Design of the EnVisS instrument optical head

The EnVisS (Entire Visible Sky) instrument is one of the payloads of the European Space Agency Comet Interceptor mission. The aim of the mission is the study of a dynamically new comet, i.e. a comet that never travelled through the solar system, or an interstellar object, entering the inner solar system. As the mission three-spacecraft system passes through the comet coma, the EnVisS instrument maps the sky, as viewed from the interior of the comet tail, providing information on the dust properties and distribution. EnVisS is mounted on a spinning spacecraft and the full sky (i.e. 360°x180°) is entirely mapped thanks to a very wide field of view (180°x45°) optical design selected for the EnVisS camera. The paper presents the design of the EnVisS optical head. A fisheye optical layout has been selected because of the required wide field of view (180°x45°). This kind of layout has recently found several applications in Earth remote sensing (3MI instrument on MetOp SG) and in space exploration (SMEI instrument on Coriolis, MARCI on Mars reconnaissance orbiter). The EnVisS optical head provides a high resolved image to be coupled with a COTS detector featuring 2kx2k pixels with pitch 5.5µm. Chromatic aberration is corrected in the waveband 550-800nm, while the distortion has been controlled over the whole field of view to remain below 8% with respect to an Fθ mapping law. Since the camera will be switched on 24 hours before the comet closest encounter, the operative temperature will change during the approaching phase and crossing of the comet’s coma. In the paper, we discuss the solution adopted for reaching these challenging performances for a space-grade design, while at the same time respecting the demanding small allocated volume and mass for the optical and mechanical design. The view expressed herein can in no way be taken to reflect the official opinion of the European Space Agency

Heat treatment procedure of the Aluminium 6061-T651 for the Ariel Telescope mirrors

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

Particle monitoring capability of the Solar Orbiter Metis coronagraph through the increasing phase of solar cycle 25

Context. Galactic cosmic rays (GCRs) and solar particles with energies greater than tens of MeV penetrate spacecraft and instruments hosted aboard space missions. The Solar Orbiter Metis coronagraph is aimed at observing the solar corona in both visible (VL) and ultraviolet (UV) light. Particle tracks are observed in the Metis images of the corona. An algorithm has been implemented in the Metis processing electronics to detect the VL image pixels crossed by cosmic rays. This algorithm was initially enabled for the VL instrument only, since the process of separating the particle tracks in the UV images has proven to be very challenging. Aims. We study the impact of the overall bulk of particles of galactic and solar origin on the Metis coronagraph images. We discuss the effects of the increasing solar activity after the Solar Orbiter mission launch on the secondary particle production in the spacecraft. Methods. We compared Monte Carlo simulations of GCRs crossing or interacting in the Metis VL CMOS sensor to observations gathered in 2020 and 2022. We also evaluated the impact of solar energetic particle events of different intensities on the Metis images. Results. The study of the role of abundant and rare cosmic rays in firing pixels in the Metis VL images of the corona allows us to estimate the efficiency of the algorithm applied for cosmic-ray track removal from the images and to demonstrate that the instrument performance had remained unchanged during the first two years of the Solar Orbiter operations. The outcome of this work can be used to estimate the Solar Orbiter instrument's deep charging and the order of magnitude for energetic particles crossing the images of Metis and other instruments such as STIX and EUI.Comment: 8 pages, 6 figure

Test of protected silver coating on aluminum samples of ARIEL main telescope mirror substrate material

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FEA testing the pre-flight Ariel primary mirror

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

SIMBIO-SYS Near Earth Commissioning Phase: a step forward toward Mercury

On December 2018, the Near Earth Commissioning Phase (NECP) has been place forSIMBIO-SYS (Spectrometers and Imagers for MPO BepiColombo Integrated Observatory - SYStem), the suite part of the scientific payload of the BepiColombo ESA-JAXA mission. SIMBIO-SYS is composed of three channels: the high resolution camera (HRIC), the stereo camera (STC) and the Vis/NIR spectrometer (VIHI) . During the NECP the three channels have been operated properly. For the three channels were checked the operativity and the performance. The commanded operations allowed to verify all the instrument functionalities demonstrating that all SIMBIO-SYS channels and subsystems work nominally. During this phase we also validated the Ground Segment Equipment (GSE) and the data analysis tools developed by the team

Enabling planetary science across light-years. Ariel Definition Study Report

Ariel, the Atmospheric Remote-sensing Infrared Exoplanet Large-survey, was adopted as the fourth medium-class mission in ESA's Cosmic Vision programme to be launched in 2029. During its 4-year mission, Ariel will study what exoplanets are made of, how they formed and how they evolve, by surveying a diverse sample of about 1000 extrasolar planets, simultaneously in visible and infrared wavelengths. It is the first mission dedicated to measuring the chemical composition and thermal structures of hundreds of transiting exoplanets, enabling planetary science far beyond the boundaries of the Solar System. The payload consists of an off-axis Cassegrain telescope (primary mirror 1100 mm x 730 mm ellipse) and two separate instruments (FGS and AIRS) covering simultaneously 0.5-7.8 micron spectral range. The satellite is best placed into an L2 orbit to maximise the thermal stability and the field of regard. The payload module is passively cooled via a series of V-Groove radiators; the detectors for the AIRS are the only items that require active cooling via an active Ne JT cooler. The Ariel payload is developed by a consortium of more than 50 institutes from 16 ESA countries, which include the UK, France, Italy, Belgium, Poland, Spain, Austria, Denmark, Ireland, Portugal, Czech Republic, Hungary, the Netherlands, Sweden, Norway, Estonia, and a NASA contribution

The Comet Interceptor Mission

Here we describe the novel, multi-point Comet Interceptor mission. It is dedicated to the exploration of a little-processed long-period comet, possibly entering the inner Solar System for the first time, or to encounter an interstellar object originating at another star. The objectives of the mission are to address the following questions: What are the surface composition, shape, morphology, and structure of the target object? What is the composition of the gas and dust in the coma, its connection to the nucleus, and the nature of its interaction with the solar wind? The mission was proposed to the European Space Agency in 2018, and formally adopted by the agency in June 2022, for launch in 2029 together with the Ariel mission. Comet Interceptor will take advantage of the opportunity presented by ESA’s F-Class call for fast, flexible, low-cost missions to which it was proposed. The call required a launch to a halo orbit around the Sun-Earth L2 point. The mission can take advantage of this placement to wait for the discovery of a suitable comet reachable with its minimum ΔV capability of 600 ms−1. Comet Interceptor will be unique in encountering and studying, at a nominal closest approach distance of 1000 km, a comet that represents a near-pristine sample of material from the formation of the Solar System. It will also add a capability that no previous cometary mission has had, which is to deploy two sub-probes – B1, provided by the Japanese space agency, JAXA, and B2 – that will follow different trajectories through the coma. While the main probe passes at a nominal 1000 km distance, probes B1 and B2 will follow different chords through the coma at distances of 850 km and 400 km, respectively. The result will be unique, simultaneous, spatially resolved information of the 3-dimensional properties of the target comet and its interaction with the space environment. We present the mission’s science background leading to these objectives, as well as an overview of the scientific instruments, mission design, and schedule

Optical and Opto-mechanical Analysis and Design of the Telescope for the Ariel Mission

The Atmospheric Remote-sensing Infrared Exoplanet Large-survey (Ariel) is the first space mission dedicated to measuring the chemical composition and thermal structures of thousands of transiting exoplanets, enabling planetary science far beyond the boundaries of the Solar System. Ariel was officially adopted in 2020 as the fourth medium size (M4) mission in the scope of ESA “Cosmic Vision” program, with launch expected in 2029. The mission will operate from the Sun-Earth Lagrangian point L2. The scientific payload consists of two instruments: a highresolution spectrometer covering the waveband 1.95–7.8 µm, and a multi-purpose fine guidance system / visible photometer / low resolution near-infrared spectrometer with wavelength coverage between 0.5 µm and 1.95 µm. The instruments are fed a collimated beam from an unobscured, off-axis Cassegrain telescope. Instruments and telescope will operate at a temperature below 50 K. The mirrors and supporting structures of the telescope will be realized in aerospace-grade aluminum. Given the large aperture of the primary mirror (0.6 m2), it is a choice of material that requires careful optical and opto-mechanical design, and technological advances in the three areas of mirror substrate thermal stabilization, optical surface polishing and optical coating. This thesis presents the work done by the author in these areas, as member of the team responsible for designing and manufacturing the telescope and mirrors. The dissertation starts with a systematic review of the optical and opto-mechanical requirements and design choices of the Ariel telescope, in the context of the previous development work and the scientific goals and requirements of the mission. The review then progresses with the opto-mechanical design, examining the most important choices in terms of structural and thermal design. This will serve as an introduction to a statistical analysis of the deformations of the optical surface of the telescope mirrors and of their alignment in terms of rigid body motions. The qualification work on thermal stabilization, polishing and coating is then presented. The three procedures have been set up and tested to demonstrate the readiness level of the technological processes employed to fabricate the mirrors. The first process, substrate thermal stabilization, is employed to minimize deformations of the optical surface during cool down of the telescope to the operating temperature below 50 K. Purpose of the process is to release internal stress in the substrate that can cause such shape deformations. Then a combined optical surface figuring/polishing process is applied to reduce residual surface shape errors and bring surface roughness to below 10 nm RMS. Polishing of large aluminum surfaces to optical quality is notoriously difficult due to softness of the material, so a dedicated polishing recipe was set up and tested. Finally, an optical coatingrecipe with protectedsilver was characterized in terms ofreflectivity and qualified for environmental stability, particularly at cryogenic temperatures, and for uniformity. Some of the coated samples are also being monitored and measured periodically for any sign of performance degradation while they age. All tests were performed on samples of the same aluminum alloy chosen as baseline for the mirror substrates and on a full-scale prototype of the Ariel primary mirror. Results from the coating characterization were also used to prepare an estimation of the various components contributing to the expected throughput of the telescope at the end of the scientific lifetime of the mission.The Atmospheric Remote-sensing Infrared Exoplanet Large-survey (Ariel) is the first space mission dedicated to measuring the chemical composition and thermal structures of thousands of transiting exoplanets, enabling planetary science far beyond the boundaries of the Solar System. Ariel was officially adopted in 2020 as the fourth medium size (M4) mission in the scope of ESA “Cosmic Vision” program, with launch expected in 2029. The mission will operate from the Sun-Earth Lagrangian point L2. The scientific payload consists of two instruments: a highresolution spectrometer covering the waveband 1.95–7.8 µm, and a multi-purpose fine guidance system / visible photometer / low resolution near-infrared spectrometer with wavelength coverage between 0.5 µm and 1.95 µm. The instruments are fed a collimated beam from an unobscured, off-axis Cassegrain telescope. Instruments and telescope will operate at a temperature below 50 K. The mirrors and supporting structures of the telescope will be realized in aerospace-grade aluminum. Given the large aperture of the primary mirror (0.6 m2), it is a choice of material that requires careful optical and opto-mechanical design, and technological advances in the three areas of mirror substrate thermal stabilization, optical surface polishing and optical coating. This thesis presents the work done by the author in these areas, as member of the team responsible for designing and manufacturing the telescope and mirrors. The dissertation starts with a systematic review of the optical and opto-mechanical requirements and design choices of the Ariel telescope, in the context of the previous development work and the scientific goals and requirements of the mission. The review then progresses with the opto-mechanical design, examining the most important choices in terms of structural and thermal design. This will serve as an introduction to a statistical analysis of the deformations of the optical surface of the telescope mirrors and of their alignment in terms of rigid body motions. The qualification work on thermal stabilization, polishing and coating is then presented. The three procedures have been set up and tested to demonstrate the readiness level of the technological processes employed to fabricate the mirrors. The first process, substrate thermal stabilization, is employed to minimize deformations of the optical surface during cool down of the telescope to the operating temperature below 50 K. Purpose of the process is to release internal stress in the substrate that can cause such shape deformations. Then a combined optical surface figuring/polishing process is applied to reduce residual surface shape errors and bring surface roughness to below 10 nm RMS. Polishing of large aluminum surfaces to optical quality is notoriously difficult due to softness of the material, so a dedicated polishing recipe was set up and tested. Finally, an optical coatingrecipe with protectedsilver was characterized in terms ofreflectivity and qualified for environmental stability, particularly at cryogenic temperatures, and for uniformity. Some of the coated samples are also being monitored and measured periodically for any sign of performance degradation while they age. All tests were performed on samples of the same aluminum alloy chosen as baseline for the mirror substrates and on a full-scale prototype of the Ariel primary mirror. Results from the coating characterization were also used to prepare an estimation of the various components contributing to the expected throughput of the telescope at the end of the scientific lifetime of the mission

Distortion calculation and removal for an off-axis and wide angle camera

For off-axis and wide angle systems, the calculation, calibration and removal of distortion effects from the images are often challenging tasks. Specific procedures have been implemented to assess and remove the distortion from the images acquired by the OSIRIS imaging instrument on-board the Rosetta ESA mission. OSIRIS consisted in a narrow and a wide angle camera. The Wide Angle Camera (WAC) is an off-axis, unobstructed and wide FoV (i.e. about 12\ub0x12\ub0) optical system. It has a peculiar optical configuration, and due to the off-axis design the camera presents a high level of intrinsic distortion, with the major component being anamorphism. The distortion has been estimated theoretically via raytracing during the design phase, then measured on-ground and inflight during the calibration campaigns. To obtain correct undistorted images, a distortion removal procedure has been implemented. The first step of the process has been to remove from the images the theoretical distortion. Then the distortion correction procedure has been refined using on-ground and in-flight calibration measurements. This work describes in detai