Introduction

The VLA calibrator manual lists $\sim$ 1860 calibrator sources at different frequencies. But this manual is mainly for high radio frequencies and there are very few good calibrators available at low frequencies. Say for 1000 good calibrators at L band (1420 MHz), there are only $\sim$ 50 sources which are good at low radio frequencies. Since GMRT is mainly operating at low radio frequencies, the need for a set of good calibrators at low radio frequencies is a necessity.

Hence, the initial list of candidate calibrators for the GMRT observation was taken from the VLA Calibrator Manual. Since VLA have A,B,C & D array configurations, two of the configurations are almost identical to the hybrid configuration of GMRT, i.e., the VLA configurations B & D, provide baseline which is similar to GMRT. ($A$ configuration is of diameter of 36 km, the size of array gradually decreases with the $B$ & $C$ configuration and for the $D$ configuration the diameter decreases up to 1 km). So most of the calibrators which are good calibrators at $L$ band or 1420 MHz could have been used by bootstrapping method at 610 MHz, but it was difficult to identify good quality phase-calibrators at low radio frequencies, in particular 235 MHz. This is because of the wider field-of-view and hence, ionosphere effects are nearly always important at these low radio frequencies.

In this following section we will discuss the criteria for the sample selection and observing setup used for the observations at GMRT.