On-board the Solar Orbiter ESA/NASA mission there is Metis, a coronagraph designed to study the solar corona by providing an artificial solar eclipse. Metis features two channels working at the ultraviolet Lyman-α (121.6 nm) and in the visible light (580-640 nm). On-ground, the Metis radiometric performance has been tested using a flat-field panel (uniform illumination); the stability of the performance can be verified in-flight through the analysis of the stars passing in the Metis Field of View. Care must be taken to ensure the quality of the calibration, both before launch and for the long period associated with the space mission lifetime. For this reason, we are carrying out long period research of stars that cross the Field of View of Metis. In this paper, we describe the vignetting function acquired: on-ground, simulated via a raytracing code and in-flight derived from on-ground measurements (performing some adjustments to account for the real Metis flight configuration). These vignetting functions are then compared with the vignetting data derived from the passage of the star Theta Ophiuchi in March and December 2021. Additional presentation content can be accessed on the supplemental content page.
Metis is the coronagraph on board the Solar Orbiter ESA/NASA mission, it is designed to study the solar corona by providing an artificial solar eclipse. Metis features two channels: the ultraviolet H I (121.6 nm) and the visible light (580-640 nm). This work is focalised on the latter. Radiometric performances have been tested on-ground using a flatfield panel (uniform illumination), and the in-flight stability can be verified through the light reflected from the instrument door. When the Sun light impacts on the spacecraft shield, a fraction is reflected in the direction of the door, which then partly reflects it inside Metis. The analysis of the door images confirms its integrity and that of its subsequent optical components, since the reflected intensity follows as expected a 1/r2 law, r being the Sun-spacecraft distance. Further analysis is being performed on such images to verify the operating status of various elements of Metis. Complementary ray-tracing simulation studies on the door retro-reflectivity properties are also in progress.
During the last two decades, Liquid Crystal Variable Retarder (LCVR) technology has matured and advanced as reliable and well-understood technology for ground applications to the point of being recently integrated in space-based optical instruments for the first time. LCVR cells use nematic liquid crystals to electronically tune the birefringence of the device in order to control the polarization of the transmitted light. The possibility to modulate the light polarization by means of an applied voltage offers the advantage of replacing the conventional rotary mechanisms, dedicated to carry the polarizing optics. Consequently, LCVR cells represent an excellent electro-optic solution to include in the design of space instruments where polarized light modulation is necessary. However, to validate the applicability of a LCVR cell to a space mission it is imperative to test its survivability in its exposure to conditions representative of the space environment. In this article, we summarize the activities performed to test the survivability of two commercial LCVR samples after their exposure to space-like environment for radiation and we report the result obtained by analyzing the radiation dose impact on the cell performances. The under-test samples have been produced by Meadowlark Optics Inc and designed for operation at 547 nm. We exposed the cells to multiple levels of gamma radiation dose, measuring their response time after each dose. To verify the impact of the accumulated radiation dose on the optical performances of the LCVR, we chose as indicators the retardance versus voltage, the transmission, and the response time. We measured these quantities before and after the whole test campaign and compared the two datasets to verify if gamma rays introduced any alterations in cell performances.
Metis coronagraph is one of the remote-sensing instruments of the Solar Orbiter mission launched in February 2020. The mission profile will allow for the first time the remote-sensing observation of the Sun from as close as 0.28 AU and from ecliptic latitudes as high as 30?. Metis, in particular, is aimed at the study and the overall characterization of the solar corona and solar wind. This instrument is an innovative inverted-occultation coronagraph that will image the solar corona for the first time simultaneously in two different wavelength band-passes: in the linearly-polarized visible-light (VL), between 580 and 640 nm, and in the ultraviolet (UV) Lyman-a line of hydrogen, HI at 121.6 nm by combining in the same telescope UV interference mirror coatings (Al/MgF2) and spectral bandpass filters. The visible channel includes a broad-band polarimeter to observe the linearly polarized component of the K corona. These measurements will allow a complete characterization of the physical parameters, such as density and outflow speed, of the two major plasma components of the corona and the solar wind: electrons (protons) and hydrogen. After a period of commissioning, by the summer of 2020, Metis will have performed the First-light Science Observations during the “Remote-Sensing Check-out Window” (RSCW) that is a telemetry contact period, specifically allocated before entering the operational phase at the end of 2021. This presentation will report the first-light science observations of Metis represented by the UV and polarized VL images of the corona. The calibration results from the commissioning will be used for the correction of the instrumental effects. The resulting first-light maps of the coronal electron and hydrogen distributions will be presented.
Metis coronagraph is one of the remote-sensing instruments of the Solar Orbiter mission launched at the begin of 2020. The mission profile will allow for the first time the remote-sensing observation of the Sun from a very close distance and increasing the latitude with respect to the ecliptic plane. In particular, Metis is aimed at the overall characterization and study of the solar corona and solar wind. Metis instrument acquires images of the solar corona in two different wavelengths simultaneously; ultraviolet (UV) and visible-light (VL). The VL channel includes a polarimeter with an electro-optically modulating Liquid Crystal Variable Retarder (LCVR) to measure the linearly polarized brighness pB) of the K-corona. This paper presents part of the in-flight calibration results for both wavelength channels together with a comparison with on-ground calibrations. The orientation of the K-corona linear polarization was used for the in-flight calibration of the Metis polarimeter. This paper describes the correction of the on-ground VL vignetting function after the in-flight adjustment of the internal occulter. The same vignetting function was adaptated to the UV channel.
PROBA-3 (PRoject for OnBoard Autonomy) is an ESA mission to be launched on beginning of 2023 where a spacecraft is used as an external occulter (OSC-Occulter Spacecraft), to create an artificial solar eclipse as observed by a second spacecraft, the coronagraph (CSC-Coronagraph Spacecraft). The two spacecrafts (SCs) will orbit around the Earth, with a highly elliptic orbit (HEO), with the perigee at 600 km, the apogee at about 60530 km and an eccentricity of ≈ 0.81. The orbital period is of 19.7 hours and the precise formation flight (within 1 mm) will be maintained for about 6 hours over the apogee, in order to guarantee the observation of the solar corona with the required spatial resolution. The relative alignment of the two spacecrafts is obtained by combining information from several subsystems. One of the most accurate subsystems is the Shadow Position Sensors (SPS), composed of eight photo-multipliers installed around the entrance pupil of the CSC. The SPS will monitor the penumbra generated by the occulter spacecraft, whose intensity will change according to the relative position of the two satellites. A dedicated algorithm has been developed to retrieve the displacement of the spacecrafts from the measurements of the SPS. Several tests are required in order to evaluate the robustness of the algorithm and its performances/results for different possible configurations. A software simulator has been developed for this purpose. The simulator includes the possibility to generate synthetic 2-D penumbra profile maps or analyze measured profiles and run different versions of the retrieving algorithms, including the “on-board” version. In order to import the “as-built” algorithms, the software is coded using Matlab. The main aspects of the simulator, such as the results of the simulations, with the inclusion of some specific case studies, will be reported and discussed in this paper.
Metis is the visible light and UV light imaging coronagraph on board the ESA-NASA mission Solar Orbiter that has been launched February 10th, 2020, from Cape Canaveral. Scope of the mission is to study the Sun up close, taking high-resolution images of the Sun’s poles for the first time, and understanding the Sun-Earth connection. Metis coronagraph will image the solar corona in the linearly polarized broadband visible radiation and in the UV HI Ly-α line from 1.6 to 3 solar radii when at Solar Orbiter perihelion, providing a diagnostics, with unprecedented temporal coverage and spatial resolution, of the structures and dynamics of the full corona. Solar Orbiter commissioning phase big challenge was Covid-19 social distancing phase that affected the way commissioning of a spacecraft and its payload is typically done. Metis coronagraph on-board Solar Orbiter had its additional challenges: to wake up and check the performance of the optical, electrical and thermal subsystems, most of them unchecked since Metis delivery to spacecraft prime, Airbus, in May 2017. The roadmap to the fully commissioned coronagraph is here described throughout the steps from the software functional test, the switch on of the detectors of the two channels, UV and visible, to the optimization of the occulting system and the characterization of the instrumental stray light, one of the most challenging features in a coronagraph.
Metis is a multi-wavelength coronagraph onboard the European Space Agency (ESA) Solar Orbiter mission. The instrument features an innovative instrument design conceived for simultaneously imaging the Sun's corona in the visible and ultraviolet range. The Metis visible channel employs broad-band, polarized imaging of the visible K-corona, while the UV one uses narrow-band imaging at the HI Ly , i.e. 121.6 nm. During the commissioning different acquisitions and activities, performed with both the Metis channels, have been carried out with the aim to check the functioning and the performance of the instrument. In particular, specific observations of stars have been devised to assess the optical alignment of the telescope and to derive the instrument optical parameters such as focal length, PSF and possibly check the optical distortion and the vignetting function. In this paper, the preliminary results obtained for the PSF of both channels and the determination of the scale for the visible channel will be described and discussed. The in-flight obtained data will be compared to those obtained on-ground during the calibration campaign.
The PROBA3 mission of the European Space Agency is the first formation flying (FF) mission that will be flown in high elliptic geocentric orbit aiming at verifying and validating different metrology control systems and algorithms in order to realize and maintain the formation of two independent spacecraft, in total autonomy. The final target accuracy for the relative and absolute alignment of the two satellites is of about 2mm over an inter satellite distance of 144.3m. During the FF, the two spacecraft will realize a giant coronagraph with the external occulter on one payload and the telescope on the other one. The Sun Corona observation will be the scientific tool for the FF validation. Between the different metrology systems that will be tested, the Shadow Position Sensor (SPS) is the most challenging one, aiming at returning the relative and absolute position of the formation with the finest accuracy: 0.5mm out of the guidance and navigation and control loop and 2mm within the loop. The mission program is now in the Phase D with the realization and the testing of the flight model. Due to the high expected performance, a fine calibration of the SPS subsystem is mandatory. In this paper, we discuss the radiometric and spectral calibration plan, the algorithm validation procedure, and the laboratory test-bed realized to reproduce the in-flight observation conditions of the SPS by using a set of calibrated LED and a mechanical set-up equivalent to the SPS system. Preliminary results are also reviewed.
Solar Orbiter, launched on February 9th 2020, is an ESA/NASA mission conceived to study the Sun. This work presents the embedded Metis coronagraph and its on-ground calibration in the 580-640 nm wavelength range using a flat field panel. It provides a uniform illumination to evaluate the response of each pixel of the detector; and to characterize the Field of View (FoV) of the coronagraph. Different images with different exposure times were acquired during the on-ground calibration campaign. They were analyzed to verify the linearity response of the instrument and the requirements for the FoV: the maximum area of the sky that Metis can acquire.
Metis is a solar coronagraph mounted on-board the Solar Orbiter ESA spacecraft. Solar Orbiter is scheduled for launch in February 2020 and it is dedicated to study the solar and heliospheric physics from a privileged close and inclined orbit around the Sun. Perihelion passages with a minimum distance of 0.28 AU are foreseen.
Metis features two channels to image the solar corona in two different spectral bands: in the HI Lyman ∝ at 121.6 nm, and in the polarized visible light band (580 – 640 nm). Metis is a solar coronagraph adopting an “inverted occulted” configuration. The inverted external occulter (IEO) is a circular aperture followed by a spherical mirror which back rejects the disk light. The reflected disk light exits the instrument through the IEO aperture itself, while the passing coronal light is collected by the Metis telescope. Common to both channels, the Gregorian on-axis telescope is centrally occulted and both the primary and the secondary mirror have annular shape.
Classic alignment methods adopted for on-axis telescope cannot be used, since the on-axis field is not available. A novel and ad hoc alignment set-up has been developed for the telescope alignment.
An auxiliary visible optical ground support equipment source has been conceived for the telescope alignment. It is made up by four collimated beams inclined and dimensioned to illuminate different sections of the annular primary mirror without being vignetted by other optical or mechanical elements of the instrument.
KEYWORDS: Surface plasmons, Space operations, Coronagraphy, Error analysis, Algorithm development, Satellites, Metrology, Position sensors, Solar processes, Sun
PROBA-3 ESA’s mission aims at demonstrating the possibility and the capacity to carry out a space mission in which two spacecrafts fly in formation and maintain a fixed configuration. In particular, these two satellites - the Coronagraph Spacecraft (CSC) and the Occulter Spacecraft (OSC) – will form a 150-meters externally occulted coronagraph for the purpose of observing the faint solar corona, close to the solar limb – i.e. 1.05 solar radii from the Sun’s center (RΘ). The first satellite will host the ASPIICS (Association de Satellites Pour l'Imagerie et l'Interférométrie de la Couronne Solaire) coronagraph as primary payload. These features give to the PROBA-3 mission the characteristics of both, a technological and a scientific mission.
Several metrology systems have been implemented in order to keep the formation-flying configuration. Among them, the Shadow Position Sensors (SPSs) assembly. The SPSs are designed to verify the sun-pointing alignment between the Coronagraph pupil entrance centre and the umbra cone generated by the Occulter Disk. The accurate alignment between the spacecrafts is required for observations of the solar corona as much close to the limb as 1.05 RΘ.The metrological system based on the SPSs is composed of two sets of four micro arrays of Silicon Photomultipliers (SiPMs) located on the coronagraph pupil plane and acquiring data related to the intensity of the penumbra illumination level to retrieve the spacecrafts relative position. We developed and tested a dedicated algorithm for retrieving the satellites position with respect to the Sun. Starting from the measurements of the penumbra profile in four different spots and applying a suitable logic, the algorithm evaluates the spacecraft tri-dimensional relative position. In particular, during the observational phase, when the two satellites will be at 150 meters of distance, the algorithm will compute the relative position around the ideal aligned position with an accuracy of 500μm within the lateral plane and 500 mm for the longitudinal measurement. This work describes the formation flying algorithm based on the SPS measurements. In particular, the implementation logic and the formulae are described together with the results of the algorithm testing.
The Optical Payload System (OPSys) is an INAF (italian National Institute for Astrophysics) facility hosted by Aerospace Logistics Technology Engineering Company (ALTEC SpA) in Turin, Italy. The facility is composed by three clean rooms having different cleanliness levels, a thermo-vacuum chamber (SPOCC, Space Optics calibration Chamber) with a motorized optical bench and several light sources covering the range from the extreme ultraviolet to the red light wavelengths. The SPOCC has been designed having in mind the very stringent requirements of the calibration of solar coronagraphs and the suppression of the stray-light. The facility and the optical performances will be described here. The calibration campaign performed on Metis space coronagraph will be reported as a case study.
The Metis coronagraph aboard the Solar Orbiter ESA spacecraft is expected to provide new insights into the solar dynamics. In detail, it is designed to address three main questions: the energy deposition mechanism at the poles (where the fast wind is originated), the source of the slow wind at lower altitude, and how the global corona evolves, in particular in relation to the huge plasma ejections that occasionally are produced. To obtain the required optical performance, not only the Metis optical design has been highly optimized, but the alignment procedure has also been subjected to an accurate evaluation in order to fulfill the integration specifications. The telescope assembling sequence has been constructed considering all the subsystems manufacturing, alignment and integration tolerances. The performance verification activity is an important milestone in the instrument characterization and the obtained results will assure the fulfillment of the science requirements for its operation in space.
The entire alignment and verification phase has been performed by the Metis team in collaboration with Thales Alenia Space Torino and took place in ALTEC (Turin) at the Optical Payload System Facility using the Space Optics Calibration Chamber infrastructure, a vacuum chamber especially built and tested for the alignment and calibration of the Metis coronagraph, and suitable for tests of future payloads.
The goal of the alignment, integration, verification and calibration processes is to measure the parameters of the telescope, and the characteristics of the two Metis channels: visible and ultraviolet. They work in parallel thanks to the peculiar optical layout. The focusing and alignment performance of the two channels must be well understood, and the results need to be easily compared to the requirements. For this, a dedicated illumination method, with both channels fed by the same source, has been developed; and a procedure to perform a simultaneous through focus analysis has been adopted.
In this paper the final optical performance achieved by Metis is reported and commented.
The Solar Orbiter/Metis visible and UV solar coronagraph redefines the concept of external occultation in solar coronagraphy. Classical externally occulted coronagraphs are characterized by an occulter in front of the telescope entrance aperture. Solar Orbiter will approach the Sun down to 0.28 AU: in order to reduce the thermal load, the Metis design switches the positions of the entrance aperture and the external occulter thus achieving what is called the inverted external occultation. The inverted external occulter (IEO) consists of a circular aperture on the Solar Orbiter thermal shield that acts as coronagraph entrance pupil. A spherical mirror, located 800 mm behind the IEO, back rejects the disklight through the IEO itself. To pursue the goal of maximizing the reduction of the stray light level on the focal plane, an optimization of the IEO shape was implemented.
The stray light calibration was performed in a clean environment in front of the OPSys solar disk divergence simulator (at ALTEC, in Torino, Italy), which is able to emulate different heliocentric distances. Ground calibrations were a unique opportunity to map the Metis stray light level thanks to a pure solar disk simulator without the solar corona. The stray light calibration was limited to the visible light case, being the most stringent. This work is focused on the description of the laboratory facility that was used to perform the stray light calibration and on the calibration results.
Metis is the solar coronagraph of the ESA mission Solar Orbiter. For the first time, Metis will acquire simultaneous images of the solar corona in linearly polarized, broadband visible light (580-640 nm) and in the narrow-band HI Ly-α line (121.6 nm). The visible light path includes a polarimeter, designed to observe and analyse the K-corona linearly polarized by Thomson scattering. The polarimeter comprises a liquid crystal Polarization Modulation Package (PMP) together with a quarter-wave retarder and a linear polarizer. The Metis PMP consists of two Anti-Parallel Nematic Liquid Crystal Variable Retarders (LCVRs) with their fast axis parallel with respect to each other and a pre-tilted angle of the molecules in opposite direction. This configuration results in an instrumental wide field of view (±7°). The LCVRs provide an electro-optical modulation of the input polarized light by applying an electric field to the liquid crystal molecules inside the cells. A given optical retardance can be induced in the LCVRs by selecting a suitable voltage value. This paper reports the polarimetric characterization of the Visible-light channel for the Metis/Solar Orbiter coronagraph. The retardance-to-voltage calibration of the electro-optical polarimeter was characterized over the entire field of view of the coronagraph yielding a complete “polarimetric flat-field” of the Metis Visible-light channel.
Metis is an inverted occulted coronagraph on-board the ESA/Solar Orbiter mission. The visible light path of the instrument will observe the "white" light (580-640 nm) linearly-polarized emission from the solar corona. The coronal polarized brightness allows retrieval of physical parameters such as the electron density and temperature of the K-corona. The Metis polarimeter comprises a quarter-wave retarder, the liquid crystal polarization modulation package (PMP) and a linear polarizer working as polarization analyser. The PMP consists of two Anti-Parallel Nematic Liquid Crystal Variable Retarders (LCVRs) with the fast axes parallels one to each other and a pre-tilted angle of the molecules in opposite direction, in order to maximize the homogeneity of the retardance across instrumental wide field of view: ±7 deg. This presentation reports the characterization of the PMP breadboard (BB), fully representative of the optical/polarimetric performances of the flight model. This characterization consisted in determining the performances of the device in terms of retardance as function of the applied voltage at different temperatures, angle of incidence and the variation of the retardance as a function of the wavelength. The calibrations were performed by measuring the complete Mueller matrix of the PMP-BB. The experimental results have been compared with the parameters of the theoretical model (e.g., depolarization, effective retardance, cells misalignment).
Metis is the solar coronagraph selected for the payload of the ESA Solar Orbiter mission. Metis will acquire simultaneous imaging in linearly polarized, broadband visible light (580-640 nm) and in the narrow-band HI Ly-α line (121.6 nm). The METIS visible light path includes a polarimeter, designed to observe and analyse the K-corona linearly polarized by Thomson scattering. The polarimeter comprises a liquid crystal Polarization Modulation Package (PMP) together with a quarter-wave retarder and a linear polarizer. The Metis PMP consists of two Anti-Parallel Nematic Liquid Crystal Variable Retarders (LCVRs) with their fast axis parallel with respect to each other and a pre-tilted angle of the molecules in opposite direction. The LCVRs provide an electro-optical modulation of the input polarized light by applying an electric field to the liquid crystal molecules inside the cells. A given optical retardance can be induced in the LCVRs by selecting a suitable voltage value. This presentation will report the polarimetric characterization of the Flight Model of the Metis polarimeter and the voltage-to-retardance calibration.
PROBA3 is the first high precision formation flying (FF) mission under responsibility of the European Space Agency (ESA). It is a technology mission devoted to in-orbit demonstration of the FF techniques, with two satellites kept at an average inter-satellite distance of 144m. The guiding scientific rationale is to realize a diluted coronagraph with the telescope (ASPIICS) on one satellite and the external occulter on the other satellite to observe the inner Solar corona at high spatial and temporal resolution, down to 1.08R⊙. The two spacecraft will be orbiting in a high eccentricity geocentric trajectory with perigee at 600km and the apogee at 60000Km and with an orbital period of 19hrs. The FF acquisition and operations will last about 6 hrs around the apogee and different metrology systems will be used for realizing and controlling the FF. The alignment active most critical sub-system is the Shadow Positioning Sensors (SPS), a series of Si-PM (Silicon Photomultiplier) disposed around the ASPIICS telescope's entrance aperture and measuring the proper positioning of the penumbra generated by the occulter at the center of the coronagraph’s optical reference frame. The FF alignment measurement accuracies required to the SPS are: 500μm for lateral movements and 50mm for longitudinal movements. This paper gives an overview of the opto-mechanical and electronic design and of the software algorithm for the FF intersatellite positioning. The expected performance of the SPS metrology system are reported.
PROBA3/ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun) is the first formation flight solar coronagraph, scheduled by ESA for a launch and currently in phase C/D. It is constituted by two spacecraft (one hosting the occulter, diameter 142 cm, and one with the telescope) separated by 144 m, kept in strict alignment by means of complex active and metrology custom systems. The stray light analysis, which is always one the most critical work packages for a solar coronagraph, has been only theoretically investigated so far due to the difficulty of replicating the actual size system in a clean laboratory environment. The light diffracted by the external occulter is the worst offender for the stray light level on the instrument focal plane, thus there is strong interest for scaling at least the occultation system of the coronagraph and test it in front of a solar simulator in order to experimentally validate the expected theoretical performance. The theory for scaling the occulter, the occulter-pupil distance and the source dimension has been developed and a scaled model is being manufactured. A test campaign is going to be conducted at the OPSys facility in Torino in front of a solar simulator (conveniently scaled). This work accounts for the description of the scaled model laboratory set-up and of the test plan.
PROBA-3 [1] [2] is a Mission of the European Space Agency (ESA) composed by two satellites flying in formation and aimed at achieving unprecedented performance in terms of relative positioning. The mission purpose is, in first place, technological: the repeated formation break and acquisition during each orbit (every about twenty hours) will be useful to demonstrate the efficacy of the closed-loop control system in keeping the formation-flying (FF) and attitude (i.e. the alignment with respect to the Sun) of the system. From the scientific side, instead, the two spacecraft will create a giant instrument about 150 m long: an externally occulted coronagraph named ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun) dedicated to the study of the inner part of the visible solar corona. The two satellites composing the mission are: the Coronagraph Spacecraft (CSC), hosting the Coronagraph Instrument (CI), and the disk-shaped (1.4 m diameter) Occulter Spacecraft (OSC). The PROBA-3 GNC (Guidance, Navigation and Control) system will employ several metrological subsystems to keep and retain the desired relative position and the absolute attitude (i.e. with respect to the Sun) of the aligned spacecraft, when in observational mode. The SPS subsystem [5] is one of these metrological instruments. It is composed of eight silicon photomultipliers (SiPMs), sensors operated in photovoltaic mode [6] that will sense the penumbra light around the Instrument’s pupil so to detect any FF displacement from the nominal position. In proximity of the CDR (Critical Design Review) phase, we describe in the present paper the changes occurred to design in the last year in consequence of the tests performed on the SPS Breadboard (Evaluation Board, EB) and the SPS Development Model (DM) and that will finally lead to the realization of the flight version of the SPS system.
PROBA-3 [1] [2] is a Mission of the European Space Agency (ESA) composed of two formation-flying satellites,
planned for their joint launch by the end of 2018. Its main purposes have a dual nature: scientific and technological. In
particular, it is designed to observe and study the inner part of the visible solar corona, thanks to a dedicated coronagraph
called ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun), and to
demonstrate the in-orbit formation flying (FF) and attitude control capability of its two satellites.
The Coronagraph payload on-board PROBA-3 consists of the following parts: the Coronagraph Instrument (CI) with the
Shadow Position Sensor (SPS) on the Coronagraph Spacecraft (CSC), the Occulter Position Sensor (OPSE) [3] [4] and
the External Occulting (EO) disk on the Occulter Spacecraft (OSC).
The SPS subsystem [5] is one of the main metrological devices of the Mission, adopted to control and to maintain the
relative (i.e. between the two satellites) and absolute (i.e. with respect to the Sun) FF attitude. It is composed of eight
micro arrays of silicon photomultipliers (SiPMs) [6] that shall be able to measure, with the required sensitivity and
dynamic range as asked by ESA, the penumbral light intensity on the Coronagraph entrance pupil.
With the present paper we describe the testing activities on the SPS breadboard (BB) and Development Model (DM) as
well as the present status and future developments of this PROBA-3 metrological subsystem.
years have raised increasing interest. Many applications of astronomical observation techniques, as coronography and
interferometry get great benefit when moved in space and the employment of diluted systems represents a milestone to
step-over in astronomical research. In this work, we present the Optical Position Sensors Emitter (OPSE) metrological
sub-system on-board of the PROBA3. PROBA3 is an ESA technology mission that will test in-orbit many metrology
techniques for the maintenance of a Formation Flying with two satellites, in this case an occulter and a main satellite
housing a coronagraph named ASPIICS, kept at an average inter-distance of 144m. The scientific task is the observation
of the Sun’s Corona at high spatial and temporal resolution down to 1.08R⊙. The OPSE will monitor the relative position
of the two satellites and consists of 3 emitters positioned on the rear surface of the occulter, that will be observed by the
coronagraph itself. A Centre of Gravity (CoG) algorithm is used to monitor the emitter’s PSF at the focal plane of the
Coronagraph retrieving the Occulter position with respect to the main spacecraft. The 3σ location target accuracy is
300μm for lateral movement and 21cm for longitudinal movements. A description of the characterization tests on the
OPSE LED sources, and of the design for a laboratory set-up for on ground testing is given with a preliminary
assessment of the performances expected from the OPSE images centroiding algorithm.
KEYWORDS: Space telescopes, Space operations, Metrology, Satellites, Telescopes, Space telescopes, Space operations, Metrology, Coronagraphy, Surface plasmons, Light emitting diodes, Calibration, Solar processes
Formation flying is one of the most promising techniques for the future of astronomy and astrophysics from the space.
The capabilities of the rockets strongly affect the dimensions and the weights of telescopes and instrumentation to be
launched. Telescopes composed by several smallest satellites in formation flying, could be the key for build big space
telescopes. With this aim, the ESA PROBA-3 mission will demonstrate the capabilities of this technology, maintaining
two satellites aligned within 1 mm (longitudinal) when the nominal distance between the two is of around 144m.
The scientific objective of the mission is the observation of the solar corona down to 1.08 solar radii. The Coronagraph
Spacecraft (CSC) will observe the Sun, when the second spacecraft, the Occulter Spacecraft (OSC) will work as an
external occulter, eclipsing to the CSC the sun disk. The finest metrology sub-systems, the Shadow Position Sensors
(SPS) and the Occulter Position Sensor Emitters (OPSE) identifying respectively the CSC-Sun axis and the formation
flying (i.e., CSC-OSC) axis will be considered here. In particular, this paper is dedicated to the test-bed for the
characterization, the performance analysis and the algorithms capabilities analysis of the both the metrology subsystems.
The test-bed is able to simulate the different flight conditions of the two spacecraft and will give the opportunity to check
the response of the subsystems in the conditions as close as possible to the flight ones.
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