The Miriad task mfcal determines antenna gains, band passes, and delay parameters from a calibrator for a multi-frequency observations. A multi-source observation with dual receivers is supported. Although it can handle dual polarization data, it assumes that the calibrator is unpolarized and that the polarization leakage terms are 0. This assumption does not affect the bandpass or delay in the calibration of solutions.
mfcal is an excellent program for calibration of antenna gains and bandpass if one can find compact (point-like) and strong calibrators. At submillimeter wavelengths, quasars become weak and phase instability appears to prevent from gaining S/N by simply integrating bandpass data. Furthermore, the location of the nulls in the visibilities of a resolved planet disk (in particular, those large planets) is a function of frequency in addition to the phase flip across a null. The complex in the visibility of planets might have lead to artifacts in the bandpass solutions calculated from software program which only good for the sources with simple visibility structure, such as a point source. Software could be imperfect in handling complicated visibility data. To reduce the problems, a Miriad task, smamfcal, has been implemented for the reduction of SMA data. A few pre-processing algorithms such as moving smooth, vector normalization, and weighting prior to solving for bandpass and/or antenna gains have been introduced and coded in.
In addition, due to some instrumental effects, slow bandpass variations in time on some SMA antennas were seen. Such a variation could cause ripple across the spectral band and damage bandpass quality. The time variation effect in bandpass need to be removed, in particular, for weak line and strong continuum emission cases which require a high spectral dynamic range. A special tool smatbpass has been supported in Miriad to correct for bandpass ripples.
The SMA correlator consists of a large number of chips which compute the chunk of spectral visibility for baselines (antenna pairs). Occationally, failures of computer chips or errors in handling the data were showed. Those errors due to the correlator are usually baseline based. The baseline-based errors can be corrected using blcal in Miriad.
We demonstrate calibration procedures for both the normal and special cases in the following subsections: