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.
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.
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.
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.
Combined with adaptive optics (AO), a kernel-phase data analysis makes it possible to further improve our understanding of astronomical images, and bypass the generally accepted limit of angular resolution, imposed by the theory of diffraction. This is sometimes refered to as “super-resolution”.
Astronomy requires large telescopes to improve the sensitivity and the angular resolution of its observations. Of these qualities, angular resolution is the most difficult to maintain in the optical and near-infrared, since the atmosphere reduces it to that of a 10 cm aperture, regardless of the telescope size. On the one-hand, Adaptive Optics (AO) actively compensates for this effect but the improvement is often partial only. On the other hand, interferometric techniques (most notably sparse aperture masking interferometry) passively allow the extraction of self-calibrating observables, that boost the angular resolution, but severely affect the sensitivity of observations.
The KERNEL project will enable every optical telescope equiped with AO to reach its ultimate angular resolution potential at full sensitivity, using a Fourier-phase framework, with applications ranging from the reinterpretation of archival science data to the development of wavefront control strategies for the giant segmented aperture of large telescopes like the space-borne JWST, or the upcoming generation of ground based extremely large telescope (ELTs).
KERNEL will achieve this objectives thanks to:
a streamlined general use and highly accessible data reduction process relying on a powerful pipeline
a coordinated effort to revisit existing archival ground-based AO (or assimilated) data sets to produce new, exclusive science
a ruggedized Fourier-phase framework that will open new use cases of currently less favorable datasets
the further development of concepts with applications to high-contrast imaging, wide-field imaging and the caracterization of complex sources
the development of prescriptions for powerful wavefront control strategies on existing and future facilities
That will require the combined development of:
an upgraded Fourier-phase extraction/processing software for archival, simulated and laboratory data
an experimental setup that will validate the concepts, strategies and prescriptions developed over the course of the project
the deployment of on-sky experiments on world class telescopes
The consequences of this project will have a major impact on the design and scientific exploitation of future high angular resolution instrumentation on the existing generation of 8-10 meter class telescopes as well as on the upcoming generation of 30-40 meter giants, championned by Europe and its E-ELT.
Astronomy and Instrumentation by Frantz Martinache
Because of the exquisite level of wavefront control they enable, extreme adaptive optics (XAO)-fed instruments like 
