European Commission logo
English English
CORDIS - EU research results

Ultimate Angular Resolution Astrophysics with kernel-phase and full-aperture interferometry

Periodic Reporting for period 4 - KERNEL (Ultimate Angular Resolution Astrophysics with kernel-phase and full-aperture interferometry)

Reporting period: 2021-04-01 to 2022-03-31

The angular resolution of a telescope - its capability to distinguish the fine details of an astrophysical scene - is fundamentally limited by the phenomenon of diffraction. This constraint is one of the two major reasons why astronomers keep wishing for larger and larger telescopes (the other being sensitivity) or that of using multiple telescopes in concert using interferometric recombination.

A special case of diffraction-dominated astronomical observation of emblematic importance concerns the direct imaging and spectral characterization of extrasolar planetary systems. Since the discovery of the first exoplanet in 1995 using the indirect radial velocity method, their direct imaging has been listed amongst the top three ambitions of modern astrophysics. Each discovery of a new exoplanet brings us closer to answering the fundamental question of the place of humanity in the grand scheme of the Universe.

The ability to reliably isolate the light of extrasolar planets remains an operational challenge. Two sources of noise do dominate the high-contrast imaging game: the photon noise of the very bright host star and the phase noise induced by the atmosphere and imperfections in the instrument. Conventional high-contrast imaging solutions can suppress the static diffractive pattern of a bright source but residual aberrations induce second order starlight leakage. This leakage is a non linear function of upstream errors that is difficult to diagnose and remedy in real time. KERNEL tackles the problem from the other corner of the room: starting from non-coronagraphic imaging where one can form observables that are robust to phase errors, the central aim of KERNEL was to develop the means to increase its contrast detection limits.

KERNEL has offered to the community a now proven diffraction-dominated data reduction framework, called kernel-phase imaging (KPI), and an open source software implementation of this framework called XARA. KPI is an observing mode that will be tested during the commissioning of the recently launched James Webb Space Telescope (JWST) and XARA will be integrated to the data reduction pipeline offered to the community of JWST users. The ERC KERNEL has expanded on the original kernel-phase concept: the project contributed to increase the sensitivity of the approach, and its ability to tackle more challenging observing scenarios.

KERNEL has also led to the formulation and the experimental validation of a technique called kernel-nulling, that produces high-contrast self-calibrating observables, opening new possibilities for the direct detection and characterization of exoplanets, from the ground as well as from space.
Over the course of the project, the team developed powerful data analysis software called XARA that interpret images in the light of the kernel-framework: members leading personal projects put this software to the test on existing archival data and were able to demonstrate its effectiveness in a wide variety of cases - with more still in preparation. Motivated by the detection problem of very faint sources in diffraction dominated images, a special focus was put on the formulation of statistical tests where the false alarm probability rate can be controlled. This framework was then tested in a series of contexts of increasing complexity: tackling partial data saturation, the handling of field rotation, the use of apodized apertures, and contributed to increase the sensitivity of the approach. These results were published in peer reviewed journal papers primarily led by students of the project and presented in international conferences.

Designing a true high-contrast application of the kernel-framework required going back to a sparse aperture scenario, with a finite number of apertures. Our concept, called the kernel-nulller, is a very exciting prospect, that simultaneously tackles for the first time, the two major sources of noise affecting high-contrast imaging. Our baseline design of a four-beam recombiner, now informs the construction of a nuller for the VLTI, called Hi-5/VIKiNG, that is a module of the ASGARD instrument suite. VIKiNG is a novel all-in-one interferometric recombiner, whose realization using conventional optics would be difficult. We therefore turned our attention to integrated optics that offers powerful, elegant and very compact solutions to such problems. Our first kernel-nuller prototype, at the moment, probably the most complex type of photonic circuitry ever developed for astronomy, was used in the laboratory to validate the theoretical concept, and is now ready to be deployed on-sky.

The original kernel-framework however also offered possibilities for metrology applications. We were able, over the course of an engineering observing run at the telescope, to demonstrate its ability to correctly diagnose a problem plaguing high-contrast imaging instruments, called the low-wind effect, which conventional adaptive optics are unfortunately unable to tackle. After developing a polychromatic extension of the same approach, we designed and prototyped of an integrated fringe-tracking and alignment stabilization solution for the VLTI interferometer: HEIMDALLR, another module of the ASGARD instrument suite, will be installed at the focus VLTI in the second part of 2024.
The project started with a framework exploiting the properties of a mathematical space called "the kernel" that is protected against small instrumental perturbations and that can be used when the effect of perturbations is linear. Our approach expanded on observing approach of aperture masking interferometry (AMI), able to produce self-calibrating observables called closure-phase, but at a high throughput cost: our work has definitely established that kernel-phase using the full aperture of a telescope, is a powerful alternative at optimum sensitivity.

We integrated a generic statistical hypothesis testing methodology, and designed powerful point-source detection algorithms. The first study published using this approach led to the obtention of 40 hours of KPI obsevations during the first cycle of JWST. Other attempts to improve the contrast detection limits were successful, but remained inherently limited. To develop a true high-contrast solution, with an optical prescription that suppresses starlight required finding the kernel of a non-linear problem.

We came up with a concept called the kernel-nuller and successfully demonstrated that our baseline design for a four-beam interferometer will be a powerful tool at the focus of VLTI. Using advanced integrated optics, we built an actual device that we tested in the laboratory to confirm its self-calibrating properties, and its resilience to aberrations.

These experiments using integrated optics solutions were also the opportunity to realize the huge, still mostly untapped potential for astronomy of technology driven by telecommunication: our device offers the means to actively control starlight with no moving parts, which is a game changer. This makes it possible to revolutionize the way instruments for astronomy are built, for imaging and interferometry, as well as for spectroscopy; for ground-based as well as for space-borne observatories.
HST image of Gl494 fixed for kernel-phase analysis
The kernel-nuller being aligned with tke KERNEL test-bench
Experimental validation of the kernel-nuller in the presence of turbulence
Kernel-apodizer and high-contrast image obtained at the Subaru Telescope with the ERC KERNEL K-CAM
Laboratory validation of the HEIMDALLR fringe-tracker for VLTI
Overview of the ERC KERNEL test-bench