If the successful proposal is scheduled to be observed in the next session, the Principal Investigator of the proposal will receive an informative email about how to prepare the observations.
The Principal Investigator is required to provide a schedule using the NRAO SCHED program (see documentation and/or source code). If help in the proposal preparation is needed, please contact one of the support scientists at JIVE.
If stations other than those of the EVN, Arecibo, NRAO, DSN and VSOP (e.g. Ny-Alesund, Matera, Fortaleza) are scheduled in the observations, please consult the code of practice for non-EVN observatories.
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 should not be run. Instead, it is fine to use auto-correlations for bandpass corrections or to use the task ACFIT.
This observation mode requires the detection of the continuum sources in a narrow band within the coherence time, especially for a lower frequency band. At higher frequencies, the cycling time should be no longer than 2 minutes (see VLBA Scientific Memo 24 for 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., Effelsberg slews 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 sources
Calibration sources are critical in VLBI observations. It is recommended to schedule at least two scans on strong, unresolved sources (aka fringe-finders) to guarantee a good signal-to-noise ratio for all baselines during the calibration. In the instance 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:
- Scheduling spectral line observations
If the project includes spectral line observations, this section details special considerations that should be taken into account during the scheduling and the calibration of the data.
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 key file, 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 centre of 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 bracketing the 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.
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. In the schedule file, it must be specified that phase calsignals are to 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 must 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 polarisation observations
There are two aspects to proper calibration of polarisation-sensitive VLBI observations: (1) D-term calibration, the calibration and removal of the instrumental polarisations, or D-terms, for each antenna and (2) Position angle calibration, the calibration of the absolute orientation of the polarisation position angles (PPAs).
Determination of the instrumental polarisations is relatively straightforward if observations of a source that is either unpolarised or is known to have a simple polarisation structure are made.
In the case of a polarised source, it is necessary to observe it over a wide range of parallactic angles. Usually, five or six scans, with a 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 polarisation calibrator, if it is known to have a simple polarisation structure. It is best to observe a different source specifically for the determination of the D-terms if the programme sources are expected to have relatively complex polarisation structures.
In the instance of a strong unpolarised source, a single observation scan of 8-10 minutes is sufficient.
Some good polarisation calibrators at frequencies up to 15 GHz are 3C84 (lots of structure but unpolarised), OQ208 (also a fair bit of structure but unpolarised), and DA193 (weakly polarised with very compact structure). For suggestions about other sources that may be suitable for D-term calibration, contact JIVE.
Position angle calibration
The most common way to calibrate the orientation of the polarisation position angles (PPAs) is to use simultaneous or nearly simultaneously integrated and VLBI observations of a source with compact polarisation (in which essentially all the integrated polarisation is detected on short VLBI baselines). By comparing the orientation of the PPAs for the total VLBI-scale polarisation with their known orientation in the integrated measurements, it is possible to determine the necessary rotation to calibrate the VLBI polarisation angles, i.e., to give them their true observed values.
For this purpose, it is necessary to obtain VLBI and integrated observations of a source with compact polarisation. In some cases, program sources may be suitable for this. If in doubt about whether the program sources have sufficiently compact polarisation, 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 when the source 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 JIVE.
There are currently no known sources with constant polarisation 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 polarisation measurements for the PPA calibration. If the phased VLA is included as part of the EVN array, integrated measurements can be derived from the data for these instruments. Otherwise, other arrangements for the acquisition of integrated polarisation measurements must be made. Possible sources for integrated measurements is the University of Michigan Radio Astronomy Observatory database, the VLA/VLBA Polarisation Calibration Page, and the Master EVLA POLCAL Database. However, in this case, there is no guarantee that observations of the selected PPA calibrator sources are conducted close in time to the EVN observations. It is important to try to have the integrated measurements as close in time as possible to the EVN observations, since the polarisation 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 polarisation structure for both D-term calibration and PPA calibration, allowing to spend less total time on the polarisation calibration. Here, it is necessary to find a source that is both relatively highly polarised and has a simple polarisation structure.
- Scheduling Target of Opportunity observations
After a positive response by the Chair of the EVN Programme Committee to a Target of Opportunity proposal:
- an observing date has to be arranged,
- manpower at the observatories has to be organised,
- disk supply or manpower at the JIVE correlator for setting up an e-VLBI run has to be organised,
- an observing schedule has to be made.
There is no guarantee that all the key people normally associated with setting up an EVN observation will be available. Therefore, the Principal Investigator must play an active role (with assistance from the EVN Scheduler, the Head of Science Operations and Support at JIVE and/or the PC Chair if they are available) in negotiating a date for the observations with the observatory contacts, ensuring that any stipulations made by the Chair of the EVN Programme Committee (minimum/maximum number of telescopes, frequency, length of time, etc) are followed.
The principal investigator, with the assistance of the same group, must assess the feasibility of using either disk recording or e-VLBI (or some combination of both). This may depend on the supply of disks at the observatories and the timescale for shipping supplies, the availability of the JIVE correlator on the agreed observing date, the current technical capabilities and limitations of disk recording and e-VLBI, and the speed with which the correlation is desired.