results are published in academic journals, all available on-line with pointers to data archives; as mentioned above, there is one single entry point for journals (ADS). The processing and analysis software maintained Observatories/Archives is made available on-line.

However, the different astronomical archives have widely different access/search interfaces and standards/conventions; and they mainly serve raw data. The widely specialized, analysis software for the various sub-branches is in general complex, thus yielding a steep learning curve, but this is needed, since multi-wavelength is now the norm to produce science. PublicationArchive links often point to raw, unprocessed data and are not complete. Furthermore, object metadata are not homogeneously defined.

3.3 The International Virtual Observatory Alliance All of these considerations require that the various players speak the same language. It is therefore necessary that common standards and protocols are defined and adopted within the whole community. This issue is tackled within the International Virtual Observatory Alliance (IVOA), which coordinates 16 world-wide national VObs projects.

The IVOA mission is “to facilitate the international coordination and collaboration necessary for the development and deployment of the tools, systems and organizational structures necessary to enable the international utilization of astronomical archives as an integrated and interoperating virtual observatory”.

The IVOA goals are the standardization of data, metadata and software, the definition of data interoperability methods, and the production of lists of available data and computing services (each provided by the individual VObs projects).

From the organizational point of view, the IVOA structure is composed of an Executive Board which includes the representatives from all national VObs projects, a dozen of Working and Interest Groups coordinated by a Technical Coordination Group. There is a total of 400-500 individuals involved in IVOA activities. The work is carried out by means of teleconferences, TWiki pages, and bi-annual meetings (the last one was carried out in Baltimore in October 2008, the next will be in Strasbourg in May 2009).

IVOA is an initiative that is based on the concept of collaborating and sharing. The absence of specific funding does not allow to force any decisions, which are basically taken unanimously.

This practice is in principle good, since it builds consensus in the community. However, competition at the personal and/or project level brings as a consequence a slow convergence in the definition of standards.

3.4 Data Centres in the VObs Era The VObs needs data, therefore astronomical data centres lie at its foundation The VObs is more than a system: it is also a “frame of mind” since it provides modern access to better data. The VObs is “convenient” for data centres as well, for various reasons: it helps archive technology keeping up with current data volume and complexity; it broadens the user base; it exposes highly processed data in a direct way through its protocols.

The VObs cannot (and does not) dictate how to manage archives. To allow an archive to be VObs-compliant, the VObs requires data centres to have a “VObs layer” to “translate” any locally defined parameter to the standard (IVOA compliant) ones (e.g., RA can be called in many different ways) and to hide any observatory/ telescope/ instrument specific detail and work in astronomical units (e.g., wavelength range/band (not grism or filter name), spectral resolution, signal-to-noise ratio, field of view, limiting magnitude). The VObs will work at best with high level “science-ready” data, thus data centres should make an effort to provide such data.

3.5 A projects perspective: the EURO-VO EURO-VO is a project successor to the Astrophysical Virtual Observatory (AVO), which was a 5 MEuro, Phase A study (2001 - 2005) on the scientific requirements and technology for building the VObs in Europe, 50% funded by European Community under the Fifth Framework Programme (FP5). It includes 8 partners: ESO, European Space Agency (ESA), plus six national nodes: INAF (Italy), INSU (France), INTA (Spain), NOVA (Netherlands), PPARC (UK), and MPG (Germany).

EURO-VO has three components: a Data Centre Alliance (an alliance of European data centres who populate the EURO-VO with data, provide the physical storage and computational fabric and who will publish data, metadata and services to the EURO-VO using VObs technologies), a Technology Centre (a distributed organization that coordinates a set of research and development projects on the advancement of VObs technology, systems and tools in response to scientific and community requirements), and a Facility Centre (an operational organization, that provides the EURO-VO with a persistent, centralized registry for resources, standards and certication mechanisms as well as community support for VObs technology take-up and scientific programs; it is also EURO-VOs public face).

Funding comes from the EC, with substantial (over 50%) partner support. The Data Centre Alliance was co-funded by the EC (EuroVO-DCA) at 1.5 MEuro level (FP6) for 2.5 yrs since Sept. 2006 with an effort of 8.5 FTE/yr. The Technical Centre was co-funded by the EC (VOTECH) at the 3.3 MEuro level (FP6) for 4.5 years since Jan. 2005, with an effort of 21 FTE/yr.

The Facility Centre (FC) is located at ESO, and is co-managed by ESO and ESA; the support is at best-effort level (∼ 2 FTE/yr).

The EURO-VO proposal “Astronomical Infrastructure for Data Access (EuroVO-AIDA)” is funded with 2.7 MEuro within the EC Seventh Framework Programme (FP7) in the “Scientific Digital Repositories” call. The project started in Feb 2008, and ensures the continuation of European-wide VObs activities until 2010. AIDA is a combination of DCA, VOTECH, and FC activities and aims at unifying the digital data collection of European astronomy integrating their access mechanisms with evolving e-technologies enhancing the science extracted from these data-sets, while moving from development to operations.

3.6 Extending the Virtual Observatory concept In the last couple of years, the VObs received support at an International level: positive statements on the importance of the initiative were made by the Organization for Economic Co-operation and Development (OECD), by the European Strategy Forum on Research Infrastructures (ESFRI), and by ASTRONET, the European initiative aiming at enhancing the coordination and cooperation between national funding and research management organizations in Europe who are responsible for astronomical research. This recognition and support is important since, to make sense, the Virtual Observatory needs to be an international effort.

In this framework, an important comment was made in the 2006 ESFRI document, where the Virtual Observatory was indicated as the example to follow to create an international federation of libraries. As a matter of fact, the VObs concepts can be, and are being, re-used in different domains. Among the various interested communities, it is worth mentioning that the planetologists have their own Inter-national Planetary Data Alliance (IPDA), the Solar community has its own VObs and DataGrid, and this Workshop shows that the High-Energy Physics community needs data preservation mechanisms as well.

4 Generalization The documents of the ASTRONET initiative have defined the Virtual Observatory as the einfrastructure project in Europe for astronomy. As a matter of fact, an e-infrastructure is a mechanism that, based on solid networking foundations, allows Computing, Data and Applications of various kinds (Data Reduction, Data Analysis, Theory, Numerical Simulations) to interoperate. Interoperability is also the key word of the Virtual Observatory.



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