Mid-IR AGPMs for ELT applications

The mid-infrared region is well suited for exoplanet detection thanks to the reduced contrast between the planet and its host star with respect to the visible and near-infrared wavelength regimes. This contrast may be further improved with Vector Vortex Coronagraphs (VVCs), which allow us to cancel the starlight. One flavour of the VVC is the AGPM (Annular Groove Phase Mask), which adds the interesting properties of subwavelength gratings (achromaticity, robustness) to the already known properties of the VVC. In this paper, we present the optimized designs, as well as the expected performances of mid-IR AGPMs etched onto synthetic diamond substrates, which are considered for the E-ELT/METIS instrument.Comment: 8 pages, 5 figures, Proc. of SPIE Vol. 9147 (2014

The VORTEX project: first results and perspectives

(abridged) Vortex coronagraphs are among the most promising solutions to perform high contrast imaging at small angular separations. They feature a very small inner working angle, a clear 360 degree discovery space, have demonstrated very high contrast capabilities, are easy to implement on high-contrast imaging instruments, and have already been extensively tested on the sky. Since 2005, we have been designing, developing and testing an implementation of the charge-2 vector vortex phase mask based on concentric subwavelength gratings, referred to as the Annular Groove Phase Mask (AGPM). Science-grade mid-infrared AGPMs were produced in 2012 for the first time, using plasma etching on synthetic diamond substrates. They have been validated on a coronagraphic test bench, showing broadband peak rejection up to 500:1 in the L band, which translates into a raw contrast of about $6\times 10^{-5}$ at $2 \lambda/D$. Three of them have now been installed on world-leading diffraction-limited infrared cameras (VLT/NACO, VLT/VISIR and LBT/LMIRCam). During the science verification observations with our L-band AGPM on NACO, we observed the beta Pictoris system and obtained unprecedented sensitivity limits to planetary companions down to the diffraction limit ($0.1''$). More recently, we obtained new images of the HR 8799 system at L band during the AGPM first light on LMIRCam. After reviewing these first results obtained with mid-infrared AGPMs, we will discuss the short- and mid-term goals of the on-going VORTEX project, which aims to improve the performance of our vortex phase masks for future applications on second-generation high-contrast imagers and on future extremely large telescopes (ELTs).Comment: To appear in SPIE proceedings vol. 914

Three years of harvest with the vector vortex coronagraph in the thermal infrared

For several years, we have been developing vortex phase masks based on sub-wavelength gratings, known as Annular Groove Phase Masks. Etched onto diamond substrates, these AGPMs are currently designed to be used in the thermal infrared (ranging from 3 to 13 {\mu}m). Our AGPMs were first installed on VLT/NACO and VLT/VISIR in 2012, followed by LBT/LMIRCam in 2013 and Keck/NIRC2 in 2015. In this paper, we review the development, commissioning, on-sky performance, and early scientific results of these new coronagraphic modes and report on the lessons learned. We conclude with perspectives for future developments and applications.Comment: To appear in SPIE proceedings vol. 990

Direct exoplanet imaging with small-angle Vortex coronagraphs

Vortex coronagraphs are among the most promising solutions to perform high contrast imaging at small angular separations from bright stars. They enhance the dynamic range at very small inner working angle (down to the diffraction limit of the telescope) and provide a clear 360 degree discovery space for high-contrast direct imaging of exoplanets. In this talk, we will report on the first scientific results obtained with Vortex coronagraphs installed on 10-m class telescopes (i.e., the VLT and the LBT) and on the recent installation of one Vortex at Keck. We will describe the in-lab and on-sky performance of the Vortex, and describe the lessons learned after a few years of operation. Finally, we will discuss the prospects of our vortices for future extremely large telescopes and space missions.VORTE

Development and exploitation of an infrared coronagraphic test bench for vortex phase mask performance assessment

Coronagraphy is a high-contrast imaging technique aiming to reduce the blinding glare of a star in order to detect a potential companion in its close environment. Vortex phase mask coronagraphy is widely recognized as one of the most promising approaches. This thesis is dedicated to the performance assessment of the vortex coronagraph. For this purpose, it was crucial to have our own dedicated facility at the University of Liège. In the first part of this thesis we describe how was built the Vortex Optical Demonstrator for Coronagraphic Applications (VODCA) to optically characterize infrared phase masks, in particular vortex masks. The layout and salient features of VODCA are presented, as well as its operations, and its limitations in terms of optical quality. The bench is then used to assess the performance of L- (3575-4125 nm) and M-band (4600 to 5000 nm) annular groove phase masks (AGPM) manufactured by our team. We discuss the results obtained with other facilities and put them in perspective with the new measurements on VODCA. We demonstrate the highest rejection ratio ever measured for an AGPM at L-band: 3.2x10^3 in a narrow band filter (3425-3525 nm) and 2.4x10^3 in a broad L band filter. We also describe the first results obtained at shorter wavelengths (H- and K-bands, from 1500 to 2400 nm). Vortex phase masks with higher topological charges VVC (Vector Vortex Coronagraph) are theoretically more resilient to aberrations and resolved stars, a key feature for next generation coronagraphs. We present here various designs for the SGVC4 (charge 4 Subwavelength Grating Vortex Coronagraph) and the performance of the first manufactured components. We finally investigate the influence of optical aberrations on the AGPM and SGVC4, and confirm the simulations results. The AGPM is sensitive to low order aberrations while the SGVC4 is significantly less affected by tip/tilt and defocus. By providing measurements close to the intrinsic limit of science-grade AGPMs and accurately describing their behavior, VODCA proves to be a step forward in terms of the evaluation of vortex phase masks performance

Post-coronagraphic PSF sharpening with the vortex coronagraph

Small-angle coronagraphs are key components for future instruments being developed for the ELTs. Because they deliver high contrast capabilities at small angular separations from the host star, they enable us to address by direct imaging one of the most important science case of the ELTs, the discovery and characterization of exoplanets and proto-planetary systems. Small-angle coronagraphs based on a focal-plane mask such as the vortex phase mask are however quite sensitive to low-order aberrations. They require precise wavefront control to avoid spurious starlight leakage that would decrease the contrast level and possibly mimic the signatures of off-axis companions. Here, we present a low-order wavefront sensor technique based on an iterative process using post-coronagraphic focal images with a vortex coronagraph. Our approach is similar to a PSF sharpening technique, which is sometimes used in classical imaging to reduce non-common path aberrations (NCPA), but applied to post-coronagraphic images with the objective of minimizing the residual light of the on-axis source. We describe the laboratory implementation of this technique on the Vodca coronagraphic bench at the University of Liège, and present experimental results validating our method. Finally, we also present a weak phase solution with the vortex coronagraph and discuss its practical implementation.VORTE

PSF sharpening & post focal sensing for the VORTEX coronagraph

Small angle coronagaphy, such as the vortex phase mask, is a key scientific driver for the coming generation of VLT-like instrument and for the future ELTs. It is appealing for the full scientific exploitation of ground-based telescopes for the detection and characterization of exoplanets and circumstellar discs. One key element to small angle coronagraphy is a precise low-order wavefront control to avoid any leak decreasing the contrast level and even possibly mimicking off-axis signal. In this contribution, we present our early on-going effort in using post- coronagraphic focal images to perform wavefront control with a vortex coron- agraph. First, I will present an empirical approach similar to PSF sharpening techniques, sometimes used in classical imaging to reduce NCPA aberrations, to post-coronagraphic images. Then, I will discuss an alternative based on the general weak-phase solution to retrieve the complex Efield in the Lyot plane using three images, and how we can use this to control the phase in the pupil plane. Finally, after presenting those two approaches and first simulations, I will discuss the possible implementation of such techniques on our bench, the vortex optical demonstrator for coronagraphic applications (VODCA)

L- and M-band annular groove phase mask in lab performance assessment on the vortex optical demonstrator for coronagraphic applications

Coronagraphy is a high-contrast imaging technique that aims to reduce the blinding glare of a star to detect a potential companion in its close environment. Vortex phase mask coronagraphy is widely recognized as one of the most promising approaches. The vortex optical demonstrator for coronagraphic application (VODCA) is a test bench currently developed at the University of Liège. Its main goal is to optically characterize infrared phase masks, in particular vortex masks. We detail the layout and salient features of VODCA and present the performance of the latest L-band (3575 to 4125 nm) and M-band (4600 to 5000 nm) annular groove phase masks (AGPMs) manufactured by our team. We obtain the highest rejection ratio ever measured for an AGPM at L-band: 3.2 × 10[SUP]3[/SUP] in a narrowband filter (3425 to 3525 nm) and 2.4 × 10[SUP]3[/SUP] in a broad L-band filter. By providing measurements close to the intrinsic limit of science-grade AGPMs, VODCA proves to be a step forward in terms of the evaluation of vortex phase masks performance. VORTE

The W. M. Keck Observatory infrared vortex coronagraph and a first image of HIP79124 B

An optical vortex coronagraph has been implemented within the NIRC2 camera on the Keck II telescope and used to carry out on-sky tests and observations. The development of this new L'-band observational mode is described, and an initial demonstration of the new capability is presented: a resolved image of the low-mass companion to HIP79124, which had previously been detected by means of interferometry. With HIP79124 B at a projected separation of 186.5 mas, both the small inner working angle of the vortex coronagraph and the related imaging improvements were crucial in imaging this close companion directly. Due to higher Strehl ratios and more relaxed contrasts in L' band versus H band, this new coronagraphic capability will enable high-contrast small-angle observations of nearby young exoplanets and disks on a par with those of shorter-wavelength extreme adaptive optics coronagraphs.VORTE

Characterization of the inner disk around HD 141569 A from Keck/NIRC2 L-band vortex coronagraphy

HD 141569 A is a pre-main sequence B9.5 Ve star surrounded by a prominent and complex circumstellar disk, likely still in a transition stage from protoplanetary to debris disk phase. Here, we present a new image of the third inner disk component of HD 141569 A made in the L' band (3.8 micron) during the commissioning of the vector vortex coronagraph recently installed in the near-infrared imager and spectrograph NIRC2 behind the W.M. Keck Observatory Keck II adaptive optics system. We used reference point spread function subtraction, which reveals the innermost disk component from the inner working distance of $\simeq 23$ AU and up to $\simeq 70$ AU. The spatial scale of our detection roughly corresponds to the optical and near-infrared scattered light, thermal Q, N and 8.6 micron PAH emission reported earlier. We also see an outward progression in dust location from the L'-band to the H-band (VLT/SPHERE image) to the visible (HST/STIS image), likely indicative of dust blowout. The warm disk component is nested deep inside the two outer belts imaged by HST NICMOS in 1999 (respectively at 406 and 245 AU). We fit our new L'-band image and spectral energy distribution of HD 141569 A with the radiative transfer code MCFOST. Our best-fit models favor pure olivine grains, and are consistent with the composition of the outer belts. While our image shows a putative very-faint point-like clump or source embedded in the inner disk, we did not detect any true companion within the gap between the inner disk and the first outer ring, at a sensitivity of a few Jupiter masses.VORTE