The primary mirror of the Ariel space telescope (an ESA M class mission aimed at the study of exoplanets, scheduled for launch in 2029) is an elliptical off-axis paraboloid. Like the entire telescope, it is built of aluminum. As a massive part of the payload, as well as one of the most delicate components of the telescope, this mirror has to be accurately designed, in order to minimize its mass while not degrading its optical performances. This paper discusses the optimization study of the primary mirror of Ariel. Starting from its optical and geometrical specifications, we have run an iterative process based on FEA dynamic analyses, in order to compute the first ”free-free” eigenfrequencies while varying the three fundamental parameters of the honeycomb structure of the mirror - the thickness of the ribs, the outer edge, and the reflecting surface. Later, the optimization routine has been improved by adding the honeycomb geometry as a variable parameter. As a result, the best configurations is identified as the ones giving the higher ratios of the first relevant eigenfrequency divided by the mass.
Ariel (Atmospheric Remote-Sensing Infrared Exoplanet Large Survey) is the adopted M4 mission in the framework of the ESA “Cosmic Vision” program. Its purpose is to conduct a survey of the atmospheres of known exoplanets through transit spectroscopy. Launch is scheduled for 2029. Ariel scientific payload consists of an off-axis, unobscured Cassegrain telescope feeding a set of photometers and spectrometers in the waveband between 0.5 and 7.8 µm and operating at cryogenic temperatures (55 K). The Telescope Assembly is based on an innovative fully-aluminum design to tolerate thermal variations avoiding impacts on the optical performance; it consists of a primary parabolic mirror with an elliptical aperture of 1.1 m of major axis, followed by a hyperbolic secondary that is mounted on a refocusing system, a parabolic re-collimating tertiary and a flat folding mirror directing the output beam parallel to the optical bench. An innovative mounting system based on 3 flexure-hinges supports the primary mirror on one side of the optical bench. The instrument bay on the other side of the optical bench houses the Ariel IR Spectrometer (AIRS) and the Fine Guidance System / NIR Spectrometer (FGS/NIRSpec). The Telescope Assembly is in phase B2 towards the Preliminary Design Review to start the fabrication of the structural model; some components, i.e., the primary mirror, its mounting system and the refocusing mechanism, are undergoing further development activities to increase their readiness level. This paper describes the design and development of the ARIEL Telescope Assembly.
The Atmospheric Remote-Sensing Infrared Exoplanet Large Survey (Ariel) is the M4 mission adopted by ESA’s ”Cosmic Vision” program. Its launch is scheduled for 2029. The purpose of the mission is the study of exoplanetary atmospheres on a target of ∼ 1000 exoplanets. Ariel scientific payload consists of an off-axis, unobscured Cassegrain telescope. The light is directed towards a set of photometers and spectrometers with wavebands between 0.5 and 7.8 µm and operating at cryogenic temperatures. The Ariel Space Telescope consists of a primary parabolic mirror with an elliptical aperture of 1.1· 0.7 m, followed by a hyperbolic secondary, a parabolic collimating tertiary and a flat-folding mirror directing the output beam parallel to the optical bench; all in bare aluminium. The choice of bare aluminium for the realization of the mirrors is dictated by several factors: maximizing the heat exchange, reducing the costs of materials and technological advancement. To date, an aluminium mirror the size of Ariel’s primary has never been made. The greatest challenge is finding a heat treatment procedure that stabilizes the aluminium, particularly the Al6061T651 Laminated alloy. This paper describes the study and testing of the heat treatment procedure developed on aluminium samples of different sizes (from 50mm to 150mm diameter), on 0.7m diameter mirror, and discusses future steps.
Ariel (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) is an ESA M class mission aimed at the study of exoplanets. The satellite will orbit in the lagrangian point L2 and will survey a sample of 1000 exoplanets simultaneously in visible and infrared wavelengths. The challenging scientific goal of Ariel implies unprecedented engineering efforts to satisfy the severe requirements coming from the science in terms of accuracy. The most important specification – an all-Aluminum telescope – requires very accurate design of the primary mirror (M1), a novel, off-set paraboloid honeycomb mirror with ribs, edge, and reflective surface. To validate such a mirror, some tests were carried out on a prototype – namely Pathfinder Telescope Mirror (PTM) – built specifically for this purpose. These tests, carried out at the Centre Spatial de Liège in Belgium – revealed an unexpected deformation of the reflecting surface exceeding a peek-to-valley of 1µm. Consequently, the test had to be re-run, to identify systematic errors and correct the setting for future tests on the final prototype M1. To avoid the very expensive procedure of developing a new prototype and testing it both at room and cryogenic temperatures, it was decided to carry out some numerical simulations. These analyses allowed first to recognize and understand the reasoning behind the faults occurred during the testing phase, and later to apply the obtained knowledge to a new M1 design to set a defined guideline for future testing campaigns.
Ariel (Atmospheric Remote-Sensing Infrared Exoplanet Large Survey) is the adopted M4 mission of ESA “Cosmic Vision” program. Its purpose is to conduct a survey of the atmospheres of known exoplanets through transit spectroscopy. Launch is scheduled for 2029. Ariel scientific payload consists of an off-axis, unobscured Cassegrain telescope feeding a set of photometers and spectrometers in the waveband between 0.5 and 7.8 μm, and operating at cryogenic temperatures. The Ariel Telescope consists of a primary parabolic mirror (M1) with an elliptical aperture of 1.1 m of major axis and 0.7 m of minor axis, followed by a hyperbolic secondary (M2) , a parabolic recollimating tertiary (M3) and a flat folding mirror (M4). The Primary mirror is a very innovative device made of lightened aluminum. Aluminum mirrors for cryogenic instruments and for space application are already in use, but never before now it has been attempted the creation of such a large mirror made entirely of aluminum: this means that the production process must be completely revised and finetuned, finding new solutions, studying the thermal processes and paying a great care to the quality check. By the way, the advantages are many: thermal stabilization is simpler than with mirrors made of other materials based on glass or composite materials, the cost of the material is negligable, the shape may be free and the possibility of making all parts of the telescope, from optical surfaces to the structural parts, of the same material guarantees a perfect alignment at whichever temperature. This paper describes the methodology and preliminary results of this manufacturing process and discusses future steps.
The performance of as-built optical instruments strongly depends on thermal and structural loads, since these boundary conditions can affect the geometry of optical surfaces. Variations of temperature influence the volume, and the shape, of the structure proportionally to the coefficient of thermal expansion of the material, while mechanical loads, like gravity, may induce deformations on the optical elements according to the set of applied constraints. Those effects can introduce aberrations that degrade the performance of the optical system. Since software for optical and thermo-structural analysis are usually different, a coupling methodology between these two fields of physics is needed. This is a step-by-step procedure through many platforms. In this work, the procedure devised and used by the authors will be presented. At first, a thermo-mechanical analysis (depending on the loads involved) has to be performed, in order to obtain the final deformed geometry of the optical structure; COMSOL Multiphysics is the finite element solver (FEM) used for these analyses. Then an output data file, containing the coordinates of points belonging to the optical surface, can be generated. The output data are elaborated by a MATLAB routine that allows to convert the set of points into an n-th polynomial expression that best fits the surface data. The fitted polynomial surface is hence imported in ZEMAX ray-tracing software to study the optical performances of the system and the effects of thermo-mechanical loads.
ARIEL, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey mission1-3 was selected in early 2018 by the European Space Agency (ESA) as the fourth medium-class mission (M4) launch opportunity of the Cosmic Vision Program, with an expected launch in late 2028. It is the first mission dedicated to the analysis of the chemical composition and thermal structures of up to a thousand transiting exoplanets atmospheres, which will expand planetary science far beyond the limits of our current knowledge.
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