The Solar Corona Imager is an internally occulted coronagraph on board the ASO-S mission, which has the advantage of imaging the inner corona in H I Lyman-α (Lyα) and white-light (WL) wavebands. However, scattering of solar disk light by the primary mirror (M1) becomes the main source of stray light. To study the methods of stray light suppression, three scattering models are used to model M1 scattering in Zemax OpticStudio. The ratio of coronal emission to predicted stray light decrease along field of view in both channels. The stray light in Lyαchannel is generally lower than coronal emission, but the stray light in WL channel tends to be one order of magnitude higher than coronal signal at 2:5Rꙩ. Optimized parameter combinations that suppress the stray light to required level are obtained, which put some limitations on the M1 manufacture. Besides, K-correlation model is recommended to simulate surface scattering.
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.
The ESA formation Flying mission Proba-3 will y the giant solar coronagraph ASPIICS. The instrument is composed of a 1.4 meter diameter external occulting disc mounted on the Occulter Spacecraft and a Lyot-style solar coronagraph of 50mm diameter aperture carried by the Coronagraph Spacecraft positioned 144 meters behind. The system will observe the inner corona of the Sun, as close as 1.1 solar radius. For a solar coronagraph, the most critical source of straylight is the residual diffracted sunlight, which drives the scientific performance of the observation. This is especially the case for ASPIICS because of its reduced field-of-view close to the solar limb. The light from the Sun is first diffracted by the edge of the external occulter, and then propagates and scatters inside the instrument. There is a crucial need to estimate both intensity and distribution of the diffraction on the focal plane. Because of the very large size of the coronagraph, one cannot rely on representative full scale test campaign. Moreover, usual optics software package are not designed to perform such diffraction computation, with the required accuracy. Therefore, dedicated approaches have been developed in the frame of ASPIICS. First, novel numerical models compute the diffraction profile on the entrance pupil plane and instrument detector plane (Landini et al., Rougeot et al.), assuming perfect optics in the sense of multi-reflection and scattering. Results are confronted to experimental measurements of diffraction. The paper reports the results of the different approaches.
In the context of environmental monitoring, it is very important to improve systems for the identification, assessment and management of environmental risks through the use and integration of analytical techniques combined with hyperspectral airborne sensing technologies. Remote-sensing applications are varied, but nevertheless an accurate mapping of the soil required the use of complex scientific instruments installed on airplanes or helicopters. The high integration of electronics, combined with the computing power of modern processors, allows the development of integrated and compact hyperspectral systems installed on drones.1 The drone era poses new challenges to optical devices design: they shall be light, compact and robust, easy to assemble and to control. This work explains the optical system design of HYDRACAM (HYperspectral DRone Advanced CAMera), an instrument devoted to hyperspectral imaging by using an electro-optical liquid crystal tunable filter combined with a commercial camera. The commercial ray tracing software Zemax/OpticStudio2 was used to perform the optical design. The main challenge was to manufacture a low cost optical device, containing the mass and the total length to suit a drone payload. A huge effort was made in order to combine strict constraints (such as the filter narrow entrance aperture and its acceptance angle) with ambitious optical performance requirements (high spatial resolution, large field of view). First, a description of the work done to find a trade-off between cost and opto-mechanical constraints is provided: the choice of the commercial objective, the choice of the custom lenses materials and shapes and some optical design original solutions are addressed. Then, the details of the optical performance analysis are discussed.
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.
METIS coronagraph is designed to observe the solar corona with an annular field of view from 1.5 to 2.9 degrees in the visible broadband (580-640 nm) and in the UV HI Lyman-alpha, during the Sun close approaching and high latitude tilting orbit of Solar Orbiter. The big challenge for a coronagraph is the stray light rejection. In this paper after a description of the present METIS optical design, the stray light rejection design is presented in detail together with METIS off-pointing strategies throughout the mission. Data shown in this paper derive from the optimization of the optical design performed with Zemax ray tracing and from laboratory breadboards of the occultation system and of the polarimeter.
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.
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.
The main goal of the Astrometric Gravitation Probe mission is the verification of General Relativity and competing gravitation theories by precise astrometric determination of light deflection, and of orbital parameters of selected Solar System objects. The key element is the coherent combination of a set of 92 circular entrance apertures, each feeding an elementary inverted occulter similar to the one developed for Solar Orbiter/METIS.1 This provides coronagraphic functions over a relevant field of view, in which all stars are observed for astrometric purposes with the full resolution of a 1 m diameter telescope. The telescope primary mirror acts as a beam combiner, feeding the 92 pupils, through the internal optics, toward a single focal plane. The primary mirror is characterized by 92 output apertures, sized according to the entrance pupil and telescope geometry, in order to dump the solar disk light beyond the instrument. The astronomical objects are much fainter than the solar disk, which is angularly close to the inner field of view of the telescope. The stray light as generated by the diffraction of the solar disk at the edges of the 92 apertures defines the limiting magnitude of observable stars. In particular, the stray light due to the diffraction from the pupil apertures is scattered by the telescope optics and follows the same optical path of the astronomical objects; it is a contribution that cannot be eliminated and must therefore be carefully evaluated. This paper describes the preliminary evaluation of this stray light contribution.
KEYWORDS: Visible radiation, Coronagraphy, Ultraviolet radiation, Coronagraphy, Astronomy, Solar processes, Sensors, Space operations, Curium, Data acquisition, Detection and tracking algorithms
Metis is the coronagraph on board Solar Orbiter, the ESA mission devoted to the study of the Sun that will be launched in October 2018. Metis is designed to perform imaging of the solar corona in the UV at 121.6 nm and in the visible range where it will accomplish polarimetry studies thanks to a variable retarder plate. Due to mission constraints, the telemetry downlink on the spacecraft will be limited and data will be downloaded with delays that could reach, in the worst case, several months. In order to have a quick overview on the ongoing operations and to check the safety of the 10 instruments on board, a high-priority downlink channel has been foreseen to download a restricted amount of data. These so-called Low Latency Data will be downloaded daily and, since they could trigger possible actions, they have to be quickly processed on ground as soon as they are delivered. To do so, a proper processing pipeline has to be developed by each instrument. This tool will then be integrated in a single system at the ESA Science Operation Center that will receive the downloaded data by the Mission Operation Center. This paper will provide a brief overview of the on board processing and data produced by Metis and it will describe the proxy-pipeline currently under development to deal with the Metis low-latency data.
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.
KEYWORDS: Diffraction, Stray light, Coronagraphy, Space telescopes, Telescopes, Space operations, Diffraction, Stray light, Solar processes, Computer simulations, Photonics, Sun
PROBA-3 is a technological mission of the European Space Agency (ESA), devoted to the in-orbit demon- stration of formation flying (FF) techniques and technologies. ASPIICS is an externally occulted coronagraph approved by ESA as payload in the framework of the PROBA-3 mission and is currently in its C/D phase. FF offers a solution to investigate the solar corona close the solar limb using a two-component space system: the external occulter on one spacecraft and the optical instrument on the other, separated by a large distance and kept in strict alignment. ASPIICS is characterized by an inter-satellite distance of ∼144 m and an external occulter diameter of 1.42 m. The stray light due to the diffraction by the external occulter edge is always the most critical offender to a coronagraph performance: the designer work is focused on reducing the stray light and carefully evaluating the residuals. In order to match this goal, external occulters are usually characterized by an optimized shape along the optical axis. Part of the stray light evaluation process is based on the diffraction calculation with the optimized occulter and with the whole solar disk as a source. We used the field tracing software VirtualLabTM Fusion by Wyrowski Photonics [1] to simulate the diffraction. As a first approach and in order to evaluate the software, we simulated linear occulters, through as portions of the flight occulter, in order to make a direct comparison with the Phase-A measurements [2].
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.
The European Space Agency (ESA) is planning to launch in 2018 the PROBA3 Mission, designed to demonstrate the inorbit formation flying (FF) attitude capability of its two satellites and to observe the inner part of the visible solar corona as the main scientific objective.
The solar corona will be observed thanks to the presence on the first satellite, facing the Sun, of an external occulter producing an artificial eclipse of the Sun disk. The second satellite will carry on the coronagraph telescope and the digital camera system in order to perform imaging of the inner part of the corona in visible polarized light, from 1.08 R⦿ up to about 3 R⦿.
One of the main metrological subsystems used to control and to maintain the relative (i.e. between the two satellites) and absolute (i.e. with respect to the Sun) FF attitude is the Shadow Position Sensor (SPS) assembly. It is composed of eight micro arrays of silicon photomultipliers (SiPMs) able to measure with the required sensitivity and dynamic range the penumbral light intensity on the Coronagraph entrance pupil.
In the following of the present paper we describe the overall SPS subsystem and its readout electronics with respect to the capability to satisfy the mission requirements, from the light conversion process on board the silicon-based SPS devices up to the digital signal readout and sampling.
PROBA3 is an ESA technology mission devoted to in-orbit demonstration of the formation flight (FF) technique, with two satellites kept at an average inter-distance by about 144 m. The ASPIIC instrument on-board PROBA3 will be the first ever space-based coronagraph working on one satellite and having the external occulter located on the second satellite, thus allowing observations of the inner solar corona with unprecedented reduction of stray light. During the observational periods, the FF configuration will be maintained with very high precision and two different techniques will be implemented: the use of Shadow Positioning Sensors (SPS) located on the Coronagraph Spacecraft (diodes measuring the penumbral light intensity on the entrance pupil plane) and the use of Occulter Position Sensor LEDs (OPSE) located on the back side of the Occulter Spacecraft. This paper will review the main instrumental requirements on the SPS needed to determine the 3-dimensional relative positioning of the two PROBA3 satellites with high precision.
KEYWORDS: Surface plasmons, Signal detection, Coronagraphy, Telescopes, Space telescopes, Stray light, Space operations, Diffraction, Metrology, Solar processes
PROBA-3/ASPIICS is a formation flying coronagraph selected by ESA and currently in its C/D phase. It is constituted by two spacecrafts (OSC, Occulter SpaceCraft, carrying the occulter, diameter 142 cm, and CSC, Coronagraph SpaceCraft, with the telescope) separated by ~144 m, kept in strict alignment by means of an active custom system. The alignment active system most critical components are the Shadow Positioning Sensors (SPS), a series of Si-PM (Silicon Photomultiplier) measuring the penumbra generated by the occulter. The arrangement of the SPSs around the telescope entrance aperture is defined as a trade-off between mechanical constraints and maximum sensitivity to misalignments. The signal detected by the SPSs can be approximately simulated with a geometrical analysis based on the variation of the penumbra generated by the external occulter. The stray light generated by the diffraction from the external occulter may affect the SPSs signal. It is mandatory to carefully evaluate its level in order to refine the active alignment adjustment algorithm. This work is dedicated to the description of the preliminary investigation performed in order to evaluate the impact of the diffraction on the SPSs signal.
The METIS coronagraph aboard the Solar Orbiter mission will undergo extreme environmental conditions (e.g.,
a thermal excursion of about 350 degrees throughout the various mission phases), due to the peculiar spacecraft
trajectory that will reach a perihelion of 0.28 AUs. METIS is characterized by an innovative design for the
occultation system that allows to halve the thermal load inside the instrument while guaranteeing the stray light
reduction that is required for a solar coronagraph. The Inverted External Occulter (IEO) concept revolutionizes
the classical scheme, by exchanging the usual positions of the entrance aperture (that is now the outermost element of the instrument facing the Sun) with the actual occulter (that is a spherical mirror inside the coronagraph
boom). The chosen material for the IEO manufacturing is Titanium, as a trade o_ between light weight, strength
and low thermal expansion coefficient. A 2 years long test campaign has been run to define the IEO geometry,
and its results are addressed in previous dedicated papers. This work describes the results of a further campaign
aimed at defining the IEO surface and edge finishing, the support flange geometry and the Titanium coating.
Various edge finishing were installed on a prototype of the instrument occulting system and their performance
in stray light reduction were compared. The support flange geometry was designed in order to reduce the overall
weight, to control the thermal load and to accentuate its stray light suppression performance. The coating is a
particularly delicate issue. A black coating is necessary in order to assess the stray light issues, typically critical
for visible coronagraphs. Black coating of Titanium is not a standard process, thus several space qualified black
coatings were experimented on Titanium and characterized. The impact of the IEO coatings was evaluated, the
reflectivity and the BRDFs were measured and are addressed in the paper.
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