Calibration sources are critical in EVN observations. It is recommended to schedule at least two scans on strong, unresolved sources (so-called fringe-finders) to guarantee a good signal-to-noise ratio for all baselines during the calibration. Fringe finders are required for every observation in order to determine the station clock delay offsets and drift rates. In addition, fringe finders can be used for bandpass calibration due to their high signal-to-noise ratios on all baselines. For the observation of faint targets or astrometry studies, phase-referencing calibrators are required. These sources must have well-known coordinates, be compact and exhibit a strong signal on all baselines (above ~0.3 Jy), and should be located close to the target source (within 2-3 degrees). This guarantees an accurate calibration of the phases for all antennas and a reliable transfer of those results to the target source. These observations require the detection of the phase-referencing calibrator sources within the coherence time to be able to obtain solutions that can be transferred to the target source (see VLBA Scientific Memo 24 for more detail). In the following table we quote the recommended cycling times (total observing time on phase-reference source and target source) for the different frequency bands of the EVN. Note that depending on the source elevation and separation to the phase calibrator, shorter or longer cycling times may be preferred.
| Frequency band | Recommended cycling time (minutes) |
|---|---|
| L (21/18 cm) | 4-6 |
| C/M (6/5 cm) | 4-6 |
| X (3.5 cm) | 2-4 |
| K & Q (1.3 & 0.7 cm) | 2 |
For polarization observations additional polarization calibrators must be scheduled to correct for the D-terms (see Polarisation observations section below). Some good polarisation calibrators at frequencies up to 15 GHz are 3C84 and OQ208 (sub-structured sources on milliarcsecond scales but unpolarised), and DA193 (weakly polarised with very compact structure).
Public catalogues of calibration sources can be found in different places. For example:
- The Astrogeo VLBI Calibrator Search 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.