Tracking near-infrared fringes on BETTII: a balloon-borne, 8m-baseline interferometer

Abstract

We present the design of a fringe tracking system for the Balloon Experimental Twin Telescope for Infrared Interferometry (BETTII). BETTII is a balloon- borne, far-infrared, 8 m-baseline interferometer with two 50 cm siderostats. Beams from the two arms are combined in the pupil plane to enable double-Fourier, spatio-spectral interferometry. To maintain the phase stability of the system, we need to actively correct of the optical path difference (OPD) between the two arms. The fringe-tracking system will work in the near-infrared and will use a reference star within the field of view to achieve two goals: overlap the beams coming from the two siderostats, and track the location of the central fringe packet, which is a measure of the OPD. The fringe tracker will share most of the optical train with the science instrument. This system is part of the overall control architecture that feeds fast steering tip/ tilt mirrors and a warm delay line to ensure proper beam combination and OPD control for the science instrument. This paper investigates the different sources of perturbations that are expected at float altitude, and derives the sensitivity of the fringe-tracking system. We show progress on validating our design using a visible light, broadband Mach-Zehnder interferometer that was developed at NASA/GSFC. This system demonstrates the viability of our OPD determination approach and provides a means of testing and characterizing several OPD determination and control algorithms.