AMPLITUDE CALIBRATION

Traditional calibration of VLBI fringe amplitudes for continuum sources requires knowing the on-source system temperature in Jy ($SEFD\/$; Moran & Dhawan 1995). System temperatures in degrees K ($T_{\rm sys}$) are measured ``frequently'' in each BB channel during observations with VLBA antennas; ``frequently'' means at least once per source/frequency combination or once every user-specified interval (default is 2 minutes), whichever is shorter. These $T_{\rm sys}$ values are required by fringe amplitude calibration programs such as ANTAB/APCAL in AIPS or CAL in the Caltech VLBI Analysis Programs; see Section 24. Such programs can be used to convert from $T_{\rm sys}$ to $SEFD\/$ by dividing by the VLBA antenna zenith gains in K Jy$^{-1}$ provided by VLBA operations, based upon regular monitoring of all receiver and feed combinations. $T_{\rm sys}$ and gain values for VLBA antennas are delivered in TY and GC tables, respectively (see Section 16). Single-antenna spectra can be used to do amplitude calibration of spectral line programs (see Section 20).

An additional loss of sensitivity may occur for data taken with 2-bit (4-level) quantization, due to non-optimal setting of the voltage thresholds for the samplers (see Kogan 1995a). This usually is a relatively minor, but important, adjustment to the amplitude calibration. In the VLBA, for instance, the system design leads to a systematic (5% to 10%) calibration offset of the samplers between even and odd BB channels; for dual polarization observations, this may lead to a systematic offset between RR and LL correlations that must be accounted for in the calibration. The combination of the antenna and sampler calibrations may be found and applied in AIPS using the procedure VLBACALA.

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, particularly at high frequencies (Moran & Dhawan 1995). The GC table 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 if weather data are available in a WX table (see Section 16).

Although experience with VLBA calibration shows that it probably yields fringe amplitudes accurate to 5% or less at the standard frequencies in the 1-10 GHz range, it is recommended that users observe a few amplitude calibration check sources during their VLBA program. Such sources can be used (1) to assess the relative gains of VLBA antennas plus gain differences among base band channels at each antenna; (2) to test for non-closing amplitude and phase errors; and (3) to check the correlation coefficient adjustments, provided contemporaneous source flux densities are available independent of the VLBA observations. These calibrations are particularly important if non-VLBA antennas are included in an observation, since their a priori gains and/or measured system temperatures may be much less accurate than for the well-monitored VLBA antennas. The VLBA gains are measured at the center frequencies appearing in Table 3; users observing at other frequencies may be able to improve their amplitude calibration by including brief observations, usually of their amplitude check sources, at the appropriate frequencies. Amplitude check sources should be point-like on inner VLBA baselines. Some popular choices in the range 13 cm to 2 cm are J0555+3948=DA193, J0854+2006=OJ287, and J1310+3220. Other check sources may be selected from the VLBI surveys available through http://www.vlba.nrao.edu/astro/obsprep/sourcelist/ . It might be prudent to avoid sources known to have exhibited extreme scattering events (e.g., Fiedler et al. 1994a, b).