GMRT is currently the largest telescope in the world operating at low radio frequencies or at meter-wavelengths. In order to observe any target source in the sky at a given radio frequency, it is necessary to observe a source with known phase corrections, say a point source with the least possible source-calibrator separation, at the same frequency, and hence calibrate the instrumental phases and this is called phase calibration.
Among the frequency bands used in GMRT, 610 MHz/235 MHz have been designed as a co-axial horn pair. Hence they can be used simultaneously. In order to observe radio sources at these frequencies one needs flux density calibrators and instrumental phase calibrators. So it is useful to observe a common set of good quality calibrators while making observations in this simultaneous dual frequency mode of observations.
Since there are only a few known phase calibrators at low radio frequencies, we have been choosing the best possible phase calibrator from this small set of sources as phase calibrators for GMRT at 610 MHz, or at 235 MHz, or in the simultaneous dual frequency mode of observation both at 610 and 235 MHz for our target source.
Also, the smaller the angular separation between a target source and a calibrator, the better it is, so as to calibrate the instrumental and line-of-sight phase errors. Or in other words, if the calibrator is nearer to the target source, say within 10 deg, atmospheric phase fluctuations will be better calibrated.
Therefore, the motivation for this project are :
We therefore observed nearly 150 possible phase-calibrator candidate sources over the region of the sky accessible with GMRT.