Bandwidth smearing in optical interferometry: Analytic model of the transition to the double fringe packet

TL;DR

This paper develops an analytic model to understand and correct bandwidth smearing effects in optical interferometry, especially for multiple stellar systems, considering atmospheric impacts.

Contribution

It introduces formalism and equations to model and mitigate bandwidth smearing in optical interferometry, extending concepts from radio to optical regimes.

Findings

Provides equations to quantify fringe contrast reduction due to smearing

Models atmospheric effects on bandwidth smearing

Offers correction methods for observational data

Abstract

Bandwidth smearing is a chromatic aberration due to the finite frequency bandwidth. In long-baseline optical interferometry terms, it is when the angular extension of the source is greater than the coherence length of the interferogram. As a consequence, separated parts of the source will contribute to fringe packets that are not fully overlapping; it is a transition from the classical interferometric regime to a double or multiple fringe packet. While studied in radio interferometry, there has been little work on the matter in the optical, where observables are measured and derived in a different manner, and are more strongly impacted by the turbulent atmosphere. We provide here the formalism and a set of usable equations to model and correct for the impact of smearing on the fringe contrast and phase, with the case of multiple stellar systems in mind. The atmosphere is briefly modeled and discussed.