C. Escolle

Improved stray light suppression performance for the solar orbiter/METIS inverted external occulter

The Solar Orbiter/METIS visible and UV coronagraph introduces the concept of occulter inversion in solar coronagraphy. Classical externally occulted coronagraphs usually have a disk in front of the telescope entrance pupil. According to the mission requirements, in order to reduce the amount of power entering the instrument and to limit the instrument dimensions, METIS is equipped with an inverted external occulter (IEO). The IEO consists of a circular aperture on the Solar Orbiter thermal shield that acts as coronagraph entrance pupil. A spherical mirror (M0), located ~800 mm behind the IEO, rejects back the disk-light through the IEO itself. A light-tight boom connects the IEO to the M0 through the thermal shield. In order to achieve high performance in stray light suppression, the IEO design needs optimization. Due to the novelty of the concept we can only use the heritage of past space-borne coronagraph occulters as a starting point to design a dedicated occulter optimization shape. A 1.5 years long, accurate test campaign has been carried out to evaluate the best optimization configuration for the IEO. Two prototypes were manufactured to take into account the impact of the boom geometry on the stray light suppression performance. Two optimization concepts were compared: the inverted cone (that derives from the conic optimization of classical occulting disks) and the serrated edge, of which several samples were manufactured, with different geometrical parameters, surface roughnesses and coatings. This work summarizes the activity we have been carrying on to define the flight specifications for the METIS occulter.

Comparative theoretical and experimental study of a Shack-Hartmann and a Phase Diversity SENSOR, for high-precision wavefront sensing dedicated to Space Active Optics

Earth-imaging or Universe Science satellites are always in need of higher spatial resolutions, in order to discern finer and finer details in images. This means that every new generation of satellites must have a larger main mirror than the previous one, because of the diffraction. Since it allows the use of larger mirrors, active optics is presently studied for the next generation of satellites. To measure the aberrations of such an active telescope, the Shack-Hartmann (SH), and the phase-diversity (PD) are the two wavefront sensors (WFS) considered preferentially because they are able to work with an extended source like the Earths surface, as well as point sources like stars. The RASCASSE project was commissioned by the French spatial agency (CNES) to study the SH and PD sensors for high-performance wavefront sensing. It involved ONERA and Thales Alenia Space (TAS), and LAM. Papers by TAS and LAM on the same project are available in this conference, too [1,2]. The purpose of our work at ONERA was to explore what the best performance both wavefront sensors can achieve in a space optics context. So we first performed a theoretical study in order to identify the main sources of errors and quantify them — then we validated those results experimentally. The outline of this paper follows this approach: we first discuss phase diversity theoretical results, then Shack-Hartmann’s, then experimental results — to finally conclude on each sensor’s performance, and compare their weak and strong points.

Preliminary occulter optimization for an innovative space coronagraph

The solar corona represents the physical link between the Sun and the Earth with its human life, because it stretches without discontinuity into the heliosphere and strongly affects its physical parameters. Observation of the solar corona is challenging because of the intrinsic weakness of the coronal radiation, which is 10 6 to 10 8 times fainter than the nearby bright photosphere. Intensive observation of the solar corona requires a coronagraph, that is, a telescope equipped with a system for occulting the light from the solar disk.1, 2 The Multi-Element Telescope for Imaging and Spectroscopy (METIS), selected to fly aboard the Solar Orbiter ESA/NASA mission,3 is a coronagraph/spectrometer designed to perform imaging and spectroscopy of the solar corona by means of an integrated instrument suite located on a single optical bench and sharing the same aperture on the satellite heat shield. Like every coronagraph, METIS requires highly effective stray light suppression. The history of coronagraphs4–6 teaches us that in the design process, particular attention must be given to optimizing the occulter. The METIS occulting system is of particular interest because of its innovative concept. The Solar Orbiter will observe the Sun from a unique point of view by reaching a perihelion of 0.28 astronomical units (AU) and orbiting up to 30 out of the ecliptic. To meet the strict thermal requirements of the Solar Orbiter, the METIS optical design has been optimized by moving the entrance pupil to the level of the external occulter on the Figure 1. 3D drawing of the METIS structure, with emphasis on the main elements of the occulting system. IEO: Inverted external occulter. M0, M1, M2: Mirrors. IO: Internal occulter.

Wave-front sensing for space active optics: Rascasse project

The payloads for Earth Observation and Universe Science are currently based on very stiff opto-mechanical structures with very tight tolerances. The introduction of active optics in such an instrument would relax the constraints on the thermo-mechanical architecture and on the mirrors polishing. A reduction of the global mass/cost of the telescope is therefore expected. Active optics is based on two key-components: the wave-front sensor and the wave-front corrector.

Adapting large lightweight primary mirror to space active optics capabilities

The next generation of space telescope will use large primary mirrors to achieve high angular resolution. Due to weight constrain, these large mirrors will have a very low mass per unit area. This ultra-light-weighting leads to deformations of the primary mirror optical surface due to gravity load difference between ground and space. Active Optics systems then become essential to maintain the quality of the output wavefront. The supporting structures and surface quality specifications of the mirror must be optimized regarding the active optics capabilities. The case of a two meters lightweight primary mirror will be presented.

Coating and surface finishing definition for the Solar Orbiter/METIS inverted external occulter

The METIS coronagraph aboard the Solar Orbiter mission will undergo extreme environmental conditions (e.g., a thermal excursion of about 350 degrees throughout the various mission phases), due to the peculiar spacecraft trajectory that will reach a perihelion of 0.28 AUs. METIS is characterized by an innovative design for the occultation system that allows to halve the thermal load inside the instrument while guaranteeing the stray light reduction that is required for a solar coronagraph. The Inverted External Occulter (IEO) concept revolutionizes the classical scheme, by exchanging the usual positions of the entrance aperture (that is now the outermost element of the instrument facing the Sun) with the actual occulter (that is a spherical mirror inside the coronagraph boom). The chosen material for the IEO manufacturing is Titanium, as a trade o_ between light weight, strength and low thermal expansion coefficient. A 2 years long test campaign has been run to define the IEO geometry, and its results are addressed in previous dedicated papers. This work describes the results of a further campaign aimed at defining the IEO surface and edge finishing, the support flange geometry and the Titanium coating. Various edge finishing were installed on a prototype of the instrument occulting system and their performance in stray light reduction were compared. The support flange geometry was designed in order to reduce the overall weight, to control the thermal load and to accentuate its stray light suppression performance. The coating is a particularly delicate issue. A black coating is necessary in order to assess the stray light issues, typically critical for visible coronagraphs. Black coating of Titanium is not a standard process, thus several space qualified black coatings were experimented on Titanium and characterized. The impact of the IEO coatings was evaluated, the reflectivity and the BRDFs were measured and are addressed in the paper.

The optimization of the inverted occulter of the solar orbiter/METIS coronagraph/spectrometer

The coronagraph/spectrometer METIS (Multi Element Telescope for Imaging and Spectroscopy), selected to fly aboard the Solar Orbiter ESA/NASA mission, is conceived to perform imaging (in visible, UV and EUV) and spectroscopy (in EUV) of the solar corona. It is an integrated instrument suite located on a single optical bench and sharing a unique aperture on the satellite heat shield. As every coronagraph, METIS is highly demanding in terms of stray light suppression. In order to meet the strict thermal requirements of Solar Orbiter, METIS optical design has been optimized by moving the entrance pupil at the level of the external occulter on the S/C thermal shield, thus reducing the size of the external aperture. The scheme is based on an inverted external-occulter (IEO). The IEO consists of a circular aperture on the Solar Orbiter thermal shield. A spherical mirror rejects back the disk-light through the IEO. The experience built on all the previous space coronagraphs forces designers to dedicate a particular attention to the occulter optimization. Two breadboards were manufactured to perform occulter optimization measurements: BOA (Breadboard of the Occulting Assembly) and ANACONDA (AN Alternative COnfiguration for the Occulting Native Design Assembly). A preliminary measurement campaign has been carried on at the Laboratoire d’Astrophysique de Marseille. In this paper we describe BOA and ANACONDA designs, the laboratory set-up and the preliminary results.

James Webb Space Telescope optical simulation testbed III: first experimental results with linear-control alignment

The James Webb Space Telescope (JWST) Optical Simulation Testbed (JOST) is a tabletop experiment designed to study wavefront sensing and control for a segmented space telescope, including both commissioning and maintenance activities. JOST is complementary to existing testbeds for JWST (e.g. the Ball Aerospace Testbed Telescope TBT) given its compact scale and flexibility, ease of use, and colocation at the JWST Science and Operations Center. The design of JOST reproduces the physics of JWST’s three-mirror anastigmat (TMA) using three custom aspheric lenses. It provides similar quality image as JWST (80% Strehl ratio) over a field equivalent to a NIRCam module, but at 633 nm. An Iris AO segmented mirror stands for the segmented primary mirror of JWST. Actuators allow us to control (1) the 18 segments of the segmented mirror in piston, tip, tilt and (2) the second lens, which stands for the secondary mirror, in tip, tilt and x, y, z positions. We present the full linear control alignment infrastructure developed for JOST, with an emphasis on multi-field wavefront sensing and control. Our implementation of the Wavefront Sensing (WFS) algorithms using phase diversity is experimentally tested. The wavefront control (WFC) algorithms, which rely on a linear model for optical aberrations induced by small misalignments of the three lenses, are tested and validated on simulations.

Optimization of baffle configuration for stray light reduction

Reducing the stray light level is one of the issues that astronomical instruments have to face. In particular, the design of baffles requires special attention in order to minimize the light scattered and diffracted by the edge of the baffles vanes. The choice of the materials and the treatments used to manufacturing those parts can significantly increase the performance of stray light suppression. This is particularly critical for instruments in which the main source of stray light is in the field-of-view and its brightness is much higher than the signal the experiment aims to measure, such as solar and stellar coronagraphs. In order to identify the best configuration to adopt in the design and manufacture of a future coronagraph, we designed a dedicated set-up that allows comparing different edge geometries and finishing in a fast and comprehensive approach. A reference edge configuration was chosen and all the other configurations were compared with it. In this paper, we describe the set-up, the characterized configurations and the obtained results.

James Webb Space telescope optical simulation testbed: experimental results with linear control alignment

The current generation of terrestrial telescopes has large enough primary mirror diameters that active optical control based on wavefront sensing is necessary. Similarly, in space, while the Hubble Space Telescope (HST) has a mostly passive optical design, apart from focus control, its successor the James Webb Space Telescope (JWST) has active control of many degrees of freedom in its primary and secondary mirrors.