Spectral Line Observations

Diamond (1995) and Reid (1995, 1999) describe the special requirements for data acquisition, correlation, and post-processing of spectroscopic VLBI observations. The transition rest frequency, approximate velocity, and velocity width for the line target must be known in order to set the observing frequency and bandwidth correctly. The schedule should include observations of one or more strong continuum sources to be used for fringe-finding, "manual" phase calibration, and bandpass calibration. In addition, scans of a continuum source reasonably close to the line target should be scheduled, for use in delay and fringe-rate calibration. The pulse cal generators should be disabled.

Post-processing steps include performing Doppler corrections for the Earth's orbital motion (a correction for Earth rotation is not necessary for VLBA observations since station-based fringe rotation is applied in the correlator); amplitude calibration using single-antenna spectra; fringe fitting the continuum calibrators and applying the results to the line target; referencing phases to a strong spectral feature in the line source itself; deciding whether to use normal synthesis imaging or fringe rate mapping; and then forming a spectral line cube. All these post-processing steps can be done in AIPS.

Data reduction techniques for VLBI spectral line polarimetry are discussed by Kemball, Diamond, & Cotton (1995) and Kemball (1999).