Scheduling

The EVN regularly observes during pre-scheduled observing sessions every year. The EVN observations may be conducted with disk recording (standard EVN) or in real-time (e-VLBI or e-EVN). The standard EVN observations (disk recording) are spread along three different sessions during the year. Each session covers approximately 21 days and the observing wavelengths are planned depending on the proposal pressure.

The coming standard EVN observing sessions are:

In the case of e-EVN observations, a number of sessions are scheduled every year. Each session observes at only one frequency, which is decided based on proposal pressure. 

* Not advertised e-VLBI date. ToO or within regular EVN session.

The block schedule for EVN observations is public within few weeks before the observations. Check the latest schedules for:

• The EVN/Global VLBI session (PDF) 
• The upcoming e-EVN session.
• Known out-of-session observations

We also keep a public archive on previous block schedules for:

• EVN/Global VLBI sessions
• e-EVN sessions. 
• Out-of-session EVN/RadioAstron observations

In case your proposal has been scheduled to be observed in the next session, you would receive an informative email about how to prepare your observations. Users are required to provide a schedule using the NRAO SCHED program (see documentation and/or source code. If you need help preparing your schedule please contact one of the support scientists at JIVE.

If you are using stations other than those of the EVN, NRAO, ARECIBO, DSN and VSOP (e.g. Ny-Alesund, Matera, Fortaleza), you should consult the code of practice for non-EVN observatories.

Calibration

Amplitude Calibration

The EVN telescopes regularly measure system temperatures using noise diodes. At present, some telescopes can measure the temperatures only in time gaps between observing scans. Therefore, individual observing scans are recommended to be reasonably short, with gaps placed every ~10 minutes (sched will provide warnings for intervals between gaps of more than 15 minutes). The data are now corrected with an improved 2-bit van Vleck correction to account for the statistics of high/low bits for each IF’s data stream at each station. Thus, the AIPS task ACCOR shouldn’t be run. It should be okay to use auto-correlations for bandpass corrections or to use the task ACFIT.

Phase-referencing Observations

This observation mode probably requires, especially for a lower frequency band, the detection of the continuum sources in a narrow band within the coherence time. At higher frequency, the cycling time should be no longer than 2 minutes (see VLBA Scientific Memo 24  in more detail). For the EVN, the practical minimum cycling time may be about 60-90 seconds, arising from the need to nod the larger (slower) telescopes (e.g., Efslews typically 20-25 s away from zenith for source separations of 1-2 degrees), while maintaining a reasonable fraction of total observing time on source.

Choosing Calibration

Calibration sources are critical in VLBI observations. It is recommended to schedule at least two scans on two strong, unresolved sources (aka fringe-finders) to guarantee a good signal-to-noise ratio for all baselines during the calibration. In case of faint targets or astrometry studies, phase-referencing calibrators can be required. These sources should also exhibit a strong signal on all baselines and should be located close to the target source (within a few degrees) to guarantee an accurate calibration of the phases for all telescopes and a reliable transfer of those results to the target source.

Public catalogues of calibration sources can be found in different places. For example:

• The RFC Calibrator Search Engine is currently one of the most complete catalogues of positions of compact radio sources.
• The VLBA fringe-finder survey
• The VLBA calibrator search tool 
• The VSOP 5-GHz Continuum Survey 
• The USNO Radio Reference Frame Image Database.

Scheduling spectral line observations

In case you want to conduct spectral line observations, this section details some special considerations that you should take into account during the scheduling but also during the calibration of the data.

Frequency Setup

The EVN stations with DBBC back-ends have standard bandpass filters of 16, 8, 4, 2, 1 MHz. Newer versions of the DBBC firmware may alter this configuration set. The velocity range of individual line sources should be taken into account when choosing the frequency setup. Observations are performed with fixed frequencies, and Doppler corrections are to be determined in the scheduling process. SCHED automatically calculates appropriate observing frequencies if, in the keyinfile, the source velocities and the rest-frame frequencies of the lines are specified (see Spectral line observation setup in SCHED. Note that target lines should be located close to the centerof the frequency band because the band edges have lower sensitivity and can have large phase offsets. In particular, an absorption-line observation needs emission/absorption-free frequency ranges brackettingthe absorption lines in the same frequency band. In addition, calibration using continuum calibrators will benefit from increased bandwidth and so the bandwidth should be as large as possible. If a line source has a wide velocity coverage, overlapping a part of the frequency (velocity) coverage between two frequency bands may be a good idea to see a part of the line components in the two bands. In that case, however, the data reduction of the individual bands should be done independently in AIPS.

Radio Frequency Interference(RFI)

The useful available bandwidth is limited by the RFI distribution, especially at L-band. By looking at total-power spectra of previous L-band observations, which are available on the EVN Pipeline Feedback page, an indication of the RFI environment may be found.

Spectroscopy

The number of spectral channels should be large enough to avoid artificial effects (e.g. RFI, unexpected spurs, etc.) and to divide the true lines into more than a few spectral channels. Currently the EVN software correlator at JIVE (SFXC) provides options for Hanning, Hamming, top-hat, and cosine spectral-weighting functions. Signal sampling with 2 bits per sample is recommended for obtaining higher sensitivity in each of the spectral channels. You should specify in the schedule file that phase calsignals be turned OFF.

Fringe Finders and Bandpass Calibrators

As always, fringe finders are required for every observation in order to determine the station clock delay offsets and drift rates. As some spectral line observations will use relatively narrow bandwidths, special care should be taken so that the fringe finders can be detected on all baselines. In addition, the same considerations hold for bandpass calibrators. When several line sources are observed with different frequency setups in one observation, AIPS requires bandpass calibrators to be observed with the same setups, so independently for each target. Thus the fringe finders and bandpass calibrators should be carefully selected, especially in observing bands with low sensitivity. 

Phase, Delay and Delay-Rate Calibrators

Relative offsets of delays, rates and phases among frequency bands and polarisations can be removed by fringe fitting of the continuum calibrators independently for individual IF bands and polarisations. Because only phase and rate offsets, not delay offsets, can be determined from maser sources, delay calibrators should be separately inserted. To do this, the continuum calibrators should be in the same part of the sky and strong enough to be detected in each of the frequency bands within a coherence time and should be observed every hour or less. Again, to facilitate transparent processing in AIPS, each frequency setup should have its own delay calibrators.

Scheduling polarization observations

There are two aspects to proper calibration of polarization-sensitive VLBI observations: (1) D-term calibration, calibration and removal of the instrumental polarizations, or D-terms, for each antenna and (2) Position angle calibration, calibration of the absolute orientation of the polarization position angles (PPAs).

(1) D-term calibration

Determination of the instrumental polarizations is relatively straightforward if observations of a source that is either unpolarized or is known to have a simple polarization structure are made. In case of a polarized source it is necessary to observe it over a wide range of parallactic angles. Usually, five or six scans of duration of several minutes spread over a range of parallactic angles exceeding about 90 degrees should be adequate. In some cases, a program source may be suitable for use as a polarization calibrator, if it is known to have a simple polarization structure. If the program sources are expected to have relatively complex polarization structures, it is best to observe a different source specifically for determination of the D-terms.

In case of a strong unpolarized source, a single observation scan of 8-10 minutes is sufficient.

Some good polarization calibrator at frequencies up to 15-GHz are 3C84 (lots of structure but unpolarized), OQ208 (also a fair bit of structure but unpolarized), and DA193 (weakly polarized with very compact structure). For suggestions about other sources that may be suitable for D-term calibration, contact one of the support scientists at JIVE or someone else you know who has experience with polarization VLBI observations.

(2) Position angle calibration

The most common way to calibrate the orientation of the polarization position angles is to use simultaneous or nearly simultaneously integrated and VLBI observations of a source with compact polarization (in which essentially all the integrated polarization is detected on short VLBI baselines). By comparing the orientation of the PPAs for the total VLBI-scale polarization with their known orientation in the integrated measurements, it is possible to determine the necessary rotation to calibrate the VLBI polarization angles, i.e., to give them their true observed values. For this purpose, it is necessary to have VLBI and integrated observations of a source with compact polarization. In some cases, program sources may be suitable for this. If you have doubts about whether your program sources have sufficiently compact polarization,
it is better to observe another source specifically for this purpose – since a source used for PPA calibration should have a relatively simple structure, five or six several-minute scans spread out over the time it is visible by most of the antennas in the array should be sufficient. For advice about sources that should be good for PPA calibration, contact one of the support scientists at JIVE or someone else you know who has experience with polarization VLBI observations.

There are currently no known sources with constant polarization position angles on VLBI scales; thus, it is not feasible to simply observe such a source and rotate the VLBI-scale PPAs to agree with the known constant value. This is why it is necessary to also obtain integrated polarization measurements for the PPA calibration. If the phased VLA or phased WSRT is included as part of the VLB array, integrated measurements can be derived from the data for these instruments. Otherwise, other arrangements for the aquisitionof integrated polarization measurements must be made. Some possible source for integrated measurements is the University of Michigan Radio Astronomy Observatory database, the VLA/VLBA Polarization Calibration Page,  and the Master EVLA POLCAL Database. However, in this case, there is no guarantee that there will be observations of your PPA calibrator sources near your VLBI observations. It is important to try to have the integrated measurements as close to the VLBI observations as possible, since the polarization of compact sources can vary on timescales of days or weeks.

Note that it may be possible to use observations of a compact source with a simple polarization structure for both D-term calibration and PPA calibration, making it possible to spend less total time on the polarization calibration. Here, it is necessary to find a source that is both relatively highly polarized and has a simple polarization structure.

At JIVE

Correlation of disk-based observations at JIVE has shifted to the EVN software correlator at JIVE (SFXC). This provides much more flexibility in terms of the output number of spectral points and integration time compared to the earlier EVN MkIV Data Processor at JIVE, and overcomes the explicit 16-station limit to a correlator pass. It also provides features that were never available on the MkIV, such as pulsar gating/binning, more than 2048 spectral points across each subband/polarization, and multiple correlation phase-centers.

At Bonn

Limited time may be available for the correlation of EVN experiments at the Bonn DiFX Correlator, but only by arrangement with MPIfR correlator staff prior to submission. Suitable projects will be those for which the scientific advantage of using the MPIfR Correlator is given in the proposal, or those which include an MPIfR collaborator who wishes to have a closer “hands-on” approach to the data flow.

Global VLBI Projects

Global VLBI projects can be processed either by the EVN correlator at JIVE or the VLBA correlator at Socorro. A specific correlator may be requested for technical reasons, which should be explained in the proposal.

Non-EVN observations

It is possible to submit an EVN correlator proposal only. The rules are given here.