Capabilities

The EVN consists of radio telescopes located across the world. The majority can be found in Europe, with additional stations in Asia, Africa and North America. A full overview of the EVN telescopes can be found here.

The network can be extended by including other VLBI networks or antennas:

• e-MERLIN

• EVN-affiliated stations (NASA Deep Space Network (e.g. Robledo), LBA) 

• NRAO telescopes (VLBA, VLA, GBT)

It is also possible to add stations that are not currently part of the EVN (or affiliated networks) - known as 'non-EVN stations'.

If you are considering to include Arecibo in your observing EVN array, then have a look at the VLBI at Arecibo user guide.

Telescope limitations:

• Hartebeesthoek can not observe sources north of declination +30 degrees.

• No more than 12 source changes per hour are possible at the Lovell telescope.

The angular resolution of an interferometer is proportional to the ratio of observing wavelength to maximum baseline -  for the EVN this is to the order of milliarcseconds. Final values depend on the network of telescopes used in the observation.

Typical values of maximum angular resolution (in milliarcseconds) for telescopes within the EVN and EVN+affiliated arrays (except the LBA):

Baseline lengths (in km) between EVN stations (marked in yellow) and other global networks:

The EVN Observation Planner can be used to determine when a target can be observed and the theoretical thermal noise for a given observation based on the participating stations, the observing band, bitrate, and the observing time and coordinates on the target source. It also provides estimations of the field of view for a given time and frequency average, expected resolution, and (u,v) coverage.

For observations where the target coordinates are not known (for example in trigger proposals to observe possible transient events that are not known beforehand), please use the previous EVN Sensitivity Calculator.

UV coverage

UV coverage indicates the resultant quality of the eventual radio image obtained during observations. To learn more about UV coverage go here. 

The EVN-Europe gives excellent UV coverage for sources above +20 degrees declination. Including EVN telescopes from Asia and South Africa, and additional networks from North America, provides an extension of UV coverage.

Examples:

From left to right the images demonstrate UV coverage at -20 degrees, 20 degrees and 60 degrees declination.

Source visibility

The observing declination limit for the EVN is -30 degrees. For declinations > 50 degrees sources are circumpolar for most EVN telescopes.

Examples of source elevation versus Greenwich Sidereal Time (GST) at various declinations for the EVN antennas (these fictional sources all have RA=12h):

From left to right, the plots show source declinations of -30 degrees, -20 degrees, +10 degrees and +50 degrees.

EVN observations from the telescope network require correlation.

By default, correlation occurs using the EVN software correlator (SFXC) at JIVE. The EVN software correlator allows pulsar gating/binning, more than 2048 spectral points across each subband/polarization, and multiple correlation phase-centers.

By arrangement with MPIfR correlator staff prior to proposal submission, limited time on the Bonn DiFX Correlator is available.

Global VLBI projects can either be processed via JIVE's correlator or by the VLBA's correlator. A specific correlator may be requested for technical reasons, which must be explained in the proposal.

An EVN correlator proposal for the correlation of observations from non-EVN stations may also be submitted.

Imaging is affected by a number of limitations. The Field of View (FoV) and smallest/largest detectable angular structures are the principal limitations that arise from the array geometry, and these are outlined below.

Field of View

The undistorted Field of View (FoV) for the EVN is much smaller than the primary beam of the individual participating VLBI antennas. The two main effects responsible for this are bandwidth smearing and time smearing. Of these, time smearing usually places the most severe limitations on the FoV. For more information see the Field of View guide.

• Bandwidth Smearing

  • Observations over a finite frequency band.

  • Averaging visibilities over frequency range leads to radially smearing of sources located far from phase center in the image.

  • Peak flux is reduced but the total flux of the source is conserved.

  • Example table (10% reduction in response to a point source):

    Channel BW	B = 2.500 km	B = 10.000 km	Comment
    256 MHz	77.3 mas	19.3 mas	Full spanned BW for 2 Gbps
    32 MHz	0.619"	0.155"	Single subband BW for 2 Gbps
    0.5 MHz	39.6"	9.9"	Typical continuum frequency-channel width

• Time Smearing

  • Stronger effect for longer baselines.

  • Depends on source position and baseline orientation.

  • Total flux density of a smeared component is not conserved.

  • Scales with increasing observing frequency.

  • Example table (10% reduction in response to a point source)

    Integration time	B = 2.500 km	B = 10.000 km
    2 s	22.2"	5.55"
    0.25 s	178"	44.4"

More detailed tables summarising bandwidth and time smearing effects can be found in the FoV Guide.

Smallest/largest detectable angular structure

The angular size of the smallest structure that can usefully be discriminated is closely related to the synthesized VLBI beam. For a truly thermal-noise limited image, the minimum size would scale as ~synthesized beam size/Signal to Noise Ratio.

The angular size of the largest structure detectable by the EVN depends on the length of the shortest (projected) baseline. As an example, a conservative estimate of the largest detectable angular size for the Ef-Wb baseline of 266 km is about 0.1 arcsec at 18cm.

If the target has a radio structure on larger angular scales than given by the shortest projected baseline of the EVN, then joint EVN + e-MERLIN observations can be proposed.

Disk observations (standard EVN)

In standard VLBI, the data are recorded on disks at the stations and then shipped to the JIVE correlator for processing.

e-VLBI observations

The e-VLBI technique uses fibre optic networks to connect EVN telescopes to the JIVE data processor, which correlates the data in real-time. This allows for a much faster delivery of the calibrated dataset.

General e-VLBI proposals can be submitted either for continuum or spectral line observations. Scheduling will be done by JIVE staff using the technical information included in the proposal.

Each e-VLBI session is 24 hours long and restricted to one frequency band which is determined by the selected projects for the session:

Current e-VLBI session,

Upcoming scheduled e-VLBI sessions.

Advertised frequencies and array configurations for e-VLBI observations:

*The QUASAR stations may be available for some of the e-VLBI runs with a limited data rate (likely 1 Gbit/s).
 

The following telescopes operate at 2 Gbit/s: Ef, Hh, Jb, Mc, Nt, On, Sh, Ys, Tr, Wb1 (due to a limitation in the IF system, Wb1 only has a bandwidth of 160 MHz). Still limited to 1 Gbit/s: Mh.

Arecibo is limited to 512 Mbit/s without UT restriction (as from October 2013), up to 72 hours per month. 

e-MERLIN outstations (Cm, Da, De, Kn, Pi) are limited to 512 Mbps.

Mixed-bandwidth observations are possible.

The current limitation for real-time e-VLBI correlation is estimated to be 8 telescopes at 2 Gbit/s, or 15 telescopes at 1 Gbit/s. For example, 6 stations at 2 Gbit/s + 3 stations at 1 Gbit/s, or 5 stations at 2 Gbit/s and 4 stations at 1 Gbit/s should be possible. 

Triggered Observations

Triggered observations can be proposed for the 10 real-time correlation e-EVN days. This class of projects is for rapid response science observations in which a (class of) source(s) is expected to become active in the radio in the foreseeable future, and requires immediate attention if this happens.

The triggering conditions (e.g. radio flux density level, entering to a specific X-ray spectral state etc.) must be clearly described in the proposal.

The triggers must be sent to the EVN PC Chair by e-mail (with reference to relevant telegrams or circulars) up to 8:00 UT the day before the e-VLBI observations start. If the conditions are fulfilled, and the project has higher grade than the already scheduled other programs in the requested GST range, then the observations are carried out.

A trigger project is valid for scheduling up to 1 year.

Trigger activation in EVN Sessions is now also possible with the following restrictions:

• performed on best effort basis; only if there is an available observing slot in the right frequency range and if media resources are sufficient
• performed in disk-recording only
• trigger activation should be done at least one week before the start of the desired frequency block in the schedule by writing a mail to the PC Chair

Note that not all gaps in the observing schedule may be available for observations!

Target of Opportunity

Target of Opportunity (ToO) observations are defined as extremely rare and/or unpredictable events where there is a limited opportunity to make scientifically important observations. 

The proprietary period for ToO proposals is six months.

Short Observations

Disk observations are <4 hours and can be proposed up to six weeks before observing session begins.

e-VLBI observations are <2 hours and can be proposed up to three weeks prior to the start of any e-VLBI run.

Out-of-Session observing time (up to a maximum of 144 hours/year), is now available to all proposals. These observing blocks should be no less than 12 hours in duration (although individual observations can be shorter), and occur no more than 10 times per year (up to a maximum of 144 hours).

It is not possible to request a special observing mode in short observation proposals. The observing mode will be set according to the other projects in the observing session or e-VLBI run.

Large Proposals

Large proposals are >48 hrs.

Proposals are subject to more detailed scrutiny and the EVN PC may, in some cases, attach conditions to the release of the data.

Out-of-Session Observations

Out-of-Session observing time (up to a maximum of 144 hours/year), is now available on all proposals. These observing blocks should be no less than 12 hours in duration (although individual observations can be shorter), and occur no more than 10 times per year (up to a maximum of 144 hours).

Continuum

Continuum observations will be run at the highest possible reliable bitrate.

Spectral Line

Real-time e-EVN spectral line observations are similar to those recorded on disk, but without the possibility of multiple correlation passes, which may limit the tactics for achieving higher spectral resolutions. The minimum data rate remains 32 Mbps (e.g. 1 dual-pol 4 MHz subbands).

Triggered proposals for spectral-line observations requiring only a single spectral line pass may be accepted if technically possible. Triggered proposals requiring multiple correlator passes will not be accepted.

Currently, the EVN software correlator at JIVE (SFXC) provides options for Hanning, Hamming, top-hat, and cosine spectral-weighting functions.

Pulsar Observations

The SFXC EVN correlator can combine pulsar gating and binning.

Pulsar gating: Accumulation of correlation results during the "on" phase of the pulsar period.

Pulsar binning: Accumulation of correlation results in multiple bins over which the pulsar period is divided.

Multiple Phase Center

Multiple correlation centers can be specified in the proposal and at the correlation stage, the correlation results will be phase shifted to each of the specific phase centers.