5.5. Very Long Baseline lnterferometric Observations and Data Reduction

Abstract

Publisher Summary This chapter presents an overview of very long baseline interferometric (VLBI) observations and data reduction. VLBI is similar to conventional interferometry except that the receiving elements are not connected in “real time.” The received signals are recorded, undetected, on magnetic tape under the control of atomic frequency standards and reproduced at a later time and processed to give interference fringes. There are important differences between VLBI and conventional interferometry. The telescopes used in VLBI can be very far apart owing to their independence, and the measurable quantities of phase, delay, and phase rate take on much larger values and play different roles in the data interpretation. Five topics of importance to VLBI observations and data reduction are discussed in the chapter. The measurable quantities are defined by tracing the received signals through the VLBI system. The problem of estimating the fringe amplitude of weak signals in the presence of phase noise is treated, and expressions for the signal-to-noise ratio for so- “broken-coherence” averaging are derived. The probability of misidentifying the interference fringes is given. The interpretation of the data in terms of simple models of brightness distribution is discussed. Some operational problems encountered in acquiring data are also described.