Towards Optical & Infrared Interferometry From Space

Abstract

The study of exoplanets is one of the fastest growing sub-fields in astronomy, and in particular one question stands above the rest: "is Earth the only planet to harbour life? Are we alone?" While there have been numerous proposed missions to answer this question, none hold as much promise as a mid-infrared (MIR) nulling space interferometer as it concurrently provides the contrast, sensitivity and angular resolution to characterise many Earth-sized planets inside their star's habitable zone (HZ). Previous space interferometry missions, such as Darwin and TPF-I, were cancelled in the mid 2000s, but in recent times there has been a revival in the field with the advent of the Large Interferometer For Exoplanets (LIFE) initiative, that aims to resurrect such a mission for launch in the 2040s. Nevertheless, numerous challenges remain including the formation flight at the appropriate precision and deep, cryogenic nulling. In this thesis, I present advances towards making a MIR space interferometer mission, such as LIFE, feasible. First, I investigate configuration and architecture options for a large-scale multi-aperture mission such as LIFE. Using the paradigm of kernel-nulling, where linear combinations of nulled outputs can create observables robust to phase errors, I find that a pentagonal array of five telescopes is superior to the default Emma X-array configuration in the photon limited regime when it comes to detecting and characterising HZ Earth-like planets. Taking this array configuration, I present a possible implementation of the beam combiner using an adaptive nuller and a cascade of beam splitters, as well as a discussion on a few alternative implementations. The base implementation is analysed for instrumental errors caused by imperfections in the beam splitters or RMS fringe tracking residuals, and I find that the short end of the MIR bandpass is much more susceptible to these errors than the zodiacal light dominated long wavelengths. The implementation also has in built redundancy for if a collector telescope were to fail. I then introduce and discuss the ground-based pathfinder interferometer Pyxis, the only visible light combiner in the Southern Hemisphere which consists of three autonomous robotic platforms. These platforms are used as placeholders for satellites, where Pyxis demonstrates the metrology, pointing and fringe tracking precision needed for a small space interferometer mission. In particular, I detail the beam combiner which makes use of a small ``tricoupler'' based photonic chip to provide a retrieval of the full complex coherence of the two-arm interferometer, allowing us to take scientific exposures while simultaneously fringe tracking on the starlight. I also detail the complex network of control systems that will allow such a formation-flying interferometer to maintain interferometric fringe stability. Together, this work demonstrates that space-based formation-flying interferometry is indeed feasible, and is a step towards a future space demonstrator mission. Further work into the effect of instrumental errors, beam combiner complexity and MIR photonics are still required, but hopefully, with due time, research and planning, we will begin to characterise Earth-like exoplanets using a MIR space interferometer in the near future. Show more