Amplitude Calibration

  Amplitude calibration for VLBI data sets is done using the tasks ANTAB and APCAL. A very large text file needs to be created outside of ${\cal AIPS}$, containing the system temperatures and gain curves of each antenna. This file will be read by ANTAB to create a TY (system temperature) table. APCAL reads the TY table and produces an SN table which can be applied to the calibration by CLCAL. APCAL can also perform opacity corrections to the gain curves, but the details will not be discussed here. This table is shipped as part of the log file for VLBA data sets. The EVN has begun providing its users with an ANTAB-ready system temperature file as well. Contact Fredrik Rantakyro (frederik@astbo1.bo.cnr.it) for further information. Errors in these tables have frequently arisen as the system is developed, so carefully check the system temperatures provided by the EVN with the telescope log files.

The system temperature section is just a list of temperatures and the times at which they were measured. For the GR2 data, the Effelsberg system temperature section looked like

TSYS EFLSBERG FT=1. TIMEOFF=26.0
INDEX ='R1','L1','R2','L2','R3','L3','R4','L4','R5','L5','R6','L6','R7','L7'/
086 06:12 47 43 47 43 47 43 47 43 48 43 48 44 48 43
086 07:09 47 43 47 44 47 43 47 43 47 43 48 45 48 43
086 07:30 52 48 52 48 52 47 52 47 52 47 53 49 52 48
086 07:44 36 34 37 34 36 34 36 34 36 34 37 35 36 34
086 08:23 48 44 48 44 49 44 49 44 49 44 49 45 49 44

The INDEX adverb is used to specify the polarization of each value entered. For the file prepared at the VLBA correlator, the index string is not provided, and must be added by hand.

After the system temperatures have been entered in the file for all antennas, the gain curves must be entered. For the VLBA antennas this information is found in the vlba_gains.key file obtained from the AOC public FTP area. Gain curves from other antennas are supplied by the stations in various formats and are entered in the file. At this time, an effort is underway to accurately and systematically determine the antenna gain curves for all of the EVN stations.

Once the gain curves are entered for all antennas, copy the file to a directory accessible by ${\cal AIPS}$ and run ANTAB and APCAL. Use SNPLT to plot the system temperatures for each antenna, flagging bad calibration points. This flagging step is important since ${\cal AIPS}$ will interpolate between all entered values to get the amplitude for each time stamp. High values will cause problems with some data as a result. The resultant SN table should be applied to the cumulative calibration with CLCAL, again using interpolation mode 'SELF' to avoid interpolating between gain values from different sources.

Other tests of the amplitude calibration can be done. For example, use VPLOT to plot the amplitude of RR/LL as a function of time. This should be close to unity for all baselines. If one antenna shows a consistently low or high value of RR/LL , you can scale either the right or left gain curve to compensate (either with the DPFU in the ANTAB text file or by using the task CLCOR). Another test would be to plot the amplitude of the parallel hand data as a function of u-v distance. If data to one antenna is consistently low or high, scale the gain curve of that antenna to fix it. Such testing will make the hybrid mapping stage much easier and faster.