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A Geometric View of Closure Phases in Interferometry

Closure phase is the phase of a closed-loop product of correlations in a $\ge 3$-element interferometer array. Its invariance to element-based phase corruption makes it invaluable for interferometric applications that otherwise require high-accuracy phase calibration. However, its understanding has remained mainly mathematical and limited to the aperture plane (Fourier dual of image plane). Here, we lay the foundations for a geometrical insight. we show that closure phase and its invariance to element-based corruption and to translation are intricately related to the conserved properties (shape, orientation, and size, or SOS) of the principal triangle enclosed by the three fringes formed by a closed triad of array elements, which is referred herein as the "SOS conservation principle". When element-based amplitude calibration is not needed, as is typical in optical interferometry, the 3-element interference image formed from phase-uncalibrated correlations is a true and uncorrupted representation of the source object's morphology, except for a possible shift. Based on this SOS conservation principle, we present two geometric methods to measure the closure phase directly from a 3-element interference image (without requiring an aperture-plane view): (i) the closure phase is directly measurable from any one of the triangle's heights, and (ii) the squared closure phase is proportional to the product of the areas enclosed by the triad of array elements and the principal triangle in the aperture and image planes, respectively. We validate this geometric understanding across a wide range range of interferometric conditions using data from the Very Large Array and the Event Horizon Telescope. This geometric insight can be potentially valuable to other interferometric applications such as optical interferometry. These geometric relationships are generalised for an $N$-element interferometer.