Introduction to the EVN

The European VLBI Network (EVN) is an interferometric array of radio telescopes spread throughout Europe (and beyond) that conducts unique, high resolution, radio astronomical observations of cosmic radio sources. It is the most sensitive VLBI array in the world, thanks to the collection of extremely large telescopes that contribute to the network. The three stations from the Russian VLBI Network KVASAR (Svetloe, Zelenchukskaya, and Badary) joined the EVN as member stations in November 2009. The Korean VLBI Network (Yonsei, Ulsan, Tamna) became an Associate Member of the EVN in January 2014. Irbene joined the EVN as a member station in October 2016. Kunming (Yunnan Astronomical Observatory) became an Associate Member in June 2019.

The map on the left shows the EVN and other participating stations, color-coded via:

• Yellow/Red: current operational EVN stations
• Cyan/Red: existing telescopes soon to be EVN stations
• Cyan/Blue: new EVN stations under construction
• Pink/Purple: non-EVN stations that have participated in EVN observations

800x800 pixel and 2048x2048 pixel versions of this map are also available. The underlying map was generated from EarthView, specifically this page to get a form to specify a "satellite-view" of the planet.

Other MERLIN out-stations besides Cambridge can also participate in EVN observations in some situations. Global VLBI observations are also often conducted in conjunction with the VLBA, the GBT, and/or the VLA telescopes in the U.S. A Google map of EVN Facilities is also available.

VLBI provides astronomers with their sharpest view of the most energetic phenomena in the universe, including expanding supernovae, pulsars, flare stars, star-forming regions in molecular clouds, the environment surrounding nearby and distant galaxies, gravitational lenses, starburst galaxies and distant active galactic nuclei.

Formation of the EVN

The European VLBI Network (EVN) was formed in 1980 by five of the major radio astronomy institutes in Europe and Geodetic Dept of the University of Bonn. The founding radio astronomy institutes were:

• MPIfR in Bonn, Germany
• IRA in Bologna, Italy
• ASTRON in Dwingeloo, The Netherlands
• OSO in Onsala, Sweden
• Jodrell Bank Observatory (formerly NRAL) near Manchester, UK

EVN Structure and Management

The EVN and the body which administers it, the European Consortium for VLBI, has expanded rapidly since those early days, and now includes a total of 14 major institutes, including the Joint Institute for VLBI ERIC (JIVE). A pictorial guide to the EVN telescopes, including links to the relevant institute web pages, is also available.

The overall policy of the EVN is set by the EVN Consortium Board of Directors (CBD).

Issues related to technical aspects of EVN operations are considered by the Technical and Operations Group (TOG).

Astronomers who wish to use the EVN, must submit observing proposals to the EVN Programme Committee. The EVN is an open facility and the PC assesses and awards observing time based on scientific merit and technical feasibility.

Both the TOG and EVN PC report directly to the EVN CBD. EVN user meetings are open to all and are normally held along-side EVN Symposia. Users are kept up-to-date with developments in the EVN via the EVNtech e-mail exploder (and the VLBI exploder), and the EVN WWW home page.

EVN observing

The EVN members operate 21 individual telescopes, which include some of the world's largest and most sensitive radio telesopes. The EVN observes for 3 periods per year known as "VLBI sessions". Each of these sessions are approximately 3-4 weeks long and typically involve 3-4 different observing frequencies. In addition, there are about ten 24-hour e-EVN runs per year, approximately once per month outside the main EVN sessions. The EVN scheduler ensures that all telescopes follow the same observing schedule. After each observing programme is completed, the VLBI friends at each EVN telescope provide feedback on the status of the observations and deposit logs and other ancilliary data on the EVN Server maintained in Bologna, Italy.

EVN Data processing

As the EVN telescopes observe the same cosmic radio source simultaneously, the data are recorded on high capacity magnetic tapes, and these are later replayed and combined at a special purpose data processor, often referred to as the "correlator". The EVN MkIV Data Processor at JIVE began operations in 1999. The EVN software correlator at JIVE (SFXC) had taken over from the MkIV in 2012, now correlates essentially all of the EVN observations (a few specialized experiments correlate on the DiFX correlator at Bonn) and about half of the global VLBI observations (usually the experiments with European PIs).

EVN users can obtain assistance and support from JIVE via its support scientists. These scientists are able to advise on many different aspects of EVN observations and data analysis, including:

• the technical feasability of a proposed observing programme,
• the creation of EVN and Global VLBI experiment schedules,
• support of observations and data correlation,
• data calibration and image analysis at JIVE.

Financial support may be available to EVN users who wish to visit JIVE in order to analyze correlated EVN data under the EC's RadioNet Trans-National Access program. See the "Access to the EVN" web page for more details and conditions.

EVN-MERLIN, Global VLBI and Space VLBI

In addition to "EVN-only" observations, the EVN array often links-up with MERLIN, an interferometer network of telescopes distributed around the southern half of the UK. In this extended mode, the coverage of the EVN-MERLIN array ranges from a few tens to many thousands of kilometers. The EVN-MERLIN array is thus sensitive to a wide range of radio structures from the arcsecond scale to the milliarcsecond scale. The EVN also observes simultaneously with the US VLBA, so-called "global VLBI", obtaining sub-milliarcsecond resolution at frequencies higher than 5 GHz. The EVN also participates in Space VLBI observations as part of a ground array of radio telescopes observing simultaneously with the Russian RadioAstron satellite.