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PROBA-3 is an ESA mission aimed at the demonstration of formation flying performance of two satellites that will form a giant coronagraph in space. The first spacecraft will host a telescope imaging the solar corona in visible light, while the second, the external occulter, will produce an artificial eclipse. This instrument is named ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun). To accomplish the payload's scientific tasks, PROBA-3 will ensure sub-millimeter reciprocal positioning of its two satellites using closed-loop on-board metrology. Several metrology systems will be used and the Shadow Position Sensors (SPS) subsystem senses the penumbra around the instrument aperture and returns the 3-D displacement of the coronagraph satellite, with respect to its nominal position, by running a dedicated algorithm. In this paper, we describe how the SPS works and the choices made to accomplish the mission objectives.

PROBA-3 is a space mission of the European Space Agency that will test, and validate metrology and control systems for autonomous formation flying of two independent satellites. PROBA-3 will operate in a High Elliptic Orbit and when approaching the apogee at 6·104 Km, the two spacecraft will align to realize a giant externally occulted coronagraph named ASPIICS, with the telescope on one satellite and the external occulter on the other one, at inter-satellite distance of 144.3 m. The formation will be maintained over 6 hrs across the apogee transit and during this time different validation operations will be performed to confirm the effectiveness of the formation flying metrology concept, the metrology control systems and algorithms, and the spacecraft manoeuvring. The observation of the Sun's Corona in the field of view [1.08;3.0]RSun will represent the scientific tool to confirm the formation flying alignment. In this paper, we review the mission concept and we describe the Shadow Position Sensors (SPS), one of the metrological systems designed to provide high accuracy (sub-millimetre level) absolute and relative alignment measurement of the formation flying. The metrology algorithm developed to convert the SPS measurements in lateral and longitudinal movement estimation is also described and the measurement budget summarized.

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'Interferometrie 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 PROBA-3 is a European Space Agency (ESA) mission devoted to obtain, as never before, highly accurate formation flying. A couple of satellites will works together as an externally occulted solar coronagraph. The distance between the two satellites is of 144 meters and the required longitudinal accuracy of the relative positioning is better than 1 mm. In this proceeding the in-flight calibration of one of the most accurate metrology subsystem, the SPS (Shadow Position Sensors), will be presented.

The development and employment of metrology controlled diluted systems for astronomical applications in space represents a cornerstone to dramatically improve the observation limits. In this scenario, the ESA mission PROBA-3 is the first technology demonstration mission aimed at verifying and validating several metrological systems to obtain high accuracy formation flying (FF) of two satellites. The two spacecraft will be aligned and maintained in a FF, around the apogee of their geocentric orbit, at an average inter-satellite distance of 144m, with an accuracy down to sub-millimetre level. The scientific task of the mission, that is the FF validation tool, is the observation of the Sun’s Corona at high spatial and temporal resolution within the field of view [1.08; 3.0]R ⊙ . To this end, one satellite will house a coronagraph telescope, named ASPIICS, and the other one, the external occulter. In this paper, we review the metrology concept applied to the mission with a special focus on the Occulter Position Sensor Emitters (OPSE) metrology sub-system, a set of 3 monochromatic Led emitters positioned on opposite-Sun surface of the occulter and imaged at the ASPIICS’s focal plane.

PROBA-3 is a two-spacecraft ESA mission carrying the space-based diluted coronagraph ASPIICS. The imaging instrument is hosted on the first spacecraft with the second acting as external occulter. In order to accomplish the payload’s scientific tasks, PROBA-3 will ensure sub-millimeter reciprocal positioning of its two satellites by means of closedloop on-board metrology. The Shadow Position Sensors (SPS) sense the penumbra around the instrument aperture and return the 3-D displacement of the coronagraph satellite with respect to its nominal position by running a dedicated algorithm. In this paper we describe how the SPS works and the choices made in order to accomplish the mission objectives.

This paper presents the recent achievements in the development of ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun), a solar coronagraph that is the primary payload of ESA’s formation flying in-orbit demonstration mission PROBA-3. The PROBA-3 Coronagraph System is designed as a classical externally occulted Lyot coronagraph but it takes advantage of the opportunity to place the 1.4 meter wide external occulter on a companion spacecraft, about 150m apart, to perform high resolution imaging of the inner corona of the Sun as close as ~1.1 solar radii. Besides providing scientific data, ASPIICS is also equipped with sensors for providing relevant navigation data to the Formation Flying GNC system. This paper is reviewing the recent development status of the ASPIICS instrument as it passed CDR, following detailed design of all the sub-systems and testing of STM and various Breadboard models.

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.

This paper presents the current status of ASPIICS, a solar coronagraph that is the primary payload of ESA’s formation flying in-orbit demonstration mission PROBA-3. The “sonic region” of the Sun corona remains extremely difficult to observe with spatial resolution and sensitivity sufficient to understand the fine scale phenomena that govern the quiescent solar corona, as well as phenomena that lead to coronal mass ejections (CMEs), which influence space weather. Improvement on this front requires eclipse-like conditions over long observation times. The space-borne coronagraphs flown so far provided a continuous coverage of the external parts of the corona but their over-occulting system did not permit to analyse the part of the white-light corona where the main coronal mass is concentrated. The PROBA-3 Coronagraph System, also known as ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun) is designed as a classical externally occulted Lyot coronagraph but it takes advantage of the opportunity to place the external occulter on a companion spacecraft, about 150m apart, to perform high resolution imaging of the inner corona of the Sun as close as ~1.1 solar radii. The images will be tiled and compressed on board in an FPGA before being down-linked to ground for scientific analyses. ASPIICS is built by a large European consortium including about 20 partners from 7 countries under the auspices of the European Space Agency. This paper is reviewing the recent development status of the ASPIICS instrument as it is approaching CDR.

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.

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.

We present the proposal for the physical instrument model of the SOXS (Son OF X-Shooter) spectroscopic facility mainly devoted to the follow-up observation of transient sources. A dedicated suitable instrument to exploit the science of these transients is lacking, resulting in severe science "dissipation". SOXS will cover the optical/NIR band (0.35-1.75 μm) with a medium resolution (R 4500), down to the limiting magnitude of R 20-20.5 (1 hr at S/N 10) that is perfectly suited to study transients from on-going imaging surveys. Imaging capabilities in the optical are also foreseen to allow for multi-band photometry of the faintest transients with a field of view of at least 2arcmin. We propose to implement a physical modelling approach in order to link the instrument parameters and behaviour to physical quantities, thus providing a description of the instrument that can be connected with measurements. The method has been already successfully applied to the X-shooter instrument. The X-shooter physical model is based on a kernel optical ray-tracing realised by means of matrix optics representation, which can handle a large number of wavelengths. This can be extended to the SOXS design. The foreseen applications of the SOXS physical model are broad, ranging from support to detailed instrument design and development of the data reduction software, wavelength calibration, evaluation of instrument performance as a function of the model parameters, instrument alignment, and support during the commissioning phase and as a tool for quality check during operations.

The GAME mission concept is aimed at testing the General Relativity, through precise measurement of the gravitational deflection of light by the Sun, by means of an optimised telescope operating in the visible and launched in orbit on a small class satellite. We describe the proposed mission profile, the preliminary payload design and the expected performance. The targeted precision on the γ parameter of the PPN formulation of General Relativity is in the range 10−6 to 10−7. The measurement principle relies on the differential astrometric signature on the stellar positions. The instrument concept is based on a dual field, multiple aperture Fizeau interferometer, observing simultaneously two sky regions close to the solar limb. A beam combiner folds the telescope line of sight on two different directions on the sky, separated by a base angle of 4°. The diluted optics approach achieves an efficient rejection of the scattered solar radiation, while retaining an acceptable angular resolution on the science targets.

SOXS (Son Of X-Shooter) will be a unique spectroscopic facility for the ESO-NTT 3.5-m telescope in La Silla (Chile), able to cover the optical/NIR band (350-1750 nm). The design foresees a high-efficiency spectrograph with a resolution-slit product of ~4,500, capable of simultaneously observing the complete spectral range 350 - 1750 nm with a good sensitivity, with light imaging capabilities in the visible band. This paper outlines the status of the project., Comment: 10 pages, submitted to SPIE Astronomical Telescopes & Instrumentation 2016, paper 9908-152

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.

ASTRI SST-2M is an end-to-end prototype of Small Size class of Telescope proposed for the Cherenkov Telescope Array (CTA). Currently under completion at the Serra La Nave observing station (Mt. Etna, Catania, Italy), the ASTRI SST- 2M telescope is the first imaging dual-mirror telescope ever realized for Cherenkov telescopes. A mini-array of nine such telescopes will form the ASTRI mini-array proposed as a precursor and initial seed of CTA to be installed at the final CTA southern site. ASTRI SST-2M is equipped with an active optics system, controlling both the segmented primary mirror and the monolithic secondary mirror, which allows optical re-alignment during telescope slew. We describe the hardware and software solution that have been implemented for optics control and the models we developed for the system.

In recent years, ESA has assessed several mission involving formation flying (FF). The great interest in this topics is mainly driven by the need for moving from ground to space the location of next generation astronomical telescopes overcoming most of the critical problems, as example the construction of huge baselines for interferometry. In this scenario, metrology systems play a critical role. PROBA3 is an ESA technology mission devoted to in-orbit demonstration of the FF technique, with two satellites, an occulter and a main satellite housing a coronagraph named ASPIICS, kept at an average inter-distance by about 144m, with micron scale accuracy. The guiding proposal is to test several metrology solution for spacecraft alignment, with the important scientific return of having observation of Corona at never reached before angular field. The Shadow Position Sensors (SPS), and the Optical Position Emitters Sensors (OPSE) are two of the systems used for FF fine tracking. The SPS are finalized to monitor the position of the two spacecraft with respect to the Sun and are discussed in dedicated papers presented in this conference. 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. By following the evolution of the emitters images at the focal plane the alignment of the two spacecrafts is retrieved via dedicated centroiding algoritm. We present an overview of the OPSE system and of the centroiding approach.

The "sonic region" of the Sun corona remains extremely difficult to observe with spatial resolution and sensitivity sufficient to understand the fine scale phenomena that govern the quiescent solar corona, as well as phenomena that lead to coronal mass ejections (CMEs), which influence space weather. Improvement on this front requires eclipse-like conditions over long observation times. The space-borne coronagraphs flown so far provided a continuous coverage of the external parts of the corona but their over-occulting system did not permit to analyse the part of the white-light corona where the main coronal mass is concentrated. The proposed PROBA-3 Coronagraph System, also known as ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun), with its novel design, will be the first space coronagraph to cover the range of radial distances between ~1.08 and 3 solar radii where the magnetic field plays a crucial role in the coronal dynamics, thus providing continuous observational conditions very close to those during a total solar eclipse. PROBA-3 is first a mission devoted to the in-orbit demonstration of precise formation flying techniques and technologies for future European missions, which will fly ASPIICS as primary payload. The instrument is distributed over two satellites flying in formation (approx. 150m apart) to form a giant coronagraph capable of producing a nearly perfect eclipse allowing observing the sun corona closer to the rim than ever before. The coronagraph instrument is developed by a large European consortium including about 20 partners from 7 countries under the auspices of the European Space Agency. This paper is reviewing the recent improvements and design updates of the ASPIICS instrument as it is stepping into the detailed design phase.

Chromatic effects are usually associated with refractive optics, so reflective telescopes are assumed to be free from them. We show that all-reflective optics still bears significant levels of such perturbations, which is especially critical to modern micro-arcsecond astrometric experiments. We analyze the image formation and measurement process to derive a precise definition of the chromatic variation of the image position, and we evaluate the key aspects of optical design with respect to chromaticity. The fundamental requirement related to chromaticity is the symmetry of the optical design and of the wavefront errors. Finally, we address some optical engineering issues, such as manufacturing and alignment, providing recommendations to minimize the degradation that chromaticity introduces into astrometry., 10 pages, 8 figures

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.

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.

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.

ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana) SST-2M is an end-to-end prototype of Small Size class of Telescope for the Cherenkov Telescope Array. It will apply a dual mirror configuration to Imaging Atmospheric Cherenkov Telescopes. The 18 segments composing the primary mirror (diameter 4.3 m) are equipped with an active optics system enabling optical re-alignment during telescope slew. The secondary mirror (diameter 1.8 m) can be moved along three degrees of freedom to perform focus and tilt correc tions. We describe the kinematic model used to predict the system performance as well as the hardware and software de sign solution that will be implemented for optics control. Keywords: Imaging Atmospheric Cherenkov Telescope, CTA, ASTRI, gamma-rays, active optics 1. INTRODUCTION The dual-mirror ASTRI (Astrofisica con Specchi a Tecnologia Replicante Italiana) Small Size Telescope prototype, named ASTRI SST-2M, is a wide field of view Schwarzschild-Couder Cherenkov telescope developed within the Cherenkov Telescope Array (CTA) framework

The Solar Orbiter/METIS visible and UV coronagraph introduces the concept of occulter inversion in solar coronagraphy. Classical externally occulted coronagraphs usually have a disk in front of the telescope entrance pupil. According to the mission requirements, in order to reduce the amount of power entering the instrument and to limit the instrument dimensions, METIS is equipped with an inverted external occulter (IEO). The IEO consists of a circular aperture on the Solar Orbiter thermal shield that acts as coronagraph entrance pupil. A spherical mirror (M0), located ~800 mm behind the IEO, rejects back the disk-light through the IEO itself. A light-tight boom connects the IEO to the M0 through the thermal shield. In order to achieve high performance in stray light suppression, the IEO design needs optimization. Due to the novelty of the concept we can only use the heritage of past space-borne coronagraph occulters as a starting point to design a dedicated occulter optimization shape. A 1.5 years long, accurate test campaign has been carried out to evaluate the best optimization configuration for the IEO. Two prototypes were manufactured to take into account the impact of the boom geometry on the stray light suppression performance. Two optimization concepts were compared: the inverted cone (that derives from the conic optimization of classical occulting disks) and the serrated edge, of which several samples were manufactured, with different geometrical parameters, surface roughnesses and coatings. This work summarizes the activity we have been carrying on to define the flight specifications for the METIS occulter.

Gaia is ESA's next-generation space mission aimed at global astrometry at the microarcsecond level. Its science case is devoted to the understanding of our Galaxy's structure, evolution and composition. The GAIA payload includes two identical telescopes separated by a Basic Angle whose variations in time must be measured with very high accuracy, to fulll the astrometry goal. To this purpose, an interferometric sub-system, the Basic Angle Monitoring Device (BAMD), has been introduced. The BAMD optical concept is based on a pair of laser beams per telescope, producing fringes on a dedicated CCD. The basic measurement principle of BAMD consists in monitoring the stability of the fringe pattern phase. We are developing a demonstrator of the BAMD for educational purposes, considered representative of the driving design concept for the Gaia microarcsecond astrometry. In this paper we describe the design guidelines and analyze some of the key elements related to the demonstration of the basic angle monitoring concept.

GAME is a recent concept for a small/medium class mission aimed at Fundamental Physics tests in the Solar system, by means of an optimised instrument in the visible, based on smart combination of coronagraphy and Fizeau interferometry. The targeted precision on the γ and β parameters of the Parametrised Post-Newtonian formulation of General Relativity are respectively in the 10 -7 -10 -8 and 10 -5 -10 -6 range, improving by one or two orders of magnitude with respect to the expectations on current or near future experiments. Such precision is suitable to detect possible deviations from the unity value, associated to generalised Einstein models for gravitation, with potentially huge impacts on the cosmological distribution of dark matter and dark energy from a Solar system scale experiment. The measurement principle is based on the differential astrometric signature on the stellar positions, i.e. based on the spatial component of the effect rather than the temporal component as in the most recent experiments using radio link delay timing variation (Cassini). The instrument concept is based on multiple field, multiple aperture Fizeau interferometry, observing simultaneously regions close to the Solar limb (requiring the adoption of coronagraphic techniques), and others in opposition to the Sun. The diluted optics approach is selected for achieving an efficient rejection of the scattered solar radiation, while retaining an acceptable angular resolution on the science targets. The multiple field observation is aimed at cost-effective control of systematic effects through simultaneous calibration. We describe the science motivation, the proposed mission profile, the instrument concept and the expected performance.

We present a new design of a four mirrors telescope for astrometric measurement to be used in the GAME mission, currently under study at the Astronomical Observatory of Turin, Italy. The main aim of GAME - Gamma Astrometric Measurement Experiment - is to measure the γ parameter of the Parameterized Post- Newtonian formulation by looking at the deflection of light produced by the Sun's gravitational curvature, as in the Dyson, Eddington et al. 1919 experiment, using a dedicated, space based dual-field telescope. A first design has been presented in recent years, based on a Cassegrain scheme with a mask in front of the primary mirror to realize multiple aperture Fizeau interferometry. The new design still implements a Fizeau interferometer, but the telescope layout is based on a Korsch-like scheme with four conical mirrors, long focal length, and without the use of exotic surfaces (aspheric or polynomial) as adopted in other long focal astrometric instruments. A different combination scheme of the two lines of sight makes the dimensioning of the primary mirror more relaxed allowing us to work with smaller surfaces and therefore to achieve a more compact payload configuration. The design of the instrument and the masked interferometry approach allow us to maximize the astrometric performances and at the same time to improve the baffling, minimizing the amount of stray light from the Sun. In this paper we describe the mission profile, the observation principle, the new instrument layout and the expected performances.

In this paper we present the status of different experiments set up at Turin Observatory on novel techniques for multiple beam combination, adopting mostly bulk optics. The goal of these experiments is to find the best scheme able to perform efficient fringe tracking operation on a densely populated (N>4) interferometer, while at the same time maximizing optical throughput and sensitivity on faint sources. One of these concepts has been proposed for the VSI fringe tracker (see Corcione et al, this conference). The schemes proposed have also the advantage of being in principle easily adapted to a large number of beams.

We present the optical and cryo-mechanical solutions for the Spectrograph of VSI (VLTI Spectro-Imager), the second generation near-infrared (J, H and K bands) interferometric instrument for the VLTI. The peculiarity of this spectrograph is represented by the Integrated Optics (IO) beam-combiner, a small and delicate component which is located inside the cryostat and makes VSI capable to coherently combine 4, 6 or even 8 telescopes. The optics have been specifically designed to match the IO combiner output with the IR detector still preserving the needed spatial and spectral sampling, as well as the required fringe spacing. A compact device that allows us to interchange spectral resolutions (from R=200 to R=12000), is also presented., 12 pages, 11 figures, SPIE Conference - Marseille 2008

The implementation of the simultaneous combination of several telescopes (from four to eight) available at Very Large Telescope Interferometer (VLTI) will allow the new generation interferometric instrumentation to achieve interferometric image synthesis with unprecedented resolution and efficiency. The VLTI Spectro Imager (VSI) is the proposed second-generation near-infrared multi-beam instrument for the Very Large Telescope Interferometer, featuring three band operations (J, H and K), high angular resolutions (down to 1.1 milliarcsecond) and high spectral resolutions. VSI will be equipped with its own internal Fringe Tracker (FT), which will measure and compensate the atmospheric perturbations to the relative beam phase, and in turn will provide stable and prolonged observing conditions down to the magnitude K=13 for the scientific combiner. In its baseline configuration, VSI FT is designed to implement, from the very start, the minimum redundancy combination in a nearest neighbor scheme of six telescopes over six baselines, thus offering better options for rejection of large intensity or phase fluctuations over each beam, due to the symmetric set-up. The planar geometry solution of the FT beam combiner is devised to be easily scalable either to four or eight telescopes, in accordance to the three phase development considered for VSI. The proposed design, based on minimum redundancy combination and bulk optics solution, is described in terms of opto-mechanical concept, performance and key operational aspects., Comment: 11 pages, to be published in Proc. SPIE conference 7013 "Optical and Infrared Interferometry", Schoeller, Danchi, and Delplancke, F. (eds.)

The design and performance of a Fizeau interferometer with long focal length and large field of view are discussed. The optical scheme presented is well suited for very accurate astrometric measurements from space, being optimised, in terms of geometry and aberrations, to observe astronomical targets down to the visual magnitude mV=20, with a measurement accuracy of 10 microarcseconds at mV=15. This study is in the context of the next generation astrometric space missions, in particular for a mission profile similar to that of the Gaia mission of the European Space Agency. Beyond the accuracy goal, the great effort in optical aberrations reduction, particularly distortion, aims at the optimal exploitation of data acquisition done with CCD arrays working in Time Delay Integration mode. The design solution we present reaches the astrometric goals with a field of view of 0.5 square degrees.

We describe a software tool developed to simulate the behaviour of the angle between two lines of sight in a dual view telescope assembly (usually referred to as basic angle) due to optical misalignments induced by thermo-mechanical fluctuations. The tool applies to a variety of reflective optical designs. In principle, not only the basic angle behaviour can be simulated, but also other optical parameters. As a practical example, we present and discuss results obtained from application of our software to the case of the Gaia baseline optical design. We show that the final error can be severely degraded by fluctuations of the basic angle due to thermo-mechanical effects.

ESO and the Astronomical Observatory of Torino (OATo) are collaborating for development of a Fringe Sensor Unit (FSU) for the Very Large Telescope Interferometer (VLTI, see [1] and references therein): FINITO (Fringe-tracking Instrument of Nice and Torino), derived from the concept demonstrated by a prototype from the Observatoire de la Cote d’Azur. It will be used for stabilisation of the Optical Path Difference (OPD) between telescope beams for the first generation scientific interferometers (AMBER, MIDI, VINCI), providing an improvement of up to five magnitudes in limiting sensitivity, thanks to the integration time increased from a few ten milliseconds up to minutes. Accordingly to current planning, FINITO is the only FSU able to support three-beam observations, allowing closure phase measurements in the near and mid IR range.

We present the design of a Fizeau interferometer to be implemented for the GAME mission. The aim is to measure the PPN γ parameter with the same technique used for the first time by Dyson, Eddington et al., but at a 10−6 accuracy level. GAME will observe about 106 sufficiently bright stars at about 2° from the Sun. A dedicated space mission has the advantage of observing the light bending without waiting for an eclipse.

We discuss the design and the performance of a Fizeau interferometer with a long focal length and a large field of view that is well suited for a global astrometry space mission. Our work focuses on the geometric optimization and minimization of aberration of such an astrometric interferometer, which is able to observe astronomical targets down to the visual magnitude (mag) mv = 20 mag, with an accuracy in the measurements of 10 micro-arcseconds at mv = 15 mag. We assume a mission profile similar to that of the Global Astrometric Interferometer for Astrophysics mission of the European Space Agency. In this framework, data acquisition is performed by an array of CCDs working in time-delay integration mode. Optical aberrations, particularly distortion and coma, play a crucial role in the efficiency of this technique. We present a design solution that meets the requirements for the best possible exploitation of the time-delay integration mode over a field of view of 0.7° × 0.7°.

PROBA-3 is the forth of the PROBA technological mission designed by the European Space Agency. The main purpose of PROBA-3 is to demonstrate the capabilities of two different spacecraft to perform in-orbit formation flight. Furthermore, the two satellites will form a scientific instrument, a solar coronagraph, for acquiring relevant data related to the corona of the Sun. In order to make the mission feasible, several novel technologies compose the mission formation flight system. The Shadow Position Sensor (SPS) system is one of the metrological systems designed for providing the satellite relative position with an high level of accuracy.This paper describes the algorithm developed for providing the spacecraft relative position to the mission Guidance and Navigation Control System (GNC). Furthermore, the achieved results in terms of precision are presented. Lastly, an overview of the implemented logic, from the measurements acquisition to the computed position, is provided.