The GMT-Consortium Large Earth Finder (G-CLEF) is a spectrograph, to be delivered as the first light scientific instrument for the Giant Magellan Telescope (GMT), and divided into a red and a blue channel.
In the frame of this project, Bertin Winlight is in charge of the manufacturing of the blue camera lenses, and entrusted CILAS to develop and realize the antireflection coatings on the 8 different lenses of this camera. The camera consist of 8 lenses with diameter in the 190 – 323 mm range made of CaF2 and other optical glass (BAL35Y, PBM18Y, BSM51Y). We present the results on the design, development and manufacturing of antireflection coating with PIAD (Plasma-Ion Assisted Deposition) technology, which is well adapted to produce very dense layers and high quality coatings for severe environments.
The design of antireflection coating, in the spectral range 350-541nm, has taken into account large diameter and high curvature of the lenses and has been experimentally validated on a dummy shape. Moreover a dedicated coating process has been implemented and qualified on CaF2 substrates, leading to the successful coating of the 8 different lenses.
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 baffle’s vanes. This is particularly critical for instruments in which the main source of stray light is in the field-of-view (such as solar and stellar coronagraphs).
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.
KEYWORDS: Astronomical imaging, Active optics, Space telescopes, Disk lasers, Wavefront sensors, Telescopes, Relays, Device simulation, Cameras, Control systems
The next generation of large lightweight space telescopes will require the use of active optics systems to enhance the performance and increase the spatial resolution. Since almost 10 years now, LAM, CNES, THALES and ONERA conjugate their experience and efforts for the development of space active optics through the validation of key technological building blocks: correcting devices, metrology components and control strategies. This article presents the work done so far on active correcting mirrors and wave front sensing, as well as all the facilities implemented. The last part of this paper focuses on the merging of the MADRAS and RASCASSE test-set up. This unique combination will provide to the active optics community an automated, flexible and versatile facility able to feed and characterise space active optics components.
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 Earth's 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.
KEYWORDS: Sensors, Space telescopes, Stars, Wavefront sensors, Astronomical imaging, Active optics, Signal to noise ratio, Device simulation, Telescopes, Numerical simulations
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.
KEYWORDS: Mirrors, Wavefronts, Space telescopes, Error analysis, Signal to noise ratio, Astronomical imaging, Telescopes, Active optics, Zernike polynomials, Spatial resolution
Discoveries in astronomy and earth science lie on the capabilities of the space observatories to see fainter objects and smaller details. This need of high collecting power and high angular resolution implies instruments with large primary mirrors. However, a simple scaling of existing space telescopes leads to bigger optical elements and structure that exceed the allocated volume and launch mass capability of medium size launchers. Due to volume, weight and cost constraints on satellites, the next generation of large telescopes must combine innovative and compact optical concepts using lightweight primary mirrors and structures. Furthermore the lightweighting of primary mirrors and structures reduce their stiffness and make them more deformable under static and dynamic load. Also, the compactness needed implies primary mirrors with low focal ratio and a small distance between primary and secondary mirrors. This leads to an optical train more sensitive to misalignment.
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.
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.
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 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.
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 baffle's 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.
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.
LAM is developing a high-contrast imaging testbeds for in-lab demonstration of new instrumental concepts requiring
high contrast imaging: in particular, for solar and stellar coronagraphy applications. In such applications, a faint target
has to be detected close to a very bright source. For these test-benches, a high-dynamic range detector is required to
characterize and/or to determine the performance of a new concept. Beyond the capability to detect the target, an
imaging detector has to be accurate, reliable and provide reproducible performances.
In order to identify a commercial camera for the development of laboratory demonstrators working with high contrast
scenes, we carried out a test campaign at the Laboratoire d’Astrophysique de Marseille (LAM) evaluating several
cameras implementing different detector technologies. This paper presents the results of the test campaign, carried out at
LAM, providing a quantitative comparison between the investigated technologies
METIS (Multi Element Telescope for Imaging and Spectroscopy investigation), 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, by means of an integrated instrument suite located on a single optical bench and sharing the same aperture on the satellite heat shield. As every coronagraph, METIS is highly demanding in terms of stray light suppression. Coronagraphs history teaches that a particular attention must be dedicated to the occulter optimization. The METIS occulting system is of particular interest due to its innovative concept. 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. A breadboard of the occulting assembly (BOA) has been manufactured in order to perform stray light tests in front of two solar simulators (in Marseille, France and in Torino, Italy). A first measurement campaign has been carried on at the Laboratoire d'Astrophysique de Marseille. In this paper we describe the BOA design, the laboratory set-up and the preliminary results.
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