C.11 Pre-EVLA VLA data

The method described in this section is only meant for VLA data from before February 2013 (before the phased-EVLA came on-line). The observation being calibrated may have incorporated either a single VLA antenna or the phased VLA, but the amplitude calibration parameters for the VLA were not transferred automatically. You will need to obtain an input text file for the VLA then run ANTAB before APCAL. Before February 2013, the gains and system temperatures for this file, in an appropriate format, were supplied in a file called xxxxxcal.y.gz’, where xxxxxis the observation code (e.g., ‘bm120’), located at http://www.vlba.nrao.edu/astro/VOBS/astronomy/mmmyy/xxxxx/. That file contains instructions on editing the file to get correct inputs. For a phased array or a 1.3-cm observation in which 3 antennas are used, follow the instructions in C.11.2; for a single antenna, use C.11.1.

C.11.1 Single VLA Antenna

The method described in this section is only meant for VLA data from before February 2013. Depending on the age of your observation, you may have to add an INDEX entry within the TSYS card (Do not separate the INDEX entry from the TSYS entry by a “/” !!!), un-comment the GAIN line for your particular observing frequency, and un-comment the TSYS line. There are examples of INDEX entries in the comments at the head of the file.

Beginning in June 2003, the INDEX, GAIN, and TSYS information in this table are reformatted to be directly acceptable to AIPS. You should check the times in the text file to make sure that your observation has been properly described. Only a few special cases will require editing of the file; in most cases you are able to invoke ANTAB with no editing. Once you are satisfied with the ANTAB file, load the data following the directions in C.10.1.

C.11.2 Phased VLA

The method described in this section is only meant for phased-VLA data from before February 2013. The VLA may be phased on a program source (‘STRONG’), or may be phased on a phase-reference source (‘CAL-PHASE’), with the resulting solutions applied to the program source (‘TARGET’). Rather than recording a system temperature, the VLA system will record a ratio of antenna temperature to system temperature, which will vary as the array phases up. In order to convert the ratio of antenna and system temperatures to a usable gain, the flux density of some source will be needed.

  1.  Load and calibrate the VLA data by standard means (see Chapter 4). Determine the flux density of a relevant strong source, usually either ‘STRONG’ or ‘CAL-PHASE’. Then, on the VLBI data set, insert the flux density of this source into the SU table using SETJY. For example, if the source is ‘CAL-PHASE’ and its flux density is 0.432 Jy, run SETJY with SOURCES = ’CAL-PHASE’; BIF = 0; EIF = 0; ZEROSP = 0.432,0; OPTYPE = ’ ’.
  2.  Edit the input file as indicated above for a single VLA antenna. Again, an INDEX line, a GAIN line, and a TSYS line must be checked (after June 2003) or be created or un-commented. The GAIN line is independent of observing band (the source flux is used to determine the gain), and the TSYS line should include the parameter ‘SRC/SYS’, indicating that the ratio of antenna temperature to system temperature is being supplied.
  3.  Run ANTAB to read in the input file of amplitude calibration parameters. Then run VLBACALA to put this in an SN table. Both steps are essentially the same as for a single VLA antenna (see C.11.1). The most likely problem is that APCAL in VLBACALA will fail because you forgot to enter a source flux density using SETJY, although the error message may not always make this obvious.
  4.  Run VLBASNPL or SNPLT to inspect the resulting SN table, as for the single VLA antenna. In this instance, you should see that the phased VLA is very sensitive. If the phasing worked well at centimeter wavelengths, the amplitude should be near 4 or 5 instead of the value of 17 or 18 seen for a single VLBA antenna. At the start of scans where the VLA is being phased, you may see a rapid change in the amplitude gain (toward smaller numbers) as the antenna phases are brought into alignment. The SN table should be inspected very carefully, because there may be data that should be flagged when the VLA phasing did not work well. Three possible reasons for poor phasing are (1) the source is too weak; (2) the troposphere is misbehaving; or (3) there was radio-frequency interference at the VLA.