All posts by Frantz

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.

Postdoctoral position in Astronomical Instrumentation for the KERNEL project

Schematic representation of an early design of the KERNEL test-bench


The KERNEL project, hosted by Observatoire de la Cote d’Azur (OCA) invites applications for a postdoctoral research position in the field of high-angular resolution astronomy starting no later than February 1, 2019. This position is funded by the European Research Council (ERC – CoG – grand agreement #683029) under the European Union’s Horizon 2020 research and innovation program.

The KERNEL project

KERNEL aims at enabling every optical and infrared astronomical facility to reach its ultimate angular resolution potential, often pushing beyond the formal diffraction limit, while preserving the full sensitivity. By looking at astronomical data as the result of an interferometric process, the KERNEL framework brings much needed robustness to high-performance observing techniques, required for instance for the direct detection of extrasolar planets.

The mission

The KERNEL framework offers a wide range applications that go from the post-processing of available archival data to high-performance focal plane metrology, partly coupled with high-contrast imaging. In order to develop and prototype the next generation of high-performance instruments and metrology monitoring tools for ground based telescopes and interferometers, the completion of the KERNEL project includes the construction of a general purpose test-bench, with elements that have already been successfully deployed for on-sky applications. The postdoc responsibility will be to oversee the completion of this KERNEL test-bench.

The test-bench primarily relies on a high-order segmented deformable mirror used to modulate the phase across a diffractive aperture and a high-cadence low-readout near-infrared camera, simultaneously in up to four complementary spectral bandpasses.

The multi-band aspect of the bench expands on the capability already offered by the KERNEL framework:

  • it extends the range of tolerated input instrumental phase, with applications such fringe tracking for long baseline interferometry and adaptive optics for large telescopes.
  • it provides further calibration capability, allowing for the acquisition of spectral differential kernel-phases

In addition, with its simple but agile high-contrast mode, the bench will also make it possible to experimentally validate observing strategies devised in the context of the project that bring robustness to aberrations to high-contrast direct detection.

How to apply

A Ph.D. in astronomy, physics, or a closely related field is mandatory. We are interested in individuals with several years of post-PhD research experience in the development and the scientific exploitation of instrumentation in the field of high angular resolution astronomy that include active wavefront compensation either in the laboratory or at the telescope. The candidate should be willing to collaborate with and assist graduate students that will use the KERNEL bench for their research projects. The candidate will also be encouraged to take advantage of the experimental setup and the KERNEL project members expertise to pursue his/her own research interests.

The candidate must also possess a strong background in the modeling, reduction and interpretation of diffraction dominated data (interferograms and/or AO-corrected images). Experience with the Python and/or the C programming language is highly desirable.

The initial appointment will be for two years, with possible extensions up to four years. The successful candidate will be hosted by the Lagrange Laboratory, with a lab located on the campus of Valrose, downtown the beautiful city of Nice, France.

To apply, please send a copy of your curriculum vitae, and a summary of your research interests. Also arrange for three reference letters to be sent to Frantz Martinache ( For full consideration, applications should be received before September 15, 2018, although applications will be reviewed up until the position is filled.

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!

K-CAM: the KERNEL camera, is installed at the focus of SCExAO!

The Subaru Telescope at sunset

Toward the end of June 2018, the C-RED-1 camera built by First Light Imaging for the KERNEL project was installed on the Nasmyth IR focus of the Subaru Telescope and coupled with the SCExAO instrument.

Custom frame designed by Romain Laugier, KERNEL project PhD student and built by the OCA’s S2M service, to couple the C-RED-1 camera to the IR side port of SCExAO.
Frantz Martinache (KERNEL PI) reconnecting the KERNEL project C-RED-1 camera after installation of the camera on the IR side port of SCExAO.
Action shot of KERNEL PhD student Romain Laugier, fixing broken connections to the pulse-tube cooler of KCAM.
K-CAM: The KERNEL project C-RED-1 camera, is finally ready to observe on the IR side port of SCExAO.

Although it took more work than anticipated, the camera was successfully integrated to the SCExAO instrument both optically and in software, now using a VisioLink F4 frame grabber sold by EDT. The optics inside SCExAO make it possible to send light in focus to this camera and the images produced by the camera are written to shared memory so as to be integrated with the rest of the real time SCExAO software environment.

In its default full frame mode, the C-RED-1 makes it possible to acquire frames at 3.5 kHz. In its smallest window mode, the camera can run a little over 71 kHz. The high sensitivity of the camera, coupled with the high frame rate, are real game changers in the realm of high-contrast imaging and really make it possible to envision driving a deformable mirror directly from the focal plane. At these speeds, speckles don’t stand a chance!

The camera was partly commissioned on-sky during unfortunately rather poor observing conditions… but engineering observations are planned for October 2018 so this camera will get its chance to shine!

The software running the “K-cam” camera is maintained on Frantz’s github page.

MEDITES: Parcours Gaia

Pour la troisième année, j’interviens dans le programme MEDITES dans un parcours pédagogique qui présente la recherche en astrophysique à des élèves du secondaire. Cette année, le parcours est focalisé sur les méthodes observationnelles en astronomie: astrométrie, photométrie et spectroscopie, et parlent en particulier de la mission Gaia. Les supports des présentations que j’ai préparés pour l’astrométrie et la photométrie sont disponibles ici.

La parallaxe: trouver notre place dans l’univers

Slide extrait d’une présentation sur l’astrométrie et la mesure de la parallaxe faite dans le cadre du programme MEDITES.

Photométrie et exoplanètes

Slide extrait d’une présentation sur la photométrie et les propriétés physiques des planètes extrasolaires faite dans le cadre du programme MEDITES.

Pour ce parcours, j’ai mis au point et construit une expérience constituée d’un simulateur motorisé de couple étoile-planète et d’une mesure photométrique temps réel par une photodiode connectée à une carte Arduino. J’ai également développé un petit programme d’acquisition des données collectées par la carte Arduino qui permet de faire du traitement a posteriori des données photométriques. Un tutoriel décrivant la fabrication et l’utilisation de cette expérience sera publié cette année par le site du service éducatif de l’Observatoire de la Côte d’Azur.

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.

December Stromlo KERNEL seminar

Presentation slides of a seminar given at Mt. Stromlo Observatory, in Canberra, Australia.

KERNEL: crack open the walnut
Slide extracted from a presentation introducing the the principles behind kernel-phase and KERNEL project. Click on image to download or access the presentation file!

Parallactic effect while driving


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Wherever I go, the Moon is following me!!

It is an experience surprisingly shared by many: Have you ever felt like the Moon was following you as you drive along a straight road at night? Are we really all that special or is there a simple explanation to this very common perception?

If you carefully look at the following animation, you will notice that, as our character drives along the road, things in the picture do not apparently move at the same speed. Right by the car, the roadside seems to fly by, while the trees, a little further away do seem to move a little slower. The far away mountains also drift, albeit very slowly.

Consider this, the distance that separates the road from the mountains is only of a few dozens of kilometers and they already barely seem to move as one drives by. With this in mind, are you really that surprised that the Moon (400,000 km away) and the stars (light years away) do seem immobile?

Of course, if you wait long enough, because of the Earth’s spin, the postion of the Moon and the stars will begin to drift. But let’s assume that for a few minutes, they don’t. As our character drives along the road, and covers some distance, by looking at how much the mountains have drifted in relation with the “fixed” sky (measuring an angle), he should be able to tell how far the mountain range is from the road.

In astronomy, this principle is used to figure out the distance to the most nearby stars.

Tackling the Low Wind Effect

Because of the exquisite level of wavefront control they enable, extreme adaptive optics (XAO)-fed instruments like SPHERE at VLT or SCExAO at the Subaru Telescope, have led to discovering new subtle effects that were previously invisible, as images were still dominated by the lesser correction. One such recently discovered effect is the “low wind effect” (LWE), so called because its impact is particularly obvious when the low altitude wind speed decreases below a threshold speed: whereas such observing conditions should in theory be ideal for the instrument, the image quality decreases so much that the instrument cannot be used productively.

This effect seems to be due to subtle interactions of the airflow and the structure bearing the secondary mirror of the telescope: when the wind speed drops, temperature gradients occur over the surface of the primary, and result in strong aberrations and even discontinuities in the wavefront that are very difficult to diagnose by conventional wavefront sensors. Given that the signature of the effect is particularly strong on the image (the resulting PSF was even nicknamed “Mickey Mouse”, since it sometimes bears strong sidelobes reminiscent of the famous mouse’s ears), the image seems like the right place to diagnose this effect: an ideal job for the phase transfer model at the heart of the KERNEL project!

Given prior experience with focal plane based wavefront sensing (reported in a publication available here) with the SCExAO instrument, the KERNEL team is investigating the relevance of the asymmetric pupil Fourier wavefront sensing (APF-WFS) technique, and its ability to account for the peculiar aberrations introduced by this LWE. The video below features a proof of concept: it shows that, at least in the context of a simulation, the technique can indeed be used in a closed-loop system, to bring these aberrations down! This adaptation of the APF-WFS to deal with the low wind effect will be the object of experimental work at the Subaru Telescope, over the next couple of weeks.