Amplitude Calibration
Traditional calibration of VLBI fringe amplitudes for continuum sources requires knowing the on-source system noise in Jy (SEFD; Moran & Dhawan 1995). System temperatures in Kelvin ([inline]T_{\rm sys}[/inline]) are measured continuously during observations at VLBA stations, with mean values tabulated at least once per source/frequency combination or once every user-specified interval (default 2 minutes), whichever is shorter. These [inline]T_{\rm sys}[/inline] values are used in fringe amplitude calibration by AIPS task APCAL, which converts [inline]T_{\rm sys}[/inline] to SEFD by dividing by the VLBA antenna gains in [inline]\rm K\: Jy^{-1}[/inline], expressed as a peak gain multiplied by a normalized "gain curve". The latter data are based on regular monitoring of all receiver and feed combinations. [inline]T_{\rm sys}[/inline] and gain values for VLBA antennas are delivered in TY and GC tables, respectively. Single-station spectra can be used for amplitude calibration of spectral line observations.
Additional amplitude adjustments may be necessary to correct for the atmospheric opacity above an antenna, which can be significant at high frequencies (Moran & Dhawan 1995). Leppänen (1993) describes a method for opacity adjustments. AIPS task APCAL uses weather data from the WX table to carry out such adjustments.
Further corrections are usually applied to observations taken with 2-bit (4-level) sampling, for the effects of non-optimal setting of the quantizer voltage thresholds (Kogan 1995a). These adjustments are usually relatively minor but can induce systematic effects. Sampling-based calibration adjustments are determined by AIPS task ACCOR. The combination of the antenna and quantizer calibrations may be found and applied in AIPS using the procedure VLBACALA.
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 sub-bands at each station; (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 stations 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 stations. The recommended technique for this situation is to restrict the gain normalization in self-calibration to a subset of trusted stations (generally some of the VLBA stations), and to high elevations. AIPS task CALIB can do both.
The VLBA gains are measured at the center frequencies appearing in column [4] of the Receiver Frequency Ranges & Performance table; 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 various VLBI surveys. It might be prudent to avoid sources known to have exhibited extreme scattering events (e.g., Fiedler et al. 1994a, b).
