C.7 Amplitude and Delay Calibration

Amplitude calibration uses measured antenna gains and system temperatures (Tsys), as well as finding a correction for voltage offsets in the samplers. Even though this is substantially still the case, we now recommend a somewhat different amplitude calibration procedure than in the past based on VLBA Scientific Memo #37 (Walker 2015).

Before amplitude calibration is done, there must be information for all antennas in the gain curve (GC), system temperature (TY), and weather (WX) tables (weather tables are needed for the opacity correction in APCAL). Missing Tsys and gain curve information can usually be obtained in ANTAB format and loaded with the AIPS task ANTAB. C.10 has information on including non-VLBA calibration information, if they are not already included in the data. C.11 has details on how to incorporate the pre-EVLA VLA Tsys and gain curves. Otherwise consult 9.5.2.3.

  1.  Correct sampler offsets and apply amplitude calibration by running VLBACCOR. The procedure VLBACCOR runs ACCOR, SNSMO, and CLCAL. ACCOR uses the autocorrelation to correct the sampler voltage offsets. After ACCOR creates an SN table, SNSMO smooths the table in order to remove any outlying points. Then the SN table is applied to the highest CL table using CLCAL (using INTERPOL=’2PT’), and a new CL table is created.
  2.  Next, the instrumental delay residuals must be removed. These offsets or “instrumental single-band delays” are caused by the passage of the signal through the electronics of the VLBA baseband converters or MkIII/MkIV video converter units. There are two different methods to remove these instrumental delays, one for the case where you have pulse-cal information for some, but not necessarily all, of your antennas; and one for the case where you have no pulse-cal information at all. Note that the preferred method for continuum experiments is to use the pulse-cals, since they correct the instrumental delay over the whole experiment, rather than on a short scan. Spectral-line observers would have switched off the pulse-cals as they interfere with line observations, so they are forced to use the second (strong source) method. For VLBA continuum experiments before April 1999, you can load the pulse-cal data using PCLOD; consult the 9.5.2.
  3.  Next you must calibrate the bandpass shapes. To do this, run VLBABPSS on the bandpass calibrator, CAL-BAND. Make sure that the spectral line data for the bandpass calibrator are clean and devoid of high points, using UVPLT or SPFLG. Inputs for VLBABPASS are CALSOUR = ’CAL-BANDand a model for your calibrator if you have one. Then you must examine the BP table using POSSM by setting APARM(8)=2.
  4.  Now it is time to finish the amplitude calibration by running VLBAAMP. This procedure runs several tasks: ACSCL, SNSMO, CLCAL, APCAL and CLCAL. After the previous steps the calibrated autocorrelation amplitudes will be offset from unity; ACSCL corrects this offset. ACSCL creates an SN which is then smoothed by SNSMO and CLCAL is run to apply the calibration to the next CL table. Then to finalize the amplitude calibration, APCAL is run on the highest TY and GC tables, and a new SN table is created. Adverb DOFIT controls whether APCAL also uses the weather tables to fit and correct for opacity. It is desirable to perform an atmospheric opacity correction at high frequencies, particularly if very accurate source fluxes are needed. See 9.5.4.6 for a more detailed discussion of APCAL. Lastly, VLBAAMP runs CLCAL to apply the amplitude calibration SN table to the CL created by the last run on CLCAL. VLBAAMP will print messages telling you about the new tables it has created. To keep track of your tables, it is important to copy these messages.
  5.  At this point it is a very good idea to examine your calibration.
  6.  For spectral-line experiments needing velocity accuracy better than 1 km/s, a Doppler correction should be performed. Use CVEL; see 9.5.4.5 and 9.5.5 for details.
  7.  This is a useful time to run TASAV to save all your ancillary tables to another file. If you foul up the calibration, the relevant tables can be copied back using TACOP.