Methodology

Following is the global methodology used for observations.

• The primary$/$flux density and bandpass calibrators used were chosen from 3C48, 3C147 & 3C286. At least one of these sources was often available in the sky at any given time of observing.

• In each observation run about 5 to 10 sources were observed over the period of 5 to 8 hours. Typically, each source was observed for 17 to 20 minutes. 13 such observations were performed to observe all 150 sources.

• The primary/bandpass calibrator were observed after 1 to 2 hours of observing of the target sources.

• The number of antennas available during any observing run ranged from 25 to 30.

• The observed flux/bandpass calibrator were used as a zeroth-order secondary calibrator in the case of 235 MHz and 610 MHz data reduction. We also assumed that the observed sources are themselves secondary calibrators, which are bright enough (with steep spectra, $S_\nu \propto \nu^{-\alpha}$) and hence have sufficient flux density to permit self-calibration at subsequent stage of data reduction.

• The data could be corrupted due to RFI (Radio Frequency Interference), hardware or software failures, power supply failure, ionospheric scintillations, etc. Such data was flagged from the database before it is was further used for analysis and subsequent mapping. The gross flagging information was generated largely by monitoring the above sources of errors caused during the observing runs.