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

Andrea, Tozzi

Anna, Brucalassi

Antonio, Scippa

Ciro, Del Vecchio

Debora, Ferruzzi

Elisa, Guerriero

Elisabetta, Tommasi

Emanuele, Pace

Emiliano, Diolaiti

Enzo, Pascale

Fausto, Cortecchia

Giampaolo, Preti

Gianluca, Morgante

Gilberto, Falcini

Giovanna, Tinetti

Giuseppe, Malaguti

Giuseppina, Micela

Luca, Carbonaro

Marcella, Iuzzolino

Mario, Salatti

Matteo, Lombini

Paola, Zuppella

Paolo, Chioetto

Paul, Eccleston

Raffaele, Piazzolla

Rodolfo, Canestrari

English

UCL Discovery

PROCEEDINGS OF SPIESPIEDigitalLibrary.org/conference-proceedings-of-spieHeat treatment procedure of thealuminium 6061-T651 for the Arieltelescope mirrors.Elisa Guerriero, Paolo Chioetto, Andrea Tozzi, PaolaZuppella, Rodolfo Canestrari, et al.Elisa Guerriero, Paolo Chioetto, Andrea Tozzi, Paola Zuppella, RodolfoCanestrari, Anna Brucalassi, Marcella Iuzzolino, Debora Ferruzzi, AntonioScippa, Ciro Del Vecchio, Gilberto Falcini, Luca Carbonaro, GianlucaMorgante, Fausto Cortecchia, Emiliano Diolaiti, Paul Eccleston, MatteoLombini, Giuseppe Malaguti, Giuseppina Micela, Emanuele Pace, EnzoPascale, Raffaele Piazzolla, Giampaolo Preti, Mario Salatti, Giovanna Tinetti,Elisabetta Tommasi, "Heat treatment procedure of the aluminium 6061-T651for the Ariel telescope mirrors.," Proc. SPIE 12180, Space Telescopes andInstrumentation 2022: Optical, Infrared, and Millimeter Wave, 1218014 (27August 2022); doi: 10.1117/12.2628178Event: SPIE Astronomical Telescopes + Instrumentation, 2022, Montréal,Québec, CanadaDownloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 16 Nov 2022  Terms of Use: https://www.spiedigitallibrary.org/terms-of-useHeat treatment procedure of the Aluminium 6061-T651 forthe Ariel Telescope mirrorsGuerriero Elisaa,b, Chioetto Paoloc,d,e, Tozzi Andreaf, Zuppella Paolac,e, Rodolfo Canestrarig,Brucalassi Annaf, Iuzzolino Marcellaf, Ferruzzi Deboraf, Scippa Antonioh, Del Vecchio Cirof,Falcini Gilbertof, Carbonaro Lucaf, Morgante Gianlucai, Cortecchia Faustoi, DiolaitiEmilianoi, Eccleston Paulj, Lombini Matteoi, Malaguti Giuseppei, Micela Giuseppinaa, PaceEmanuelef,h, Pascale Enzok, Piazzolla Raffaelel, Preti Giampaoloi, Salatti Mariom, TinettiGiovannan, and Tommasi ElisabettamaINAF-Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, ItalybDipartimento di Fisica e Chimica, Università degli studi di Palermo, Via Archirafi, 36, 90123Palermo PA, ItalycCNR-Istituto di Fotonica e Nanotecnologie di Padova, Via Trasea 7, 35131 Padova, ItalydCentro di Ateneo di Studi e Attività Spaziali “Giuseppe Colombo”- CISAS, Via Venezia 15,35131 Padova, ItalyeINAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, ItalyfINAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, ItalygINAF-Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo, Via Ugo La Malfa 153,90146 Palermo, ItalyhDipartimento di Fisica ed Astronomia-Università degli Studi di Firenze, Largo E. Fermi 2,50125 Firenze, ItalyiINAF-Osservatorio di Astrofisica e Scienza dello spazio di Bologna, Via Piero Gobetti 93/3,40129 Bologna, ItalyjRAL Space, STFC Rutherford Appleton Laboratory, Didcot, Oxon, OX11 0QX, UKkDipartimento di Fisica, La Sapienza Università di Roma, Piazzale Aldo Moro 2, 00185 Roma,ItalylINAF-IAPS,Via del Fosso del Cavaliere 100, I-00133 Rome, ItalymASI, Agenzia Spaziale Italiana, Via del Politecnico snc, Roma, ItalynDepartment of Physics and Astronomy, University College London, Gower Street, LondonWC1E 6BT, UKABSTRACTThe 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 exoplan-etary atmospheres on a target of ∼ 1000 exoplanets. Ariel scientific payload consists of an off-axis, unobscuredCassegrain telescope. The light is directed towards a set of photometers and spectrometers with wavebandsbetween 0.5 and 7.8 µm and operating at cryogenic temperatures. The Ariel Space Telescope consists of a pri-mary parabolic mirror with an elliptical aperture of 1.1· 0.7 m, followed by a hyperbolic secondary, a paraboliccollimating tertiary and a flat-folding mirror directing the output beam parallel to the optical bench; all inbare 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, analuminium mirror the size of Ariel’s primary has never been made. The greatest challenge is finding a heatFurther author information:E-mail: elisa.guerriero@inaf.itSpace Telescopes and Instrumentation 2022: Optical, Infrared, and Millimeter Wave, edited by Laura E. Coyle, Shuji Matsuura, Marshall D. Perrin, Proc. of SPIE Vol. 12180, 1218014 · © 2022 SPIE · 0277-786X · doi: 10.1117/12.2628178Proc. of SPIE Vol. 12180  1218014-1Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 16 Nov 2022Terms of Use: https://www.spiedigitallibrary.org/terms-of-usetreatment procedure that stabilizes the aluminium, particularly the Al6061T651 Laminated alloy. This paperdescribes the study and testing of the heat treatment procedure developed on aluminium samples of differentsizes (from 50mm to 150mm diameter), on 0.7m diameter mirror, and discusses future steps.Keywords: space telescope, Ariel mission, aluminium mirror, bare aluminium, Al6061T651, heat treatment,thermal cycle, laminated1. INTRODUCTIONFigure 1. Artistic picture of Ariel. Credit: ESA, STFC, RAL Space, UCL, Europlanet-Science Office.Ariel is the M4 mission adopted in 2020 by ESA as part of the ”Cosmic Vision Program”. Its launch isscheduled for 2029 (see Fig 1). The questions that the mission will seek to answer are; what exoplanets are madeof and how planetary systems formed and evolved. To do this, Ariel will observe ∼ 1000 Neptunian exoplanetsand Super-Earths, studying their atmosphere. The observations will take place in spectroscopy and photometryand will cover a range from 0.5µm to 7.8µm.1Ariel’s payload consists of a Cold Payload Unit (PLM) with an afocal, unobscured Cassegrain-type telescopeinside. The PLM is passively cooled down to ∼55K and, through an active cooling system, can go <42K whenneeded.2Optics and optical bench are entirely in Al6061T651 aluminium to maximize heat exchange and to have thesame thermo-mechanical properties between the parts.A mirror the size of Ariel’s M1 (1.1·0.7m) has never been made of aluminium for the space. Therefore,there are no already consolidated manufacturing processes. Heat treatment is essential to stabilize a low-densitymaterial such as aluminium and make it mechanically workable.2. ALUMINIUM RISKS OVERVIEWSome technology development activities (TDA) was run during the project phase B to face and solve the tech-nology risks related to the Ariel telescope aluminium mirrors and bringing the technology readiness level (TRL)of the M1 to 6.Proc. of SPIE Vol. 12180  1218014-2Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 16 Nov 2022Terms of Use: https://www.spiedigitallibrary.org/terms-of-useThese activities highlighted the main problems of the material, in particular the Al6061T651 Laminatedaluminium of which M1 is made. The low density (2.7g/cm3) makes it light and easily workable, but for thatalso malleable and elastic. Futhermore, the laminated alloy has internal defects that compromise the polishingof the surface.2.1 Stress DeformationThe primary mirror is subjected to various kinds of stress throughout its manufacturing and putting into orbitprocess:• the processing environment is at 1g; therefore, it is affected by its own weight (∼160Kg);• launch operations stress the entire telescope up to 10g• the same mechanical processes (machining, diamond turning, and polishing) add further stress while work-ing the surface.Stress, for low-density materials, tends to accumulate inside the machined piece and is released over time,modifying its shape and roughness; the result is that the mirror becomes out of specification. Therefore, the alloyneeds to be tempered through a series of thermal cycles to be repeated after each processing. The treatmentallows both to release the stress of the previous process (in a controlled environment) and to harden the alloyfrom time to time.2.2 Si-Mg AggregatesAriel’s primary mirror requires the use of Al6061T651 Laminated aluminium. Only the laminated processingallows creates blocks larger than 1.2m. There are other methods to produce the aluminium Al6061T651 whosedifferent production makes the alloy better to realize mirrors, but it is not currently possible to create largeblocks; e.g., for the RSA the overall billet dimensions are 57cm in diameter by 15.8cm thick, too little for M1.However, the alloy produced by the rolling process has a strong presence of Si-Mg aggregates with dimensionsto ∼1µm (see Fig 2). These affect the polishing of the optical surface. An aggressive polishing to improve theshape can tear off the aggregates leaving holes that generate a diffuse opacity over the entire surface and amarked increase in the roughness value (See Fig 3).Figure 2. Metallographic analysis of a sample of aluminium Al6061T65, with a scale of 40µm. The bright areas areaggregates of Al and Fe, while the dark areas are aggregates of Mg, Si, and O.Aggregates are generated in the alloy because the rolling procedure requires heating the raw aluminium blockto 400◦C before each pass in the roll. The slow temperature variation allows the elements in solution with theProc. of SPIE Vol. 12180  1218014-3Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 16 Nov 2022Terms of Use: https://www.spiedigitallibrary.org/terms-of-useFigure 3. Normasky image obtained with the electron microscope and 5X magnification.The holes are due to thedetachment of the aggregates.aluminium to re-aggregate in structures, mainly Si, Mg and Fe. This defect is therefore intrinsic to the laminatedalloy. It must be considered in the development of the heat treatment if we want to improve the polishing of theM1 mirror.It was decided to perform a Solution treatment at 530◦C to bring the aggregates back into the solution inthe alloy.3. HEAT TREATMENT SETUPThe developed heat treatment is based on the recipe developed by the NASA/Goddard Space Flight Center3and is distributed throughout the mirror production flow.The whole heat treatment can be divided into three blocks (see Fig 4):• the reproduction of the T6 process of the alloy with Solution treatment with Rapid quench andAgeing;• the hardening of the alloy with the Uphill quench;• the Cold thermal cycles before and after processing the optical surface and the Figure test.3.1 Solution Treatment, Rapid Quench and AgeingThe mirror receives the first treatments after the Rough Machining, in which the shape of the surface withallowance is given, and the lightening pockets are made.The Solution treatment and the Quench in water are made in the same process. The mirror is brought to atemperature of 530◦C, maintained for 90min. After 90min, the furnace is opened to allow water to enter andgets the temperature to 29/35◦C in 15s.This treatment reduces the quantity and size of Si-Mg aggregates inside the rolled aluminium. The aggre-gates at temperatures above 500◦C, in fact, dissolve and return to the solution with the alloy. The solution issubsequently frozen with the rapid cooling of the material, fixing the alloy.Ageing is performed at 175◦C in the furnace for 500min to stabilize the mirror after the Quench in water.Proc. of SPIE Vol. 12180  1218014-4Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 16 Nov 2022Terms of Use: https://www.spiedigitallibrary.org/terms-of-useFigure 4. Manufacturing flow of the Ariel telescope mirrors based on Al 6061-T6513.2 Uphill QuenchThe Uphill quench is a cold transfer with thermal shock. The treatment involves cooling from 23◦C to -190◦Cusing liquid nitrogen. Once the temperature has stabilized, the mirror is immersed in another chamber withdistilled water boiling at 100◦C. The process hardens the material and releases internal stresses.Once the Uphill quench is finished, the mirror is subjected to Final machining in which the stock is removed,corroded by the Rapid quench in water.3.3 Thermal Cycles and Figure TestThe Thermal cycles are used to release the stress accumulated by the pressure exerted by mechanical processing.Cold thermal cycles are repeated two times during mirror manufacturing. The first time after Final machining,and the second time after Polishing.The two thermal cycles are performed as follows.3 cryogenic thermal cycles -190/+150◦C:• Cooling to -190◦C (rates <1.7◦C/min) from room temperature (with liquid nitrogen);• Keeping at -190◦C for 30 minutes;• Heating at room temperature (rates <1.7◦C/min);• Keeping at room temperature for 15 minutes;• Heating at 150◦C (rates <1.7◦C/min);• Keeping at 150◦C for 30 minutes;• Cooling at room temperature (rates <1.7◦C/min).Proc. of SPIE Vol. 12180  1218014-5Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 16 Nov 2022Terms of Use: https://www.spiedigitallibrary.org/terms-of-useThe Figure test consists of 3 cryogenic thermal cycles from 20◦C to -223◦C through liquid oxygen.After the Diamond turning, the mirror surface has a reflectivity that can be measured at the interferometerto take the Surface error (SFE). By comparing these values, especially those of the second Thermal cycle andthe Figure test, if these are very similar, it can be said that the mirror is stable.4. HEAT TREATMENT APPLICATIONThe heat treatment was first applied to disks with diameters from 50mm to 150mm, to understand how to handlealuminium and what to expect between one treatment and the next. Subsequently, it was applied to a second setof prototype mirrors, two breadboards (BB) with a circular shape instead of elliptical with a diameter of 0.7m.This is the maximum size of the available diamond turning machine (LT Ultra Precision, model MTC650). Themachine for the larger elliptical mirror is still under development, and it will be ready to manufacture the M1Engineering Qualification Model (EQM) (LT Ultra Precision, model MTC1200).4.1 DisksThe disks were made from a block representative of those of the BBs. They were designed to simulate heattreatment on a small scale and develop a polishing recipe (in Media Lario S.r.l. agency). The heat treatmentis considered representative of the M1, due to the similar thickness between the discs and the primary mirrorwith the lightening (∼19mm). Therefore, they have the same heat penetration inside them. It was possible tolearn how to handle aluminium before and after the treatments so as not to damage the surface and quantifythe necessary excess metal ∼0.5mm (see Fig 5).Figure 5. At left, disks with limescale deposit marks, footprints, and grid marks; at right, same disks after the Finalmachining and Diamond turning process, removing 0.5mm of excess metal.4.2 BreadBoards MirrorThe two M1 breadboards are following different development tasks because it is needed for the Ariel programto obtain a preliminary technology assessment by the Payload Preliminary design review (PDR) (on September2022), that will be completed by the Instrument PDR (on January 2023).BB1 is needed to mitigate the risk on the overall manufacturing process and the performance of a large-sizeAl mirror; it will follow the same manufacturing process, as Ariel M1, including the heat treatment.The mirror after the Diamond turning has a surface roughness Sq=10nm RMS and a shape error SFE=838nmRMS (see Fig 6). The specification of M1 is Sq <10nm RMS and SFE<80nm RMS. The polishing will have towork mainly on the shape, trying not to deteriorate the roughness. The heat treatment is expected to dissolvethe aggregates of the alloy, making this process easier.Proc. of SPIE Vol. 12180  1218014-6Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 16 Nov 2022Terms of Use: https://www.spiedigitallibrary.org/terms-of-useBB2 is needed to de-risk the manufacturing processes, Rough machining, Diamond turning, and Polishingwithout the heat treatment. Furthermore, it will be a means of comparison to quantify the effectiveness of thedeveloped heat treatment.The mirror after the Diamond turning has a surface roughness Sq=7nm RMS and SFE=348nm RMS (seeFig 6). Difficulty in achieving form specifications is expected due to undissolved aggregates.Figure 6. SFE interferometric measure of the BB1, at left , and BB2, at right.5. CONCLUSIONS AND NEXT STEPSThe values obtained from the two BBs after the Diamond turning do not state that the processing is better inthe BB2. The Diamond turning can bring an aluminium mirror to Sq ∼ 10nm RMS and SFE< 1µm values. Thefinal check on the effectiveness of the heat treatment on aluminium will be with polishing.The mirrors will be polished between July and September 2022, primarily, the BB2, which is the de-riskingmirror of the process, and then the BB1. Machining to shape specification of the BBs is expected to be faster inthe BB1 than in the BB2, due to the solubilization of the aggregates and the tempering of the mirror. The SFEand Sq final values should be better in BB1 than in BB2. Once this result has been verified, it will be possibleto assign TRL6 to the payload PDR to the heat treatment.Once TRL6 is reached, it will be possible to apply the process to the EQM of the actual dimensions of Ariel’sprimary mirror scheduled in 2023.6. ACKNOWLEDGMENTSThis activity has been realized under the Implementation Agreement n. 2021-5-HH.0 of the Italian Space Agency(ASI) and the National Institute for Astrophysics (INAF) Framework Agreement “Italian Participation to Arielmission phase B2/C” and under the ASI contract n. 2021-21-I.0 “ARIEL TA Phase B Industrial Activities” toLeonardo S.p.A and Media Lario s.r.l. companies.REFERENCES[1] Tinetti, G. and et al, “A chemical survey of exoplanets with ariel,” Experimental Astronomy 46, 135–209(November 2018).[2] Deppo, V. D., Middleton, K., Focardi, M., Morgante, G., Claudi, R., Pace, E., and Micela, G., “The opticalconfiguration of the telescope for the ariel esa mission,” Proc. SPIE 10698, 40 (2018).[3] Ohl IV, Raymond G and al., “Comparison of Stress Relief Procedures for Cryogenic Aluminum Mirrors,” in[Cryogenic Optical Systems and Instruments IX SPIE ], 4822 (2002).Proc. of SPIE Vol. 12180  1218014-7Downloaded From: https://www.spiedigitallibrary.org/conference-proceedings-of-spie on 16 Nov 2022Terms of Use: https://www.spiedigitallibrary.org/terms-of-use

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

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

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