Calibrator Quality - Our Criteria

As discussed earlier, since a calibrator is used to correct for systematic artifacts, like instrumental effects, atmospheric and ionospheric effects through which the signal propagates, we need to define appropriate criteria to decide which of the observed sources can be used as GMRT calibrators. In addition to the intrinsic properties of the sources themselves, since the field of view at low frequency is large, the decision will also depend on the properties of the background sources within the field-of-view, which also has to be understood.

The three basic qualities required of a calibrator are that,

• it should be a strong source.
• it should be a point source.
• there should be no other sources in the field of view.

In the following sections we will try to convert these requirements into quantifiable criteria.

1. Strength of the source : We would like the calibrator to be so strong that we can spend as little time as possible on it and still get sufficient signal-to-noise ratio on the calibrator so that it does not worsen the sensitivity of the final image. The noise on a given baseline is given by . Assuming a typical integration time of 5 minutes for the calibrator, a bandwidth of 1 MHz ( to cater for both line and continuum observations), and typical system temperatures of 100K and 300K at 610 and 235 MHz, we find that the noise on given baseline is typically 100mJy at 610MHz and 300 mJy at 235MHz (assuming 1 Jy gives an antenna temperature of $\sim$0.3K for the GMRT antennas). Based on this, we have set a lower limit of 0.75Jy at 610MHz and 2.5Jy at 235MHz for a source to be strong enough to be classified as a calibrator for GMRT. While this corresponds to a signal-to-noise ratio of only 7-8 per baseline, given that 29 baselines are used to determine the gain of an antenna, the signal-to-noise ratio on the gain solution should be better than 35-40 which should be satisfactory.

2. Compactness of the source : An ideal calibrator should be a point source so that the visibility is constant and the signal-to-noise ratio does not depend on the baseline. This translates to the source having a flat $uv$-distance plot and its deconvolved size being much smaller than the beam even with the highest resolution ( see Appendices A and B).

3. Field of view of the source : Ideally the calibrator should be the only source in the field of view. However this is not a practical definition at low frequencies and we relax it to demand that the calibrator is the dominant source in the field. A quantitative measure of this could be that there is no other source in the field that is stronger than 10% of the calibrator so that its effect on the assumed flatness of the calibrator's $uv$-distance plot can be ignored.

We note that criteria 2 and 3 could be relaxed if there is a desperate shortage of calibrators. For instance, a source which was not a point source but its structure was known so that its visibility at any baseline could be predicted could be used as a calibrator if the signal-to-noise ratio was high on all the baselines. Similarly, if there are relatively strong source in the field of a point source, it could still be used as a calibrator if the positions and structures of the secondary sources were known.