The properties of metal thin films have been largely overlooked in discussions of the technical limitations and problems that arise in the field of direct detection of exoplanets. Here, polarization properties and anisotropy properties of highly reflecting thin metal films are examined within the context of the requirements for the ultra-low scattered-light system performance of coronagraphs applied to space and ground-based high-contrast, white-light astronomy. Wavelength-dependent optical constants for highly reflecting thin metal films, taken from the literature are used to calculate the polarization-dependent transmissivity of a typical coronagraph. The effects of degraded performance on the astronomical science are examined. Suggestions are made for future work.

https://arxiv.org/pdf/astro-ph/0309399.pdf

Polarization Effects in Reflecting Coronagraphs for White Light Applications in Astronomy

The temperature-dependent dispersion of the refractive index of optical materials is analyzed by introducing an appropriate formulation of what we call their "normalized thermo-optic coefficients" (NTOC). These parameters are obtained experimentally by performing accurate interferometric measurements of both thermal expansion and changes in optical thickness of parallelepipedic shaped samples of millimetric size which are submitted to a linear ramp of temperature. Changes in optical thickness are recorded as function of temperature at a few discrete laser wavelengths; they are expressed as power series of temperature and the use of a simple vectorial formalism allows then to determine the thermal behavior of the dispersion law over the whole transparency range of the material. The validity and reliability of the proposed method has been confirmed by modeling the temperature dependence of the type II second harmonic generation 1.064 μm - 0.532 μm in a KTiOPO4 single crystal, recorded up to 120°C.

Dispersion and temperature dependence of thermo-optic coefficients of optical materials over their whole transparency range: vectorial formalism and application to KTiOPO4 and HgGa2S4

The Large Interferometer For Exoplanets (LIFE) is an envisioned nulling interferometry space mission to characterize the atmospheres of terrestrial exoplanets in the mid-infrared (MIR) wavelength range (∼4-18.5 μm.) The star-to-planet flux contrast for an Earth-twin exoplanet is ≈ 107 at these wavelengths. Previous studies have shown that a “raw” null-depth of 105 provided by the interferometer is sufficient as long as the residual starlight can be removed through signal modulation, phase-chopping and data post processing. Two main technological challenges for a nulling interferometer are instrument stability and sensitivity. Several test-benches were built for LIFE’s ancestral mission concepts DARWIN and TPF-I. Operating at ambient conditions, they demonstrated excellent stability and suppressed the artificial starlight by up to 106 (depending on the spectral bandpass). However, instrument sensitivity/throughput for astronomical sources can not be characterized at background dominated ambient conditions. Cooling the instruments to cryogenic conditions reduces the thermal background and enables sensitivity driven instrument characterization. The Nulling Interferometer Cryogenic Experiment (NICE) is a single Bracewell nulling interferometer test-bench for LIFE. The ultimate aim of this test-bench is to attain a sensitivity level that demonstrates the feasibility of detecting an Earth-twin around a Sun-like star at 10 pc with a spectral bandwidth of 10% at 10 μm. The development of NICE is divided into two phases, the warm and cold phase. The warm phase focuses on the alignment of the optical components and maintaining their position and angular stability to achieve a null depth of 10−5 − 10−6 at 4 μm over several hours. In the cold phase, NICE will be cooled to 15 K to suppress the thermal background, and the throughput and sensitivity of the instrument will be characterized. This paper describes the development plan of NICE and presents the optical layout of the NICE warm phase. It also presents the preliminary null-depth reached by the NICE warm phase and the residual alignment errors in the system.

The Nulling Interferometer Cryogenic Experiment: I

The Large Interferometer For Exoplanets (LIFE) is a proposed space mission that enables the spectral characterization of the thermal emission of exoplanets in the solar neighborhood. The mission is designed to search for global atmospheric biosignatures on dozens of temperate terrestrial exoplanets and it will naturally investigate the diversity of other worlds. Here, we review the status of the mission concept, discuss the key mission parameters, and outline the trade-offs related to the mission's architecture. In preparation for an upcoming concept study, we define a mission baseline based on a free-formation flying constellation of a double Bracewell nulling interferometer that consists of 4 collectors and a central beam-combiner spacecraft. The interferometric baselines are between 10–600m, and the estimated diameters of the collectors are at least 2m (but will depend on the total achievable instrument throughput). The spectral required wavelength range is 6–16μm (with a goal of 4–18.5μm), hence cryogenic temperatures are needed both for the collectors and the beam combiners. One of the key challenges is the required deep, stable, and broad-band nulling performance while maintaining a high system throughput for the planet signal. Among many ongoing or needed technology development activities, the demonstration of the measurement principle under cryogenic conditions is fundamentally important for LIFE. 

The Large Interferometer For Exoplanets (LIFE): a space mission for mid-infrared nulling interferometry

In the literature, one can find requirements for nulling IR interferometry in terms of nulling ratio nl and stability of this nulling ratio σ nl for the observation of an exo-Earth in a 1AU orbit around a solar type star located at 10pc from the Earth: nl = 10 -5 between 7 and 20 μm and σ nl = 3 · 10 -9 over 10 days. 1 These requirements are very demanding. We report on studies made to obtain the technological requirements in terms of nulling ratio and stability of this nulling ratio as function of the targeted planets and stars. In addition we present methods of dithering developed for the NULLTIMATE test bench to achieve the maximum stability.

Toward the stability required for direct observations of exoplanets with nulling interferometry technique

The NULLTIMATE project developed and realized three concepts of achromatic phase shifters for nulling interferometry.
One of the concepts is based on dispersive plates made of three materials which where fully
characterized regarding their refractive index and thermo-optic behavior between 100K and 330 K. The other
two concepts are based on mirror optics, one of which uses the phase shift of π when crossing a focus, the
other the reversal of electric fields at reflection. An optical bench has been set up to test and characterize these
phase shifters at wavelengths 2 − 2.4 μm with the option of changing to the 10 μm domain. We summarize the
development of the achromatic phase shifters and report on the current status of the test bench.

The NULLTIMATE test bench: achromatic phase shifters for nulling interferometry

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The NULLTIMATE test bench: achromatic phase shifters for nulling interferometry