The PLAnetary Transits and Oscillations of stars mission (PLATO) is an ESA M3 mission planned to detecting and characterizing extrasolar planetary systems as Earth-like exoplanets orbiting around the habitable zone of bright solartype stars. PLATO consists of 26 cameras (CAM) mounted on the same instrument platform in order to cover a large field of view (FoV) with the highest possible photon detection statistics. Each PLATO CAM consists of a telescope Optical Unit (TOU), the FPA, and the detector read-out Front End Electronics (FEE). The FPA is the structure located at the focal plane position of the CAM that supports four identical CCDs and the mechanical interface parts to match with the TOU and FEE. Due to the critical repeatability aspect of the mission, each FPAs must be identical with very stringent specifications which includes strict opto-mechanical positioning tolerances. Also the number of FPAs that have to be manufactured, integrated and tested at the same time requires a special space industrialization process and an optimized metrology verification due to the very restrictive design and schedule constraints. In order to solve this challenge a flight-representative QM has been developed in order to validate a manufacturing, assembly, integration and verification (AIV) on-ground processes. As well, an innovative metrology system has being developed for improving the alignment and verification under the tightly AIV requirements before, during and after a proper qualified campaign in a very demanding environment. INTA has adapted into an ISO6 cleanroom facility a high accuracy and vast performance non-contact CNC vision dimensional measuring system, and has developed a Ground Support Equipment (GSE) for a real-time alignment step in order to reach that requirements.
PLATO is an exoplanet hunting mission from the European Space Agency. It is a medium-class mission, with a launch foreseen in 2026. Its prime objective is to uncover Earth-sized planets residing in the habitable zone of their host star. The payload consists of 26 cameras with a very wide field-of-view. While the operational temperature of the cameras will be -80°C, the focal plane of each camera will be integrated with its telescope assembly (bearing the optics) at room temperature. The degradation of the optical quality at ambient, combined with the detector dark current and with the very high accuracy required from the alignment process bring a number of interesting challenges. In the present article, we review the alignment concept, present optical simulations of the measurements at ambient along with their analysis, and present an error budget for the optical measurements. The derivation of this error budget is easily applicable to all optical measurements to be performed during the alignment, i.e. the definition of the best image plane at the operational temperature and the optical alignment itself, at room temperature.
PLATO, PLAnetary Transits and Oscillation of stars, is an ESA mission mainly devoted to survey the Galaxy searching for and characterizing Earth-like exoplanets, and their host stars. This will be achieved using continuous and extremely accurate photometry for both exoplanetary transits and asteroseismology analysis. Current design plans to mount 26 cameras in the same instrument bench in order to cover a large field of view with the highest possible photon statistics. Each PLATO camera consists of the telescope (TOU, Telescope Optical Unit), the focal plane assembly (FPA), and the detector and camera read out electronics (FEE). Four CCDs (Charge Coupled Devices) will be included in each FPA, which implies a really delicate assembly and integration verification (AIV) process due to the stringent scientific requirements breakdown into hard engineering ones (among others, CCDs co-alignment in terms of tip and tilt and roll with respect to the optical axis). In the following lines, the FPA current opto-mechanical design is briefly presented and an integration process conceptual proposal is reported on, discussing the error budgets associated to the main requirements to be verified during FPAs AIV, and the main results obtained during the prototype first AIV round.
Due to the difficulty in studying the upper layer of the troposphere by using ground-based instrumentation, the conception of a space-orbit atmospheric LIDAR (ATLID) becomes necessary. ATLID born in the ESA’s EarthCare Programme framework as one of its payloads, being the first instrument of this kind that will be in the Space. ATLID will provide vertical profiles of aerosols and thin clouds, separating the relative contribution of aerosol and molecular scattering to know aerosol optical depth. It operates at a wavelength of 355 nm and has a high spectral resolution receiver and depolarization channel with a vertical resolution up to 100m from ground to an altitude of 20 km and, and up to 500m from 20km to 40km. ATLID measurements will be done from a sun-synchronous orbit at 393 km altitude, and an alignment (co-alignment) sensor (CAS) is revealed as crucial due to the way in which LIDAR analyses the troposphere. As in previous models, INTA has been in charge of part of the ATLID instrument co-alignment sensor (ATLID-CAS) electro-optical characterization campaign. CAS includes a set of optical elements to take part of the useful signal, to direct it onto the memory CCD matrix (MCCD) used for the co-alignment determination, and to focus the selected signal on the MCCD. Several tests have been carried out for a proper electro-optical characterization: CAS line of sight (LoS) determination and stability, point spread function (PSF), absolute response (AbsRes), pixel response non uniformity (PRNU), response linearity (ResLin) and spectral response. In the following lines, a resume of the flight model electrooptical characterization campaign is reported on. In fact, results concerning the protoflight model (CAS PFM) will be summarized. PFM requires flight-level characterization, so most of the previously mentioned tests must be carried out under simulated working conditions, i.e., the vacuum level (around 10-5 mbar) and temperature range (between 50°C and -30°C) that are expected during ATLID Space operation.
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