AMPLITUDE CALIBRATION

 

Traditional calibration of VLBI fringe amplitudes for continuum sources requires knowing the on-source system temperature in Jy (; Moran & Dhawan 1995). System temperatures in degrees K () are measured ``frequently'' in each BB channel during observations with VLBA antennas; ``frequent'' means at least once per observation or once every 2 minutes, whichever is shortest. These values are currently delivered to VLBA users in machine readable files that can be read by fringe amplitude calibration programs such as ANCAL or ANTAB in the NRAO Astronomical Image Processing System (AIPS) or CAL in the Caltech VLBI Analysis Programs; see Section 21. The parameters and file style expected by these programs are documented in their respective help files. Such programs can be used to convert from to by dividing by the VLBA antenna zenith gains in K Jy provided by VLBA operations, based upon regular monitoring of all receiver and feed combinations. Anonymous FTP can be used to access these VLBA zenith gains; consult file ``vlba_gains.key'' in directory ``pub'' on host ``ftp.aoc.nrao.edu'' (146.88.1.103). For projects processed on the VLBA correlator, such amplitude calibration data will eventually be delivered directly as tables in the FITS files archived and distributed by NRAO. Single-antenna spectra can be used to do amplitude calibration of spectral line projects (see Section 17).

Post-observing amplitude adjustments might be necessary for an antenna's position dependent gain (the ``gain curve'') and for the atmospheric opacity above an antenna (Moran & Dhawan 1995). File ``vlba_gains.key'', described above, contains gain curves for VLBA antennas. A scheme for doing opacity adjustments is desribed by Leppänen (1993). Such adjustments can be made with AIPS task APCAL.

Although experience with VLBA calibration shows that it probably yields fringe amplitudes accurate to 5 percent or less, it is recommended that users observe a few amplitude calibration check sources during their VLBA project. Table 4 gives a suggested list of such sources, selected because they are likely to be point-like on inner VLBA baselines at wavelengths of 6 cm and 4 cm; other sources, and other wavelengths, will be added in the future. In the meantime users may want to consult major published VLBI surveys (e.g., Taylor 1994; Polatidis 1995; Thakkar 1995; Henstock 1995; Fey 1996; plus references therein), or compendia summarizing VLBI results (e.g., Valtaoja, Lähteenmäki, & Teräsranta 1992). It might be prudent to avoid sources known to have exhibited extreme scattering events (e.g., Fiedler 1994a, b). VLBA observations of the sources in Table 4, or other suitably selected sources, can be used (1) to assess the relative gains of VLBA antennas; (2) to test for non-closing amplitude and phase errors; and (3) to check the correlation coefficient scaling factor (traditionally called the b-factor), provided simultaneous source flux densities are available independent of the VLBA observations. Positions for the sources in Table 4, used by default by the NRAO program SCHED (Walker 1997) and the VLBA correlator, are given in the standard source catalog available as an ancillary file with SCHED. The source names in Table 4 are as they appear in the standard source catalog.