.We describe the main tasks of the Product Assurance process for the Telescope Optical Unit (TOU) of the ESA PLATO mission, that starts from the design phase and proceeds through all phases, up to the final product, with the aim of improving the likelihood of success of the mission. When dealing with the opto-mechanical components of the TOU, several aspects regarding safety and performance have to be analyzed and tracked. From the PA point of view, we focus in this paper on materials and processes selection that shall be suitable and robust enough for the space environment. Cleanliness and contamination control is needed to overcome loss of optical performance. Validations and qualifications on prototypes is fundamental to assess the reliability of the instrument for its purpose and for the lifetime of the mission.
PLATO (PLAnetary Transits and Oscillation of stars) is the ESA Medium size dedicated to exo-planets discovery, adopted in the framework of the Cosmic Vision program. The PLATO launch is planned in 2026 and the mission will last at least 4 years in the Lagrangian point L2. The primary scientific goal of PLATO is to discover and characterize a large amount of exo-planets hosted by bright nearby stars, constraining with unprecedented precision their radii by mean of transits technique and the age of the stars through by asteroseismology. By coupling the radius information with the mass knowledge, provided by a dedicated ground-based spectroscopy radial velocity measurements campaign, it would be possible to determine the planet density. Ultimately, PLATO will deliver the largest samples ever of well characterized exo-planets, discriminating among their ‘zoology’. The large amount of required bright stars can be achieved by a relatively small aperture telescope (about 1 meter class) with a wide Field of View (about 1000 square degrees). The PLATO strategy is to split the collecting area into 24 identical 120 mm aperture diameter fully refractive cameras with partially overlapped Field of View delivering an overall instantaneous sky covered area of about 2232 square degrees. The opto-mechanical sub-system of each camera, namely Telescope Optical Unit, is basically composed by a 6 lenses fully refractive optical system, presenting one aspheric surface on the front lens, and by a mechanical structure made in AlBeMet.
The exoplanet revolution is well underway. The last decade has seen order-of-magnitude increases in the number of known planets beyond the Solar system. Detailed characterization of exoplanetary atmospheres provide the best means for distinguishing the makeup of their outer layers, and the only hope for understanding the interplay between initial composition chemistry, temperature-pressure atmospheric profiles, dynamics and circulation. While pioneering work on the observational side has produced the first important detections of atmospheric molecules for the class of transiting exoplanets, important limitations are still present due to the lack of systematic, repeated measurements with optimized instrumentation at both visible (VIS) and near-infrared (NIR) wavelengths. It is thus of fundamental importance to explore quantitatively possible avenues for improvements. In this paper we report initial results of a feasibility study for the prototype of a versatile multi-band imaging system for very high-precision differential photometry that exploits the choice of specifically selected narrow-band filters and novel ideas for the execution of simultaneous VIS and NIR measurements. Starting from the fundamental system requirements driven by the science case at hand, we describe a set of three opto-mechanical solutions for the instrument prototype: 1) a radial distribution of the optical flux using dichroic filters for the wavelength separation and narrow-band filters or liquid crystal filters for the observations; 2) a tree distribution of the optical flux (implying 2 separate foci), with the same technique used for the beam separation and filtering; 3) an 'exotic' solution consisting of the study of a complete optical system (i.e. a brand new telescope) that exploits the chromatic errors of a reflecting surface for directing the different wavelengths at different foci. In this paper we present the first results of the study phase for the three solutions, as well as the results of two laboratory prototypes (related to the first two options), that simulate the most critical aspects of the future instrument.
PLATO stands for PLAnetary Transits and Oscillation of stars and is a Medium sized mission selected as M3 by the
European Space Agency as part of the Cosmic Vision program. The strategy behind is to scrutinize a large fraction of the
sky collecting lightcurves of a large number of stars and detecting transits of exo-planets whose apparent orbit allow for
the transit to be visible from the Earth. Furthermore, as the transit is basically able to provide the ratio of the size of the
transiting planet to the host star, the latter is being characterized by asteroseismology, allowing to provide accurate
masses, radii and hence density of a large sample of extra solar bodies. In order to be able to then follow up from the
ground via spectroscopy radial velocity measurements these candidates the search must be confined to rather bright stars.
To comply with the statistical rate of the occurrence of such transits around these kind of stars one needs a telescope with
a moderate aperture of the order of one meter but with a Field of View that is of the order of 50 degrees in diameter. This
is achieved by splitting the optical aperture into a few dozens identical telescopes with partially overlapping Field of
View to build up a mixed ensemble of differently covered area of the sky to comply with various classes of magnitude
stars. The single telescopes are refractive optical systems with an internally located pupil defined by a CaF2 lens, and
comprising an aspheric front lens and a strong field flattener optical element close to the detectors mosaic. In order to
continuously monitor for a few years with the aim to detect planetary transits similar to an hypothetical twin of the Earth,
with the same revolution period, the spacecraft is going to be operated while orbiting around the L2 Lagrangian point of
the Earth-Sun system so that the Earth disk is no longer a constraints potentially interfering with such a wide field
continuous uninterrupted survey.
The project PLAnetary Transits and Oscillations of stars (PLATO) is one of the selected medium class (M class)
missions in the framework of the ESA Cosmic Vision 2015-2025 program. The main scientific goal of PLATO is the
discovery and study of extrasolar planetary systems by means of planetary transits detection.
According to the current baseline, the scientific payload consists of 34 all refractive telescopes having small aperture
(120mm) and wide field of view (diameter greater than 37 degrees) observing over 0.5-1 micron wavelength band. The
telescopes are mounted on a common optical bench and are divided in four families of eight telescopes with an
overlapping line-of-sight in order to maximize the science return. Remaining two telescopes will be dedicated to support
on-board star-tracking system and will be specialized on two different photometric bands for science purposes.
The performance requirement, adopted as merit function during the analysis, is specified as 90% enclosed energy
contained in a square having size 2 pixels over the whole field of view with a depth of focus of +/-20 micron. Given the
complexity of the system, we have followed a Montecarlo analysis approach for manufacturing and alignment
tolerances. We will describe here the tolerance method and the preliminary results, speculating on the assumed risks and
Thermal effects in PLATO are analyzed in terms of uniform temperature variations, longitudinal and lateral temperature gradients. We characterize these effects by evaluating the PSF centroid shifts and the Enclosed Energy variations across the whole FoV. These patterns can then be used to gauge the thermal behavior of each individual telescope in order to improve the local photometric calibration across the PLATO field of view.
The project PLAnetary Transits and Oscillations of stars (PLATO) is one of the selected medium class (M class)
missions in the framework of the ESA Cosmic Vision 2015-2025 program. The mean scientific goal of PLATO is the
discovery and study of extrasolar planetary systems by means of planetary transits detection. The opto mechanical
subsystem of the payload is made of 32 normal telescope optical units (N-TOUs) and 2 fast telescope optical units (FTOUs).
The optical configuration of each TOU is an all refractive design based on six properly optimized lenses. In the
current baseline, in front of each TOU a Suprasil window is foreseen. The main purposes of the entrance window are to
shield the following lenses from possible damaging high energy radiation and to mitigate the thermal gradient that the
first optical element will experience during the launch from ground to space environment. In contrast, the presence of the
window increases the overall mass by a non-negligible quantity. We describe here the radiation and thermal analysis and
their impact on the quality and risks assessment, summarizing the trade-off process with pro and cons on having or
dropping the entrance window in the optical train.
Hot slumping technology is under development by several research groups in the world for the realization of grazing-incidence segmented mirrors for x-ray astronomy, based on thin glass plates shaped over a mold at temperatures above the transformation point. The performed thermal cycle and related operations might have effects on the strength of the glass, with consequences for the structural design of the elemental optical modules and, consequently, on the entire x-ray optic for large astronomical missions such as IXO and ATHENA. The mechanical strength of glass plates after they underwent the slumping process was tested through destructive double-ring tests in the context of a study performed by the Astronomical Observatory of Brera with the collaboration of Stazione Sperimentale del Vetro and BCV Progetti. The entire study was done on more than 200 D263 Schott borosilicate glass specimens of dimensions 100 mm×100 mm and a thickness 0.4 mm, either flat or bent at a radius of curvature of 1000 mm through the pressure-assisted hot slumping process developed by INAF-OAB. The collected experimental data have been compared with nonlinear finite element model analyses and treated with the Weibull statistic to assess the current IXO glass x-ray telescope design, in terms of survival probability, when subjected to static and acoustic loads characteristic of the launch phase. The paper describes the activities performed and presents the obtained results.
The mirrors of the International X-ray Observatory (IXO) were based on a large number of high quality segments,
aiming at achieving a global spatial resolution better than 5” HEW while giving a large collecting area (around 3m2@ 1
keV). A study concerning the hot slumping of thin glass foils was started in Europe, funded by ESA and led by the Brera
Astronomical Observatory (INAF-OAB), for the development of a replication technology based on glass material. The
study is currently continuing even after the IXO program has been descoped and renamed ATHENA, in the perspective
of using the technology under development for other future missions or applications. INAF-OAB efforts have been
focused on the "Direct" slumping approach with convex moulds, meaning that during the thermal cycle the optical
surface of the glass is in direct contact with the mould surface. The single mirror segments are made of thin glass plates
(0.4 mm thick), with a reflecting area of 200 mm × 200 mm. The adopted integration process foresees the use of glass
reinforcing ribs for bonding together the plates in such a way to form a rigid and stiff stack of segmented mirror shells;
the stack is supported by a thick backplane. During the bonding process, the plates are constrained in close contact with
the surface of a precisely figured integration master by the application of vacuum pump suction. In this way, the springback
deformations and the low frequency errors still present in the plates' profile after slumping can be corrected. The
status of the technology development is presented in this paper, together with the description and metrology of the
prototypes already realized or under construction at the Observatory laboratories.