Large Interferometer For Exoplanets (LIFE): VII. Practical implementation of a five-telescope kernel-nulling beam combiner with a discussion on instrumental uncertainties and redundancy benefits

Abstract

Context. In the fourth paper in this series, we identified that a pentagonal arrangement of five telescopes, using a kernel-nulling beam combiner, shows notable advantages for some important performance metrics for a space-based mid-infrared nulling interferometer over several other considered configurations for the detection of Earth-like exoplanets around solar-type stars. Aims. We aim to produce a physical implementation of a kernel-nulling beam combiner for such a configuration, as well as a discussion of systematic and stochastic errors associated with the instrument. Methods. We developed a mathematical framework around a nulling beam combiner, and then used it along with a space interferometry simulator to identify the effects of systematic uncertainties. Results. We find that errors in the beam combiner optics, systematic phase errors and the root-mean-squared (RMS) fringe tracking errors result in instrument-limited performance at ∼4-7 μm, and zodiacal light limited at ≳10 μm. Assuming a beam splitter reflectance error of |ΔR| = 5% and phase shift error of Δϕ = 3°, we find that the fringe tracking RMS error should be kept to less than 3 nm in order to be photon limited, and the systematic piston error be less than 0.5 nm to be appropriately sensitive to planets with a contrast of 1×10−7 over a 4-19 μm bandpass.We also identify that the beam combiner design, with the inclusion of a well-positioned shutter, provides an ability to produce robust kernel observables even if one or two collecting telescopes were to fail. The resulting four-telescope combiner, when put into an X-array formation, results in a transmission map with a relative signal-to-noise ratio equivalent to 80% of a fully functioning X-array combiner. Conclusions. The advantage in sensitivity and planet yield of the Kernel-5 nulling architecture, along with an inbuilt contingency option for a failed collector telescope, leads us to recommend this architecture be adopted for further study for the LIFE mission.