Flux density calibration

The flux density scale is usually set by observing an unresolved reference source whose flux density is believed to be stable and whose intensity has previously been measured on an absolute scale. Furthermore, time variability of these sources should be very small. During our GMRT observations, the flux density calibration was done using observations of one of the flux density calibrators 3C147 or 3C48 or 3C286, also recognized by VLA.

The table (3.4) shows the flux densities derived for the observed flux density calibrators using the task SETJY in AIPS.

Table 3.4: Flux Densities Derived from AIPS task SETJY.

Source	3C48		3C147		3C286
Frequency (MHz)	610	235	610	235	610	235
Flux density (Jy)	29.30	51.05	38.12	59.41	21.06	28.16

At least one flux density calibrator was typically observed either at the beginning and/or at the end of each observation depending on the availability, typically for $\sim$20 min. The flux density calibrator scans were used to derive the flux densities of the observed sources in case of 610 MHz using the AIPS task GETJY.

The task QUACK flags the selected portions of scans in a multi-source UV data set. The parameters used are, OPCODE='BEG', APARM(1)=0.1,APARM(2)=0.5. OPCODE=BEG is to flag first APARM(2) minute of each scan and APARM(1) is the cutting time (min) from of each scan.

The task UVFLG reads the data flagging information from a disk file supplied via the INFILE keyword. This task was used to apply all the flagging information generated by online monitoring and off-line data analysis. All the information regarding the bad data which need to be edited was noted in this input file, INFILE.

The visibility amplitude of an unresolved source remains constant as a function of baseline length. Therefore, a plot of visibility amplitude as a function of UV distance, $\sqrt{u^{2}+v^{2}}$ for the flux density calibrator was examined using AIPS task UVPLT. AIPS task UVFLG was then used subsequently to flag the discrepant data points seen in earlier plot using AIPS task UVPLT.

The primary task for calibration in AIPS is CALIB. This task is sensitive to the presence of bad data. Therefore, initial efforts to identify and flag the bad data provide good dividends at this stage of processing. The basic need for calibration is to correct the measured visibilities to approximate as closely as possible with the true visibilities.

The task CALIB was used to compute the antenna based complex gain solution for the flux density calibrator. This generates the SN (ver 1) table containing complex gain solutions. Firstly this task was run for the single channel and the task was run using following parameters: CALSOUR='3C286', '3C48', '3C147', REFANT=9, SOLINT=1, SOLMODE = 'A&P', BCHAN=ECHAN=40. Reference antenna, 9, corresponds to antenna C09. The solution interval of 1 min was used and solution mode of `amplitude and phase' was used in order to determine the complex gains, both amplitude and phase.