Category Archives: interferometry

KERNE publication: Kernel-nullers for all interferometers!

In this new study, led by PhD candidate Romain Laugier, our team has looked into what one needs to do in order to build a kernel-nuller, that is a high-contrast interferometric recombiner robust to residual piston excursions, for an array featuring an arbitrary number of telescopes.

The paper takes advantage of nice graphical representations of the inner workings of a nuller called “complex matrix plots” (but that we’ve nicknamed “Laugier-grams”) to find the properties a recombiner must exhibit in order to be robust to piston perturbations.

The same graphs were also featured a few months ago in a video recorded in French (first announced in this blog post) and later translated into English that explains why the 4-beam kernel-nuller of Martinache & Ireland (2018) works. Both videos were posted on Youtube:

Congratulations to Romain for another published paper to append to his upcoming PhD dissertation! The preprint version of the paper is available for download on the website: Romain’s PhD defense will take place in a little over a month on September 22, 2020.

KERNEL-Nuller: vidéo explicative

Il y maintenant bientôt deux ans, j’annonçais sur ce site l’acceptation d’un article publié avec mon collègue Mike Ireland présentant un mode d’observation interférométrique haut contraste robuste aux petites erreurs de correction par un suiveur de franges: le kernel-nuller.

Notre équipe à Nice est, en collaborant avec l’entreprise Bright Photonics en train de faire fabriquer un premier prototype de kernel-nuller, sous forme de composant d’optique intégrée. La situation sanitaire du COVID-19 complique un peu le calendrier de cette activité et de la thèse de doctorat qui y est liée, mais nous devrions pouvoir annoncer cette année, des résultats partiels d’une première intégration d’un tel composant sur un banc optique.

En attendant de voir ce composant en action, voici une vidéo mise en ligne il y a quelques jours, expliquant ce qui distingue le kernel-nuller du nuller interférométrique initialement imaginé par Ronald Bracewell à la fin des années 1970… et illustre comment le concept fonctionne!

Kernel-nulling paper accepted for publication by Astronomy & Astrophysics

It is my great pleasure to be able announce that the paper Mike Ireland (ANU) and I wrote, entitled “Kernel-nulling for a robust direct interferometric detection of extrasolar planets” has been accepted for publication by Astronomy & Astrophysics.

The paper introduces a baseline class of nulling-interferometers producing outputs that can be robustly calibrated. These new observable quantities exhibit properties that are similar to closure- and kernel-phase, while taking advantage of the use of a true nulling stage. The first version of our paper had been previously announced. The (updated) preprint of the paper is now available on arXiv.

It is fantastic to have this piece accepted: the quest for robust high-contrast solutions has been on my mind for a while… And now that we know that at least one solution exists, surely others must do too!

Conclusion slide from a presentation I gave, describing Kernel-nulling and the VIKiNG instrument concept it leads to for VLTI.

Habilitation à Diriger des Recherches

Dans le but de soutenir mon Habilitation à Diriger des Recherches (HDR), j’ai soumis à mes rapporteurs la première version de cette thèse, intitulée: “Repousser les limites de la diffraction pour l’astronomie à haute résolution angulaire”.

Page de garde de la HDR de Frantz

Deux versions de cette thèse sont téléchargeables ici au format PDF:

Si mes rapporteurs m’y autorisent, la soutenance de cette thèse de HDR devrait avoir lieu le jeudi 4 octobre 2018, à l’Observatoire de la Côte d’Azur, sur le site du Mont Gros.

Entrée de la Grande Coupole de l’Observatoire de la Côte d’Azur, sur la colline du Mont Gros, à Nice.

Pour les curieux, une page de Wikipedia explique ce qu’est la HDR et quel est le rôle de cette tradition qui ne semble exister qu’en Europe ainsi que dans quelques pays d’Afrique du Nord.

La thèse est évidemment écrite en LaTeX. Pour la mise en page, j’ai choisi d’utiliser la classe tufte-latex, inspirée par les publications de Edward Tufte. La lecture de son livre intitulé The Visual Display of Quantitative Information et de son essai intitulé Essay: The Cognitive Style of Powerpoint: Pitching Out Corrupts Within durant mon premier contrat de post-doctorat à l’Université de Cornell ont eu une forte influence sur ma méthode de communication scientifique: je les recommande tous les deux!

Kernel-nulling talk in Grenoble

These are the slides of a presentation given on March 8, 2018 at IPAG, where I present research activities related to the KERNEL project, in particular the most recent development concerning the idea of kernel-nulling interferometry.

Predicted contrast detection limits for the L-band VIKiNG instrument concept proposed by Martinache & Ireland.

You can access the presentation file directly here.

Kernel-nulling for a robust direct interferometric detection of extrasolar planets

A new paper posted on arxiv by Frantz Martinache & Mike Ireland.


Combining the resolving power of long-baseline interferometry with the high-dynamic range capability of nulling still remains the only technique that can directly sense the presence of structures in the innermost regions of extrasolar planetary systems. Ultimately, the performance of any nuller architecture is constrained by the partial resolution of the on-axis star whose light it attempts to cancel out, and the design of nullers focuses on increasing the order of the extinction to reduce the sensitivity to this effect. However from the ground, the effective performance of nulling is dominated by residual time-varying instrumental phase errors that keep the instrument off the null. This is similar to what happens with high-contrast imaging, and is what we aim to ameliorate. We introduce a modified nuller architecture that enables the extraction of information that is robust against piston excursions. Our method generalizes the concept of kernel, now applied to the outputs of the modified nuller so as to make them robust to second order pupil phase error. We present the general method to determine these kernel-outputs and highlight the benefits of this novel approach. We present the properties of VIKiNG: the VLTI Infrared Kernel NullinG, an instrument concept within the Hi-5 framework for the 4-UT VLTI infrastructure that takes advantage of the proposed architecture, to produce three self-calibrating nulled outputs. Stabilized by a fringe-tracker that would bring piston-excursions down to 50 nm, this instrument would be able to directly detect more than a dozen extrasolar planets so-far detected by radial velocity only, as well as many hot transiting planets and a significant number of very young exoplanets.

The theory of optical stellar interferometry

These videos are part of Section 2 of the on-line course “Eagle Eye Astronomy”, initially released on France Université Numérique. These have recently been re-uploaded on youtube and close-captioned, to help better understand the audio track despite the French accent.

Turn the constraint of diffraction into an advantage: the interferometric trick

Understanding interference fringes: an electromagnetic theory of light

Stars are not lasers: a better model for ordinary light sources

Self-coherence and spatial incoherence of astronomical sources

Coherence length

Coherence length and bandwitdh

A finite spectral bandwidth results in some spectral decorrelation of the electric field emanating from a source: even with a true point source, you will only observe the interference phenomenon (mutual coherence of the field) over a finite range of optical path difference (OPD) that is constrained by the coherence length.

Coherence length: Λ0 = λ²/Δλ

An purely monochromatic and phase-calibrated signal like the one coming out of a very good laser would exhibit an infinite coherence length, represented by the green curve plotted below.

Change the bandpass of the filter (here expressed as 1/R=Δλ/λ) to see its effect on the coherence length (the blue curve).

For a given bandpass (say 0.05 and 0.1), how many fringes can you make out in the fringe packet? How do the two quantities (bandpass, number of fringes) relate to each other?


This tool was developed as a part of the on-line course Eagle Eye Astronomy, hosted on the France Université Numérique website.


Interferometric arrays and spatial frequencies

This post features a pretty fun simulation tool that will allow you to experience “by hand” the relation between the geometry of an interferometric array (that is the location of the telescopes or apertures making up the array) and the corresponding so called “uv-coverage”, that shows all the spatial frequencies the interferometric array gives access to.

The left-hand side plot shows the arangement of apertures. The user can select from several pre-set configurations (Y-shaped, hexagonal-grid, non-redundant) and modify them by moving, adding or removing individual apertures.

Moving one aperture by hand and observing what simultaneously happens on the right-hand side display is particularly “enlightening”: for one, you develop a more intuitive of the relation between one aperture and the different interferometric baselines it is involved with. In addition, you can also observe that geometries laying on a regular grid pattern sometimes result in overlapping points in the uv-plane. The only pattern that strictly avoids this situation is the non-redundant geometry.

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Resulting uv-coverage

This tool was developed as a part of the on-line course Eagle Eye Astronomy, hosted on the France Université Numérique website.