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Nulling interferometry is still a promising method to characterize spectra of exoplanets. One of the main issues is to cophase at a nanometric level each arm despite satellite disturbances. The bench PERSEE aims to prove the feasibility of that technique for spaceborne missions. After a short description of PERSEE, we will first present the results obtained in a simplified configuration: we have cophased down to 0.22 nm rms in optical path difference (OPD) and 60 mas rms in tip/tilt, and have obtained a monochromatic null of 3E-5 stabilized at 3E-6. The goal of 1 nm with additional typical satellite disturbances requires the use of an optimal control law; that is why we elaborated a dedicated Kalman filter. Simulations and experiments show a good rejection of disturbances. Performance of the bench should be enhanced by using a Kalman control law, and we should be able to reach the desired nanometric stability. Following, we will present the first results of the final polychromatic configuration, which includes an achromatic phase shifter, perturbators and optical delay lines. As a conclusion, we give the first more general lessons we have already learned from this experiment, both at system and component levels for a future space mission.
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We present here a new observational technique, Phase Closure Nulling (PCN), which has the potential to obtain very high contrast detection and spectroscopy of faint companions to bright stars. PCN consists in measuring closure phases of fully resolved objects with a baseline triplet where one of the baselines crosses a null of the object visibility function. For scenes dominated by the presence of a stellar disk, the correlated flux of the star around nulls is essentially canceled out, and in these regions the signature of fainter, unresolved, scene object(s) dominates the imaginary part of the visibility in particular the closure phase. We present here the basics of the PCN method, the initial proof-of-concept observation, the envisioned science cases and report about the first observing campaign made on VLTI/AMBER and CHARA/MIRC using this technique.
Stabilizing a nulling interferometer at a nanometric level is the key issue to obtain deep null depths. The PERSEE breadboard has been designed to study and optimize the operation of a cophased nulling bench in the most realistic disturbing environment of a space mission. This presentation focuses on the current results of the PERSEE bench. In terms of metrology, we cophased at 0.33 nm rms for the piston and 80 mas rms for the tip/tilt (0.14% of the Airy disk). A Linear Quadratic Gaussian (LQG) control coupled with an unsupervised vibration identification allows us to maintain that level of correction, even with characteristic vibrations of nulling interferometry space missions. These performances, with an accurate design and alignment of the bench, currently lead to a polychromatic unpolarised null depth of 8.9E-6 stabilized at 3E-7 on the [1.65-2.45] mum spectral band (37% bandwidth).
The behavior of an adaptive optics (AO) system for ground-based high contrast imaging (HCI) dictates the achievable contrast of the instrument. In conditions where the coherence time of the atmosphere is short compared to the speed of the AO system, the servo-lag error becomes the dominate error term of the AO system. While the AO system measures the wavefront error and subsequently applies a correction (taking a total of 1 to 2 milli-seconds), the atmospheric turbulence above the telescope has changed. In addition to reducing the Strehl ratio, the servo-lag error causes a build-up of speckles along the direction of the dominant wind vector in the coronagraphic image, severely limiting the contrast at small angular separations. One strategy to mitigate this problem is to predict the evolution of the turbulence over the delay. Our predictive wavefront control algorithm minimizes the delay in a mean square sense and has been implemented on the Keck II AO bench. In this paper we report on the latest results of our algorithm and discuss updates to the algorithm itself. We explore how to tune various filter parameters on the basis of both daytime laboratory tests and on-sky tests. We show a reduction in residual-mean-square wavefront error for the predictor compare to the leaky integrator implemented on Keck. Finally, we present contrast improvements for both day time and on-sky tests. Using the L-band vortex coronagraph for Kecks NIRC2 instrument, we find a contrast gain of 2.03 at separation of 3~$lambda/D$ and up to 3 for larger separations (4-6~$lambda/D$).
In this paper, we present the infrared coronagraphic test bench of the University of Li`ege named VODCA (Vortex Optical Demonstrator for Coronagraphic Applications). The goal of the bench is to assess the performances of the Annular Groove Phase Masks (AGPMs) at near- to mid-infrared wavelengths. The AGPM is a subwavelength grating vortex coronagraph of charge two (SGVC2) made out of diamond. The bench is designed to be completely achromatic and will be composed of a super continuum laser source emitting in the near to mid-infrared, several parabolas, diaphragms and an infrared camera. This way, we will be able to test the different AGPMs in the M, L, K and H bands. Eventually, the bench will also allow the computation of the incident wavefront aberrations on the coronagraph. A reflective Lyot stop will send most of the stellar light to a second camera to perform low-order wavefront sensing. This second system coupled with a deformable mirror will allow the correction of the wavefront aberrations. We also aim to test other pre- and/or post-coronagraphic concepts such as optimal apodization.
Vortex fiber nulling is a method for spectroscopically characterizing exoplanets at small angular separations, $lesssimlambda/D$, from their host star. The starlight is suppressed by creating an optical vortex in the system point spread function, which prevents the stellar field from coupling into the fundamental mode of a single-mode optical fiber. Light from the planet, on the other hand, couples into the fiber and is routed to a spectrograph. Using a prototype vortex fiber nuller (VFN) designed for monochromatic light, we demonstrate coupling fractions of $6times10^{-5}$ and $>0.1$ for the star and planet, respectively.
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