Marc Ferrari

COMPASS: an efficient, scalable and versatile numerical platform for the development of ELT AO systems

The main objective of the COMPASS project is to provide a full scale end-to-end AO development platform, able to address the E-ELT scale and designed as a free, open source numerical tool with a long term maintenance plan. The development of this platform is based on a full integration of software with hardware and relies on an optimized implementation on heterogeneous hardware using GPUs as accelerators. In this paper, we present the overall platform, the various work packages of this project, the milestones to be reached, the results already obtained and the first output of the ongoing collaborations.

Lab results of the circular phase mask concepts for high-contrast imaging of exoplanets

Circular phase mask concepts represent promising options for high contrast imaging and spectroscopy of exo-planets. Depending on their design, they can either work as a diffraction suppression system or as a focal plane wavefront sensor. While the apodized Roddier coronagraph uses a π-phase mask to obtain complete suppression of the star image in monochromatic light, the Zernike sensor uses a π/2-phase mask to measure the residual aberrations in the focal plane by encoding them into intensity variations in the relayed pupil. Implementations of the Zernike sensor can be considered in exoplanet imagers such as VLT-SPHERE, Gemini planet imager, Palomar-P1640 or Subaru-SCExAO to enlarge their capabilities. However, such concepts have not been validated experimentally up to now. Our goal is to perform lab demonstration of this concept on our visible coronagraph testbed at LAM and to propose an upgrade design for SPHERE. In this communication, we report on results of lab measurements of the Zernike sensor and determine its sensitivity to small wavefront errors.

In-flight aberrations corrections for large space telescopes using active optics

The need for both high quality images and light structures is a constant concern in the conception of space telescopes. The goal here is to determine how an active optics system could be embarked on a satellite in order to correct the wave front deformations of the optical train. The optical aberrations appearing in a space environment are due to mirrors deformations, with three main origins: the thermal variations, the weightlessness in space with respect to the Assemblage, Integration and Testing (AIT) conditions on ground and the use of large weightlighted primary mirrors. We are developing a model of deformable mirror as minimalist as possible, especially in term of number of actuators, which is able to correct the first Zernike polynomials in the specified range of amplitude and precision. Flight constraints as weight, volume and power consumption have to be considered. Firstly, such a system is designed according to the equations from the elasticity theory: we determine the geometrical and mechanical characteristics of the mirror, the location of the forces to be applied and the way to apply them. The concept is validated with a Finite Element Analysis (FEA), allowing optimizing the system by taking into account parameters absent from the theory. At the end of the program the mirror will be realized and characterized in a representative optical configuration.

Toric mirrors and active optics: degenerated configuration for spherical monomode deformable mirrors

Providing toric mirrors with Active Optics techniques will allow generating aspheric surfaces which optical quality avoid high spatial frequencies errors. In order to demonstrate the feasibility of this technique, a link has been established between analytical calculations, finite element modelling and experimental validation. A particular configuration of a flat mono-mode deformable mirror (MDM), called degenerated configuration, has been analytically calculated, showing how to generate a third order astigmatism aberration (Astm 3) by active deformation. This mirror has been manufactured and tested. A finite element model has been produced in order to correlate simulations with experiments. The deformed optical surface is projected on a Zernike polynomial base, indicating that Astm 3 mode is, within a very high precision, the only aberration generated on the optical surface. Another spherical concave MDM has been modelled as a VLT-SPHERE toric mirror of diameter 133mm, to demonstrate the feasibility of toric surfaces from active optics deformation of a spherical shell. Projection on Zernike base shows that the simulated deformed surface is a combination of a sphere and a quasi pure Astm 3 mode, corresponding to a toroidal surface. Other terms generated, like Astm 5, could benefit of a fine adjustment from the geometry of the substrate.

Simultaneous phase and amplitude retrieval with COFFEE: from theory to laboratory results

The final performance of current and future instruments dedicated to exoplanet detection and characterization (such as SPHERE on the VLT, GPI on Gemini North or future instruments on the ELTs) is limited by intensity residuals in the scientific image plane, which originate in uncorrected optical aberrations. After correction of the atmospheric turbulence, the main contribution to these residuals comes from the quasi-static aberrations introduced upstream of the coronagraph which create long-lived speckles in the detector plane that can easily be mistaken for a planet. In order to reach very high contrast such as the ones required to image earth-like planets, these aberrations needs to be compensated for. We have recently proposed a dedicated focal-plane wave-font sensor called COFFEE (for COronagraphic Focal-plane wave-Front Estimation for Exoplanet detection), which consists in an extension of conventional phase diversity to a coronagraphic system: aberrations both upstream and downstream of the coronagraph are estimated using two coronagraphic focal-plane images, recorded from the scientific camera itself, without any differential aberration. Such a system has been successfully validated on the SPHERE instrument, where COFFEEs estimation has been used to compensate for the phase aberration upstream of the coronagraph, leading to a contrast optimization in the whole focal plane area controlled by the AO loop. If compensating for phase aberrations only was acceptable to reach levels of contrast of 10U001000006, it will no longer be the case for instruments that aim at imaging earth-like planets. Such targets, which would be the ones of a planet-finder instrument integrated on an ELT, require a level of contrast better than 10U001000009. To reach this level, neglecting amplitude aberrations (inhomogeneous intensity in the pupil, Fresnel propagation effect) is no longer possible. In this communication, we present an extension of COFFEE able to perform a simultaneous estimation of both phase and amplitude aberration from three focal plane images. After a theoretical presentation, we present a study of its performances. Notably, we analyze the contrast that can be achieved in a compensation process when this estimation method is combined with our non-linear dark hole method, demonstrating that the nanometric precision estimation that can be achieved with COFFEE allow one to reach very high contrast levels. It is worth mentioning that both estimation (COFFEE) and compensation (the nonlinear dark hole) methods are model based, and thus easily adaptable to a broad class of coronagraphic device. Lastly, we validate our complex field estimator on the LAM (Laboratoire dAstrophysique de Marseille) XAO test bench, described in this communication. We introduce calibrated phase and amplitude aberration in the entrance pupil plane. Then, we demonstrate the ability of our extended version of COFFEE to estimate both phase and amplitude aberration from three coronagraphic focal plane images that differs from a known aberration.

Modeling highly aspherical optical surfaces using a new polynomial formalism into Zemax

We developed a new mathematical formalism to model highly aspherical optical surfaces opening the possibility to explore innovative optical designs. This formalism is based on Bernstein polynomials allowing to describe from low to high order deformations of the optical surface. It has been implemented into Zemax making use of the User-Defined Surface (UDS-DLL) Zemax capability. In this case, the mathematical definition of the surface is imported into Zemax then allowing to apply classical optimization and analysis functionalities. This paper presents the UDS-DLL tool based on Bernstein polynomials together with an initial optical analysis performed to evaluate the gain obtained in using such a new formalism.

Stress polishing of toric mirrors for the VLT-SPHERE common path

The very challenging goal of the Vlt-Sphere instrument, Exoplanet direct detection and characterisation, requires high contrast imaging and extreme adaptive optics.1 In order not to limit the overall imaging performances of the instrument, all the optics in the common path optical train need to be of the better quality over each range of spatial frequencies. Three Toric mirrors are placed in the common path to relay the beam to the deformable mirror and to the instruments. This paper details the Stress polishing principle developed at Laboratoire dAstrophysique de Marseille (Lam) to get the better optical quality on the toric surfaces, using a spherical polishing with full size tools. The elasticity theory giving the optimisation of the blank geometry to be warped during the stress polishing process is detailed from analytical calculation to finite element analysis. The use of an angular thickness distribution allows us to reach the better optical quality of the deformation by canceling higher order terms. We also present the polishing results for the 366mm diameter Toric Mirror manufacturing.

Active Optics Theory : Compensation of aberration using the single actuator principle

Active Optics allows the possibility of using the generation of complex variable optical surfaces to keep the optical layout of future instruments relatively simple, something which could be of great interest to future telescopes such as E-Elt, Tmt. The aim of this article is to describe the development of the single actuator - single mode principle that makes it possible to generate single optical modes on a circular mirror using a single actuator at a specific location. We show the progress from design analysis (elasticity theory, finite element analysis etc) through to experimental validation for Variable Curvature Mirrors and Variable Astigmatism Mirrors. Current and future applications of these active mirrors are discussed in the framework of the EAGLE instrument for E-Elt and we present plans for further development of the technique.

Experimental validation of the high-order coronagraphic phase diversity (COFFEE) on the SPHERE system

The final performance of current and future instruments dedicated to exoplanet detection and characterization (such as SPHERE on the VLT, GPI on Gemini North or future instruments on the E-ELT) is limited by intensity residuals in the scientific image plane, which originate in uncorrected optical aberrations. After correction of the atmospheric turbulence, the main contribution to these residuals comes from the quasi-static aberrations introduced upstream of the coronagraph. In order to measure and compensate for these aberrations, we propose a dedicated focal-plane sensor called COFFEE (for COronagraphic Focal-plane wave-Front Estimation for Exoplanet detection), which consists in an extension of conventional phase diversity to a coronagraphic system: aberrations both upstream and downstream of the coronagraph are estimated using two coronagraphic focal-plane images, recorded from the scientific camera itself, without any differential aberration. This communication gathers COFFEE’s improvements: the phase estimation is performed on a pixel-wise map coupled with a dedicated regularization metric. This allows COFFEE to estimate very high order aberrations, making possible to estimate and compensate for quasi-static aberrations with nanometric precision, leading to an optimization of the contrast on the scientific detector in the whole AO corrected area. Besides, COFFEE has been modified so that it can be used with any coronagraphic focal plane mask. Lastly, we use COFFEE to measure and correct the wavefront on the SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research) instrument during its integration phase: COFFEE’s estimation is used to compensate for the quasi-static aberrations upstream of the coronagraph, leading to a contrast improvement on the scientific camera.

Active Optics techniques and complex instrumentation for future ELTs

In the frame of the future European Extremely Large Telescope, the Laboratoire d’Astrophysique de Marseille is developing manufacturing methods and complex instrumentation for astronomy, based on the active bending of mirrors.