Ariel (Atmospheric Remote-Sensing Infrared Exoplanet Large Survey) is ESA's M4 mission of the "Cosmic Vision" program, with launch scheduled for 2029. Its purpose is to conduct a survey of the atmospheres of known exoplanets through transit spectroscopy. Ariel is based on a 1 m class telescope optimized for spectroscopy in the waveband between 1.95 and 7.8 mu m, operating at cryogenic temperatures in the range 40-50 K. The Ariel Telescope is an off-axis, unobscured Cassegrain design, with a parabolic recollimating tertiary mirror and a flat folding mirror directing the output beam parallel to the optical bench. The secondary mirror is mounted on a rototranslating stage for adjustments during the mission. The mirrors and supporting structures are all realized in an aerospace-grade aluminum alloy T6061 for ease of manufacturing and thermalization. The low stiffness of the material, however, poses unique challenges to integration and alignment. Care must be therefore employed when designing and planning the assembly and alignment procedures, necessarily performed at room temperature and with gravity, and the optical performance tests at cryogenic temperatures. This paper provides a high-level description of the Assembly, Integration and Test (AIT) plan for the Ariel telescope and gives an overview of the analyses and reasoning that led to the specific choices and solutions adopted.

Ariel (Atmospheric Remote-Sensing Infrared Exoplanet Large Survey) is the fourth medium-class mission (M4) of the ESA's Cosmic Vision Program. Its launch is planned for 2029. Ariel will observe a large and well selected sample of transiting gas giants, neptunes and super-earths around a wide range of host star types, with the objective to study planetary atmospheres and to understand composition and evolving processes of the planetary systems. A Structural, Thermal, and Optical Performance (STOP) analysis is conducted at Payload level to estimate the thermo-elastic induced degradation of the system performance for a number of selected environmental load cases. In particular, this document presents the general approach followed and the results of the optical design analysis performed to predict the performance of the Ariel Telescope Assembly for the in-flight operational cases during Cycle C-1.

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 survey the atmospheres of known exoplanets through transit spectroscopy. The launch is scheduled for 2029. The scientific payload consists of an off-axis, unobscured Cassegrain telescope feeding a set of photometers and spectrometers in the waveband 0.5-7.8 mu m and operating at cryogenic temperatures (55 K). The Telescope Assembly is based on an innovative fully aluminium design to tolerate thermal variations to avoid impacts on the optical performance; it consists of a primary parabolic mirror with an elliptical aperture of 1.1 m (the 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 of the optical bench sides. 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 Critical Design Review; the fabrication of the structural and engineering models has started; some components, i.e., the primary mirror and its mounting system are undergoing further qualification activities. This paper aims to update the scientific community on the progress concerning the development, manufacturing and qualification activity of the ARIEL Telescope Assembly.

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.

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 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.

PROBA-3 is a space mission of the European Space Agency that will test, and validate metrology and control systems for autonomous formation flying of two independent satellites. PROBA-3 will operate in a High Elliptic Orbit and when approaching the apogee at 6.10(4) Km, the two spacecraft will align to realize a giant externally occulted coronagraph named ASPIICS, with the telescope on one satellite and the external occulter on the other one, at inter-satellite distance of 144.3 m. The formation will be maintained over 6 hrs across the apogee transit and during this time different validation operations will be performed to confirm the effectiveness of the formation flying metrology concept, the metrology control systems and algorithms, and the spacecraft manoeuvring. The observation of the Sun's Corona in the field of view [1.08;3.0]R-Sun will represent the scientific tool to confirm the formation flying alignment. In this paper, we review the mission concept and we describe the Shadow Position Sensors (SPS), one of the metrological systems designed to provide high accuracy (sub-millimetre level) absolute and relative alignment measurement of the formation flying. The metrology algorithm developed to convert the SPS measurements in lateral and longitudinal movement estimation is also described and the measurement budget summarized. (C) 2020 COSPAR. Published by Elsevier Ltd. All rights reserved.

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 development and employment of metrology controlled diluted systems for astronomical applications in space represents a cornerstone to dramatically improve the observation limits. In this scenario, the ESA mission PROBA-3 is the first technology demonstration mission aimed at verifying and validating several metrological systems to obtain high accuracy formation flying (FF) of two satellites. The two spacecraft will be aligned and maintained in a FF, around the apogee of their geocentric orbit, at an average inter-satellite distance of 144m, with an accuracy down to sub-millimetre level. The scientific task of the mission, that is the FF validation tool, is the observation of the Sun's Corona at high spatial and temporal resolution within the field of view [1.08; 3.0]R ⊙ . To this end, one satellite will house a coronagraph telescope, named ASPIICS, and the other one, the external occulter. In this paper, we review the metrology concept applied to the mission with a special focus on the Occulter Position Sensor Emitters (OPSE) metrology sub-system, a set of 3 monochromatic Led emitters positioned on opposite-Sun surface of the occulter and imaged at the ASPIICS's focal plane.

This paper presents the recent achievements in the development of ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun), a solar coronagraph that is the primary payload of ESA's formation flying in-orbit demonstration mission PROBA-3. The PROBA-3 Coronagraph System is designed as a classical externally occulted Lyot coronagraph but it takes advantage of the opportunity to place the 1.4 meter wide external occulter on a companion spacecraft, about 150m apart, to perform high resolution imaging of the inner corona of the Sun as close as similar to 1.1 solar radii. Besides providing scientific data, ASPIICS is also equipped with sensors for providing relevant navigation data to the Formation Flying GNC system. This paper is reviewing the recent development status of the ASPIICS instrument as it passed CDR, following detailed design of all the sub-systems and testing of STM and various Breadboard models. ASPIICS is built by a large European consortium including about 20 partners from 7 countries under the auspices of the European Space Agency.

The ESA formation Flying mission Proba-3 will y the giant solar coronagraph ASPIICS. The instrument is composed of a 1.4 meter diameter external occulting disc mounted on the Occulter Spacecraft and a Lyot-style solar coronagraph of 50mm diameter aperture carried by the Coronagraph Spacecraft positioned 144 meters behind. The system will observe the inner corona of the Sun, as close as 1.1 solar radius. For a solar coronagraph, the most critical source of straylight is the residual diffracted sunlight, which drives the scientific performance of the observation. This is especially the case for ASPIICS because of its reduced field-of-view close to the solar limb. The light from the Sun is first diffracted by the edge of the external occulter, and then propagates and scatters inside the instrument. There is a crucial need to estimate both intensity and distribution of the diffraction on the focal plane. Because of the very large size of the coronagraph, one cannot rely on representative full scale test campaign. Moreover, usual optics software package are not designed to perform such diffraction computation, with the required accuracy. Therefore, dedicated approaches have been developed in the frame of ASPIICS. First, novel numerical models compute the diffraction profile on the entrance pupil plane and instrument detector plane (Landini et al., Rougeot et al.), assuming perfect optics in the sense of multi-reflection and scattering. Results are confronted to experimental measurements of diffraction. The paper reports the results of the different approaches.

This paper presents the current status of ASPIICS, a solar coronagraph that is the primary payload of ESA’s formation flying in-orbit demonstration mission PROBA-3. The “sonic region” of the Sun corona remains extremely difficult to observe with spatial resolution and sensitivity sufficient to understand the fine scale phenomena that govern the quiescent solar corona, as well as phenomena that lead to coronal mass ejections (CMEs), which influence space weather. Improvement on this front requires eclipse-like conditions over long observation times. The space-borne coronagraphs flown so far provided a continuous coverage of the external parts of the corona but their over-occulting system did not permit to analyse the part of the white-light corona where the main coronal mass is concentrated. The PROBA-3 Coronagraph System, also known as ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun) is designed as a classical externally occulted Lyot coronagraph but it takes advantage of the opportunity to place the external occulter on a companion spacecraft, about 150m apart, to perform high resolution imaging of the inner corona of the Sun as close as ~1.1 solar radii. The images will be tiled and compressed on board in an FPGA before being down-linked to ground for scientific analyses. ASPIICS is built by a large European consortium including about 20 partners from 7 countries under the auspices of the European Space Agency. This paper is reviewing the recent development status of the ASPIICS instrument as it is approaching CDR.

In the framework of development of ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun), the Centre Spatial de Liege is responsible of the optical design of the coronagraph and the optics will be manufactured by TOPTEC.The particularity of this coronagraph is to have an external occulter located 150 m ahead of the first imaging lens. This external occulter is re-imaged on an internal occulter which function is - as in a classical externally occulted Lyot coronagraph - to block the sun light diffracted by the external occulter and to reduce the straylight on the detector. The selection of this configuration is driven by the requirement to observe the corona as close as possible to the solar limb (i.e. 1 R-Sun) without imaging the limb itself. A requirement of 1.08 R-Sun is specified at optical design level to grant 1.2 R-sun at instrument level. The coronograph instrument is designed to have a field of view of 1.6 degrees x 1.6 degrees with a resolution of less than 6 arcsec. Its performances are limited by diffraction in a 530 - 590 nm wavelength range.This paper presents the optical design and demonstrates that by design the requirements are fulfilled within the misalignment, manufacturing and thermo-elastic error contributions.