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 μm). 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.
On March 2015 an L'-band vortex coronagraph based on an Annular Groove Phase Mask made up of a diamond sub-wavelength grating was installed on NIRC2 as a demonstration project. This vortex coronagraph operates in the L' band not only in order to take advantage from the favorable star/planet contrast ratio when observing beyond the K band, but also to exploit the fact that the Keck II Adaptive Optics (AO) system delivers nearly extreme adaptive optics image quality (Strehl ratios values near 90%) at 3.7μm. We describe the hardware installation of the vortex phase mask during a routine NIRC2 service mission. The success of the project depends on extensive software development which has allowed the achievement of exquisite real-time pointing control as well as further contrast improvements by using speckle nulling to mitigate the effect of static speckles. First light of the new coronagraphic mode was on June 2015 with already very good initial results. Subsequent commissioning nights were interlaced with science nights by members of the VORTEX team with their respective scientific programs. The new capability and excellent results so far have motivated the VORTEX team and the Keck Science Steering Committee (KSSC) to offer the new mode in shared risk mode for 2016B.
The exoplanet direct imagers Gemini/GPI and VLT/SPHERE are built around extreme adaptive optics (ExAO)
to correct the atmospheric turbulence and the aberrations associated with the optical surfaces. However, additional
strategies are necessary to correct the non-common path aberrations (NCPA) between the ExAO and
science paths that can limit the instrument contrast performance. To perform an adequate calibration, we have
developed ZELDA, a Zernike sensor to achieve NCPA measurements with nanometric accuracy. We report the
results of a new design analysis that maximizes the dynamic range, and from laboratory demonstrations on the
LAM high-contrast testbed and on VLT/SPHERE during its integration.
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.
Vortex coronagraphs are among the most promising solutions to perform high contrast imaging at small angular separations from bright stars. They feature a very small inner working angle (down to the diffraction limit of the telescope), 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 sub-wavelength 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 x 10-5 at 2λ=D. Three of them have now been installed on world-leading diffraction-limited infrared cameras, namely 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 imager and on future extremely large telescopes (ELTs). In particular, we will briefly describe our current efforts to improve the manufacturing of mid-infrared AGPMs, to push their operation to shorter wavelengths, and to provide deeper starlight extinction by creating new designs for higher topological charge vortices. Within the VORTEX project, we also plan to develop new image processing techniques tailored to coronagraphic images, and to study some pre- and post-coronagraphic concepts adapted to the vortex coronagraph in order to reduce scattered starlight in the final images.
In this paper, we present the infrared coronagraphic test bench of the University of Liège 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.