Fringe tracking controller modeling for a stellar imaging CubeSat optical interferometer

Abstract

Terrestrial optical interferometers have produced the first resolved images of main-sequence stellar surfaces by using long co-phased baselines and correcting for atmospheric errors using fringe tracking and closure phases. These stellar surface images are helpful for understanding the formation, dynamical structure, and evolution of stars, and also for evaluating the potential habitability of exoplanetary systems. However, the astronomical targets suitable for ground based interferometric imaging remain limited due to restricted baseline diversity, atmospheric absorption in the ultraviolet and much of the infrared, and shot noise limits on the fringe tracker when observing faint objects through tropospheric turbulence. A spaceborne formation flying optical interferometer could potentially image dimmer targets with longer baselines, but must still contend with disturbances like propulsive stationkeeping and attitude control system noise. This work simulates on-orbit fringe tracking controller performance under representative disturbances for a small three-spacecraft Michelson interferometer, and evaluates the effects of imperfect fringe tracking on the measured visibilities. We investigate fringe tracker design options, including the choice of optical path delay estimator(s), number of spectral channels, and integration time. Assuming 10 cm subapertures, we compute limiting target magnitudes and the maximum baseline beyond which noise dominates the interferometric visibility measurements. Based on this analysis, we identify design parameters for a cost effective demonstration mission that could complement ground-based stellar surface imaging capabilities.