Fringe Tracking and Group Delay Tracking Methods for MIDI

The tracking of the white light (central) fringe in a broadband optical/IR interferometer, allows the possibility of long coherent integrations of fringe visibility. Fringe tracking involves the determination of absolute OPD o sets (not just narrowband phase) and either real-time correction of the interferometer's delay lines, or determination of the estimated OPD time series to be applied to the interferometric data o -line to e ect coherent integration of fringe visibility. Algorithms of this sort will be included in the data reduction software package being developed for MIDI, the 10 micron interferometric instrument of the VLTI. Group-delay tracking is a technique which is somewhat more robust than true phase tracking, but supplies a cruder estimate of OPD variations. Such incoherent techniques are useful for coarse adjustments to an interferometer's delay lines, and will be part of the real-time software operating in support of the MIDI instrument. General characteristics of both estimators are compared as regards sensitivity, detection bandwidth, and behavior in response to dispersion.