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Capabilities OLD

Here we present a summary of the main capabilities of the EVN. To obtain a more detailed and updated feedback about the working status of all the EVN stations, please check the EVN Status Table.

Telescopes

The EVN consists of a large number of telescopes located worldwide. Whereas the core of the EVN is based in Europe, the network also connects several stations in Asia, Africa, and America. Occasionally, the EVN is joined by other VLBI networks, such as e-MERLIN, the VLBA, or the LBA in global VLBI observations.

Angular resolution

The EVN images can reach a resolution of the order of milliarcseconds. In the following table, we present typical values of the maximum angular resolution (in milliarcseconds) from typical EVN and Global VLBI arrays. Note that the final values would depend on several factors such as the participating antennas or the coordinates of the source.

Array  90 cm 18 cm 6 cm 3.6 cm 1.3 cm 0.7 cm
EVN   -   15   5   3   1   0.6
EVN (inc. Sh/Ur)   30     5   1.5   1   0.3   0.15
EVN+VLBA     19     3   1   0.7   0.25   0.13

 

We note that the finest angular resolution of an interferometer (in radians) is given by the ratio of the observing wavelength to the maximum baseline. This table shows the baseline lengths between different EVN stations.

Frequency coverage

We show the frequencies available at each of the EVN stations in the following table, marked with gray colors. Frequencies at each station that can observe in real-time EVN observations (e-EVN) are marked in blue. The last column specifies the maximum bitrate at each station available for e-EVN projects.

Frequency coverage table

Sensitivity, UV-coverage & Source visibility

In order to obtain an accurate estimation of the sensitivity that the EVN can reach in a given observation, use the EVN Sensitivity Calculator. It provides the theoretical thermal noise for a given observation based on the participating stations, the observing band, bitrate, and the observing time on the target source. It also provides an estimation of the field of view for a given time and frequency average.

 

uv-coverage

The quality of a VLBI image is usually determined by the density and distribution of UV tracks in the UV plane. These tracks are formed by the 2-D projection of the various interferometer baselines on a plane (the so-called “uv-plane”) which is perpendicular to the source direction.

In the following links, we provide the resulting uv coverage of the EVN at 18cm for different source declinations:

As can be seen from these plots, the uv coverage of the EVN at the main observing frequencies (18,6,3.6 and 1.3 cm) is excellent for sources above +20 deg declination. Note that the inclusion of the Chinese telescope (Tianma65 and Urumqi) as well as the South African telescope in Hartebeesthoek extends the uv coverage considerably (marked in red in both images).

The superb uv coverage obtained with global VLBI array (EVN+VLBA), is ideal for producing high dynamic range, milliarcsecond resolution images of complex radio sources. "Snap-shot" observations of many sources can also take advantage of the dense uv coverage:

At lower source declinations (<+15 degrees) the coverage of the EVN array becomes foreshortened, resulting in an increasingly elongated beam and poorer uv-coverage. This can be improved by including VLBI telescopes at southern latitudes, for example the South African telescope in Hartebeesthoek (marked in red in the image):

 

Source visibility

In designing an experiment and preparing a schedule, it is important to know when the source of interest is visible to the array. Here, we present plots of source elevation vs GST at various declinations for the EVN antennas (these fictional sources all have RA=12h).

From the source visibility plots presented above, we can conclude that the observing declination limit of the EVN is around -30 degrees. At this declination, a source is visible (elevation > 5 degrees) by more than 3 antennas simultaneously, for only a few hours. The uv-coverage is very poor and the elevation of the source is typically around 10 degrees, only rising above 20 degrees for Noto and Tianma. In addition, there is very little mutual visibility between the European and Chinese stations at this declination. The same is true for the EVN and the VLBA. The situation is much better for Hartebeesthoek, which has a similar longitude to the European telescopes.

At source declinations greater than -20 degrees, the mutual visibility coverage of the EVN is much improved. The source is visible by the vast majority of telescopes for around 6 hours. The mutual visibility between the European telescopes and both the Chinese telescopes and VLBA telescopes is still poor (<2 hours). This improves at higher declination. At Dec +10 degrees, the mutual visibility of the EVN and the Chinese telescopes is 7 hours for Urumqi and 5 hours for Tianma. Similarly, the mutual visibility between the EVN and VLBA is 6 hours for Hancock and 3 hours to Owens Valley.

At declinations greater than 50 degrees sources are circumpolar for most of the EVN telescopes (except Urumqi -55 deg, Noto -60 deg and Tianma -65 deg).

EVN Sensitivity Calculator

In order to obtain an accurate estimation of the sensitivity that the EVN can reach in a given observation, use the EVN Sensitivity Calculator. It provides the theoretical thermal noise for a given observation based on the participating stations, the observing band, bitrate, and the observing time on the target source. It also provides an estimation of the field of view for a given time and frequency average.

Image limitations

The technique of Synthesis Imaging is affected by a number of limitations. In this section, we outline some arising from the array geometry: the field of view such that bandwidth- and time-smearing effects are held below a specified value, and the smallest/largest usefully detectable angular structures. Other limitations, such as propagation effects through the neutral atmosphere or the ionosphere and instrumental effects are not treated here. For more information on aperture-synthesis imaging in general, refer to

  • the various European Radio Interferometry Schools (linked from EVN Meetings page)
  • transcripts from the various NRAO Synthesis Imaging Summer Schools, e.g., "Synthesis Imaging in Radio Astronomy", eds. R.A. Perley, F.R. Schwab, and A.H. Bridle (1988, ASP Conf. Ser. v6); "Synthesis Imaging in Radio Astronomy II", eds. G.B. Taylor, C.L. Carilli, and R.A. Perley (1999, ASP Conf. Ser. v180)
  • lectures in "VLBI and the VLBA", eds. J.A. Zensus, P.J. Diamond, and P.J. Napier (1995, ASP Conf. Ser. v82)
  • "Interferometry and Synthesis in Radio Astromony", A.R. Thompson, J.M. Moran, G.W. Swenson (multiple editions)

 

Field of View (FOV) Limitations

For VLBI the undistorted FoV is always much smaller than the primary beam of the individual participating 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. Both are discussed below. There is also a separate EVN Field of View Guide that discusses these issues in more detail.

 

Bandwidth Smearing

Bandwith smearing arises because the telescopes observe over a finite frequency band (i.e., the larger the bandwidth, the higher the sensitivity). Averaging the visibilities over frequency range is equivalent to averaging over a short radial cut in the uv-plane. If the response of the interferometer varies appreciably over this area or cut through the uv-plane, then structure corresponding to this variation will be reduced in amplitude or "smeared out" altogether.

Rapidly varying components in the uv-plane correspond to sources that are located far from the phase centre. The effect of bandwidth smearing on the final image is thus to radialy smear sources located far from the phase centre. The observed peak for the smeared source is reduced (compared to the true peak) but the total flux of the source is conserved.

The effects of bandwidth smearing can be minimised (at the expense of computer processing time and disk space) by not averaging in frequency. The number of frequency points per subband used in the correlation will of course set an fundamental level of BW-smearing, but any subsequent frequency averaging will in turn further reduce the unsmeared FoV.

Here we tabulate the radial distance from the phase-centre for which bandwidth smearing is limited to a 10% reduction in the response to a point source for a sampling of frequency-channel widths for typical "short" (2500 km) and "long" (10000km) EVN baselines. Note that BW-smearing FoV is independent of frequency when expressed as an angle on the sky.

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 

 

The FoV Guide contains a more detailed table, keyed by the observing subband bandwidth and number of frequency points per SB in the correlation, along with the formulae by which the FoV values were computed.

 

Time Smearing

Whereas BW-smearing involved averaging radially in the uv-plane, in time smearing the averaging is along the elliptical tracks in the uv-plane. The effect of time-smearing is also stronger on longer baselines, since these sweep through the uv-plane more quickly than do shorter baselines. Time-smearing is a much more complicated process than bandwidth smearing, being dependent on the source position and baseline orientation. The total flux density of a smeared component is not conserved. The effects of time-smearing also scale directly with increasing observing frequency. We assume an observing frequency of 5 GHz (6cm) here, and tabulate some indicative values for the radial distance from the phase-centre for which time smearing is limited to a 10% reduction in the response to a point source, for a sampling of correlation integration times and for typical "short" (2500 km) and "long" (10000km) EVN baselines.

integration time B = 2.500 km

B = 10.000 km

2 s       22.2"        5.55"
0.25 s       178"        44.4"

 

Again, the FoV Guide contains a more detailed table covering other standard EVN observing bands, along with the formulae by which the FoV values were computed.

 

Largest Detectable Angular Structure

The angular size of the smallest strcture that can usefully be discriminated is of course closely related to the synthesized VLBI beam. For a truly thermal-noise limited image, the minimum size would scale as ~beam/SNR (but of course, this ideal condition is often not met...)

The angular size of the largest structure detectable (mappable) by the EVN depends on the length of the shortest (projected) baseline (typically Ef-Wb, 266 km). A conservative estimate of the largest detectable angular size is therefore about 0.1 arcsec at 18cm. (Note however that for the time being, Wb is limited to a single 25m antenna, which is currently reflected in the EVN status table and EVN calculator.)

If the source of interest has radio structure on a larger angular scale-size, then joint EVN + e-MERLIN observations can be proposed. In joint EVN + e-MERLIN observing, a number of e-MERLIN out-stations participate in the array and are correlated as individual antennas along with all the other EVN stations, providing a number of baselines in the range of 11-217 km. Currently, all five out-stations can be incorporated at 512 Mbps each (other EVN stations can be observing at higher bit-rates in joint EVN + e-MERLIN observing). VLBI baselines to the Jb, Wb, and Eb telescopes. The inclusion of these shorter baselines allows larger-scale structure on the scales of several arcseconds to be recovered.

Further, it is possible to propose for (near-)contemporaneous separate EVN and e-MERLIN observations, for which e-MERLIN can observe alone at higher bit-rate than it can in a joint observation within a single EVN array. The downside of this would be that there is no baseline in common between the two arrays to help with the mutual calibration of the distinct inner/outer regions of the uv-plane. 

Observing modes

Continuum

Continuum observations will be run at the highest possible reliable bit rate.

Continuum observations can be proposed for only one of the available frequency bands in any given 24 hour session.

 

Spectral Line

Real-time e-EVN spectral line observations would be 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. 2 dual-pol 2 MHz subbands).

Note that General e-VLBI observations and short observation proposal types are mostly suited for spectral line observations. Triggered proposals (see below) 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.

e-VLBI observations that can take place on the scheduled runs fall into the three classes as defined below. Time within these classes will only be allocated in response to proposals submitted for the standard proposal deadlines of 1st Feb, 1st June or 1st October. Proposals should make clear in the proposal text which class of observations is being requested.

 

Pulsar Observations

The SFXC EVN data processor can combine pulsar gating and binning, PDF. Gating: Accumulation of correlation results during the "on" phase of the pulsar period. 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 correlation stage, the correlation results will be phase shifted to each of the specific phase centers.

 

Operational modes

Disk observations (standard EVN)

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

 

e-VLBI observations

e-VLBI, or electronic Very Long Baseline Interferometry, uses fibre optic networks to connect radio telescopes to a central data processor, which correlates the data from the telescopes in real-time. With this technique, astronomers can inspect their results almost immediately after observation.

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

The current limitation for real-time e-VLBI correlation is estimated to be eight telescopes at 2 Gbit/s, or 15 telescopes at 1 Gbit/s. Mixed bandwidth observations are possible.

More information about the current status of offered e-VLBI observations can be found here: http://www.evlbi.org/evlbi/e-vlbi_status.html.

 

ToO

Targets-of-opportunity (ToO) are defined to be extremely rare and/or unpredictable events where there is a limited opportunity to make scientifically important observations. The policy to apply for ToO time can be found here.

 

Triggered Observations

These e-VLBI observations are only conducted during an e-VLBI run if a specific triggering criterion is met. Continuum observations, or spectral line observations requiring a single correlator pass (when technically possible) can be proposed for within this class.

Automated scheduling of these observations is possible as well. The expected response time to execute a new program may be as low as 10 minutes. The station experiment setup, including frequency, will be the same as the interrupted program. Only continuum observations can be proposed for within this proposal class.

 

Short Observations in standard VLBI sessions

“Short” observations (< 4 hours) may be proposed up to six weeks before an observing session begins, by means of a brief justification to the EVN Program Committee Chair. They can only be granted limited resources (number of telescopes, disks, correlator time) and must use standard recording modes. They should not involve any special observing set-ups.

 

Short Observations in e-VLBI sessions

Short e-VLBI observations may be requested for checking calibrator or target source compactness in preparation for or as part of a larger VLBI observation or proposal. These projects are limited to 2 hours or less in length. Such requests may be submitted up to three weeks prior to the start of any e-VLBI run directly to the EVN Program Committee Chair.

 

Large Projects

The EVN Program Committee (PC) encourages larger projects (>48 hrs); these will be subject to more detailed scrutiny, and the EVN PC may, in some cases, attach conditions on the release of the data.

 

Out-of-Session Observing Time

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).