VLBA Phase-Referencing

Residual delay errors in VLBI data limit the image sensitivity and dynamic range attainable using current calibration techniques such as self-calibration. Applying phase corrections derived from regular observations of nearby calibrators (phase-referencing), the residual delay errors on a target source can be reduced, increasing coherent integration times. In this paper we present the results of a series of observations made with the VLBA to examine the effectiveness of phase-referencing for improving instrumental phase stability. Optimum observing strategies when studying weak (~ mJy) sources are discussed.