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The Einstein Probe (EP) is a mission of the Chinese Academy of Sciences (CAS) dedicated to time-domain high-energy astrophysics. Its primary goals are to discover high-energy transients and monitor variable objects. The ESA Science Programme Committee (SPC) approved on 19 June 2018 the participation of ESA to the CAS EP mission as a mission of opportunity. Among other elements, CAS has requested ESA participation for the provision of the mirror modules of the follow-up x-ray telescope (FXT).
FXT is a pair of Wolter-I telescopes operating in the 0.5-10 keV energy range, inheriting the design from eROSITA [2][3]. It provides field of view of about 1 deg diameter. The source localization error will be of 5-15 arcsec depending on the source strength [1]. The FXT is responsible for the quick follow-up observations of the triggered sources and will also observe other interested targets during the all-sky survey at the rest time.
Three FXT mirror modules were produced: structural and thermal model (STM), qualification model (QM) and flight model (FM). Media Lario could leverage on the manufacturing and integration infrastructure still available at its premises from the eROSITA programme [3][4], including the complete set of 54 mandrels needed for the mirror repliforming, property of MPE.
Media Lario produced and integrated the FXT mirror modules, each comprising 54 nested repliformed mirror shells; Max-Planck Institute (MPE) conducted the x-ray optical tests at the PANTER facility, for the acceptance of all the different models. This efficient collaboration enabled the on-time and in-quality delivery of the FXT mirror modules.
Leonardo Avionics & Space System Division is the prime contractor for the FLORIS Instrument for which Media Lario is manufacturing the QM unit of the spherical mirror included in the High-Resolution Spectrometer (HRSPE), hereafter called HRM mirror.
The High-Resolution Mirror is a 250-mm diameter spherical mirror with a radius of curvature of approximately 440 mm. For the mirror substrate, Leonardo has selected the Aluminium alloy AlSi40, a special alloy with 40% Silicon content, coated with a hard polishing layer of Nickel Phosphorus (NiP), deposited by electroless chemical process. The Silicon content allows this special Aluminium alloy to have the same coefficient of thermal expansion (CTE) of the NiP layer, therefore preventing thermal deformations deriving from the bimetallic effect. The mirror structure is light-weighted to approximately 2.8 kg. The required wave-front error of the mirror is better than 0.5 fringes PV, while the surface microroughness has been specified at 0.5 nm RMS due to stringent straylight requirements of the FLORIS instrument.
Media Lario has been selected for the mirror development phase because of their experience in the design and manufacturing of AlSi/NiP mirrors demonstrated in the development of the Earth Observation optical payload for small satellites (called STREEGO), based on an AlSi40 TMA telescope. The manufacturing process includes precision diamond turning, optical figuring and super-polishing. The optical coating will be done by Leonardo at their thin-films facility of Carsoli, Italy. Since the recipe prescribes to pre-heat the mirror surface at 100° C, Media Lario will qualify the mirror substrate with -25/+110°C thermal cycles to ensure adequate thermal stability for the coating process.
The payload is based on a 1-m class telescope ahead of a suite of instruments: two spectrometric channels covering the band 1.95 to 7.80 μm and four photometric channels working in the range 0.5 to 1.9 μm.
The production of the primary mirror (M1) is one of the main technical challenges of the mission. A trade-off on the material to be used for manufacturing the 1-m diameter M1 was carried out, and aluminium alloys have been selected as the baseline materials both for the telescope mirrors and structure. Aluminium alloys have demonstrated excellent performances both for IR small size mirrors and structural components, but the manufacturing and thermo-mechanical stability of large metallic optics still have to be demonstrated especially at cryogenic temperatures.
The ARIEL telescope will be realized on-ground (1 g and room temperature), but it shall operate in space at about 50 K. For this reason a detailed tolerance analysis was performed to assess the telescope expected performance.
M1 is an off-axis section of a paraboloidal mirror and will be machined from a single blank as a stand-alone part. To prove the feasibility of such a large aluminium mirror, a pathfinder mirror program has been started. The prototype has been realized and tested, so far at room temperature, by Media Lario S.r.l.. Cryogenic testing of the prototype will be performed during Phase B1.
Media Lario used their in-house coordinate measuring machine to adjust the surface during assembly, with the reflector panels facing upwards. As part of the Final Acceptance Review measurements of the surface were undertaken by LMT staff at the Media Lario factory, using both a laser tracker and photogrammetry. Measurements were also made of the electroforming mold for the central panel. The reflector was mounted on a rotating stand allowing surface measurements to be performed according to the respective gravitational load cases. Measurements at the Media Lario factory provided a useful reference for repeat data taken at the LMT site, since the reflector was shipped as a fully assembled unit, designed to require no further adjustment after leaving the factory.
In this paper we present the surface measurements conducted during the review, and comparisons of the observed gravitational load deformations with those predicted by FEA. Although the latter were often at the level of measurement uncertainty, we were able to verify specific cases, as well as performing a sanity check on the manufacturer's design analysis. The measurements confirmed final surface error values leading to reflector acceptance by the project. An RMS surface error of the order of 25 microns over the entire reflector was recorded at 60 degrees elevation using photogrammetry data after adjusting to the best-fit parabola, showing compliance with the LMT specification. Acceptance review measurements also provided a baseline for surface measurements at site prior to installation.
ARIEL has been selected by the European Space Agency (ESA) as the next medium-class science mission (M4) to be launched in 2028. The aim of the ARIEL mission is to study the atmospheres of a selected sample of exoplanets.
The payload is based on a 1-m class telescope ahead of a suite of instruments: two spectrometric channels covering the band 1.95 to 7.80 μm without gaps, three photometric channels working in the range 0.5 to 1.2 μm, and a low-resolution spectrometer in the range 1.25 to 1.95 μm.
The telescope layout is conceived as an eccentric pupil two-mirror classic Cassegrain configuration coupled to a tertiary off-axis paraboloidal mirror. The telescope will be realized on-ground, i.e. subjected to gravity and at room temperature, but it shall operate in space, at 0 g, and at a temperature of about 50 K. For this reason, the telescope expected “as-built” in-flight performance has to be determined via a detailed thermo-elastic analysis.
A trade-off on the material to be used for manufacturing the 1-m diameter primary mirror (M1) was carried out, and aluminum alloys have been selected as the baseline materials for both the telescope mirrors and structure.
The use of metals, like aluminum alloys, is nowadays frequently considered for the fabrication of space telescopes observing in the infrared wavelength range. Small-size aluminum parts have been proved to be popular both for IR mirrors and structural components, but the manufacturing and stability of large metallic optics still have to be demonstrated. The production of a large aluminum mirror such as that of ARIEL is a challenge, and to prove its feasibility a dedicated study and development program has been started. A prototype, with the same size of the M1 flight model but a simpler surface profile, has been realized and tested.
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