Luc Damé, Eleanore Fringhian rupert, Patrick Lacroix, Pierre Gilbert, Nicolas Caignard, Mustapha Meftah, Nicolas Rouanet, Pierre Etcheto, Jacques Berthon
SUAVE (Solar Ultraviolet Advanced Variability Experiment) is a far UV imaging solar telescope (Lyman Alpha, 121.6 nm, Herzberg continuum, 200-242 nm, etc.) of novel design for ultimate thermal stability and long lasting performances over several years instead of, often, a few weeks or months in this wavelength range. SUAVE is a 80 mm Ritchey-Chrétien off-axis telescope with "mushroom type" SiC mirrors and no entrance window for long and uncompromising observations in the UV (no coatings of mirrors, flux limited to less than 2 solar constants on filters to avoid their degradation), associated with an ultimate thermal control (no central obscuration resulting in limited thermal gradients and easier heat evacuation, focus control, stabilization). Design and performances will be detailed as well as results of thermal/optical tests performed on the SiC primary mirror and its regulated support plate also in SiC. Plans for the realization of a representative breadboard for testing of both optical and thermal properties are presented. SUAVE is the main instrument of the Solar/Climate microsatellite SoSWEET mission (Solar ultraviolet variability and Space Weather Extreme EvenTs).
There is a surge in interest for the ultra wide bandgap (Eg ~ 4.9 eV) semiconductor gallium oxide (Ga2O3). A key driver for this boom is that single crystal wide area bulk β-Ga2O3 substrates have become commercially available and that a variety of methods have been shown to give high quality epitaxial growth. Amongst a whole range of potential applications (power/switching electronics, solar transparent electrodes, etc.) extreme solar blindness photodetectors in β- Ga2O3, the more stable monoclinic phase of Ga2O3, offers the most exciting perspectives for deep ultraviolet observations below 250 nm. We present an overview of the complete realization process (epitaxy, photolithography, singulation and packaging), and space qualification (to TRL8) of a series of β-Ga2O3 photodetectors We insist on their remarkable solar blindness performance (factor 1000 or more between peak responsivity around 215-220 nm and 250 nm) and dynamical properties (rise and fall times < 5 ms), along with their enhanced responsivity, excellent thermal behaviour and radiation hard properties. 4 of these detectors have been integrated on the INSPIRE-SAT 7 nanosatellite expected to be launched in April 2023 to reach the TRL9 "demonstrated in flight" qualification.
This conference presentation was prepared for the Space Telescopes and Instrumentation 2022: Ultraviolet to Gamma Ray conference at SPIE Astronomical Telescopes + Instrumentation, 2022.
We present measurement protocols of performances, test and calibrations of new compact solid-state photodetectors based on β-Ga2O3 oxides, and optimized for the UVC. They present reduced dark currents, permitting room temperature operation suppressing need for a cooling system (mass and power savings) and avoiding cold surfaces that traps environmental contamination. Detectors' response peak around 215-220 nm with a bandpass of 30 nm, allowing to observe the UVC wavelength band responsible of ozone creation in the stratosphere (Herzberg continuum, 200-242 nm) and to achieve solar-blindness for wavelengths above 250 nm. Other key assets of β-Ga2O3 detectors are their radiation hard properties (longer lifetime), and possible sensitivity (several hundreds mA/W at -5 V) that allows operation at lower voltages (reduced power), a key asset for Space applications. These detectors, evaluated, tested and calibrated, will be integrated on the INSPIRE-7 nanosatellite to be launched in 2023.
Ga2O3 layers were grown on c-sapphire substrates by pulsed laser deposition. Optical transmission spectra were coherent with a bandgap engineering from 4.9 to 6.2 eV controlled via the growth conditions. X-ray diffraction revealed that the films were mainly β-Ga2O3 (monoclinic) with strong (-201) orientation. Metal-Semiconductor-Metal photodetectors based on gold/nickel Inter- Digitated-Transducer structures were fabricated by single-step negative photolithography. 240 nm peak response sensors gave over 2 orders-of-magnitude of separation between dark and light signal with state-of-the-art solar and visible rejection ratios ((I240 : I290) of > 3 x 105 and (I240 : I400) of > 2 x 106) and dark signals of <50 pA (at a bias of -5V). Spectral responsivities showed an exceptionally narrow linewidth (16.5 nm) and peak values exhibited a slightly superlinear increase with applied bias up to a value of 6.5 A/W (i.e. a quantum efficiency of > 3000%) at 20V bias.
SUAVE (Solar Ultraviolet Advanced Variability Experiment) is a far UV imaging solar telescope (Lyman Alpha, 121.6 nm, Herzberg continuum, 200-242 nm, etc.) of novel design (off-axis telescope with "mushroom type" SiC mirrors) for ultimate thermal stability and long lasting performances over years instead of, often, a few weeks or months in this wavelength range. SUAVE has no entrance window for long and uncompromising observations in the UV (no coatings of mirrors, flux limited to less than 2 solar constants on filters to avoid their degradation), associated with an ultimate thermal control (no central obscuration resulting in limited thermal gradients and easier heat evacuation, focus control, stabilization). Design and anticipated performances will be detailed as well as results of representative thermal/optical tests performed on the SiC primary mirror and support regulated plate (SiC also). SUAVE is the main instrument of the Solar/Climate SoSWEET-SOUP constellation mission.
STARTIGER stands for "Space Technology Advancements by Resourceful, Targeted and Innovative Groups of Experts and Researchers". STARTIGER is an ESA initiative within the Basic Technology Research Program (TRP) aiming at facilitating innovative and breakthrough research. The STARTIGER approach encourages the investigation of novel ideas, the aim being to achieve technology breakthroughs with the maximum likelihood of success in the shortest period of time (6 months or less). In the framework of the ESA STARTIGER initiative, Laboratoire d'Astrophysique de Marseille (LAM, France) associated with Laboratoire Atmosphères, Milieux, Observations Spatiales (LATMOS, France), and together with Centre Spatial de Liège (CSL, Belgium), Osservatorio Astronomico di Torino (INAF-OATo, Italy), University of Athens (UoA/IASA, Greece), and Rutherford Appleton Laboratory (RAL, UK), proposed to study the new generation of formation flying solar coronagraphs incorporating the most sophisticated capabilities to achieve diagnostics measurements in the solar corona in a unified, coordinated and progressive approach.
PICARD is a mission devoted to solar variability observation, which aims at perpetuating valuable historical time-series of the solar radius. PICARD contains a double program with in-space and on-ground measurements using Ritchey-Chrétien telescopes. The PICARD spacecraft was launched on June 15, 2010, commissioned in-flight in October of the same year, and was retired in April 2014. PICARD ground-based observatory is functional since May 2011 in the Plateau de Calern (France), and is still operational today. We shall give an overview of the PICARD instrumentation and the performances of the existing ground-based telescope. We will also present our current results about solar radius variations after eight years of solar observations.
SUAVE (Solar Ultraviolet Advanced Variability Experiment) is a far UV imaging solar telescope (Lyman Alpha, 121.6 nm, Herzberg continuum, 200-220 nm, etc.) of novel design for ultimate thermal stability and long lasting performances. SUAVE is a 80 mm Ritchey-Chrétien off-axis telescope with SiC mirrors and no entrance window for long and uncompromised observations in the UV (no coatings of mirrors, flux limited to less than 2 solar constants on filters to avoid degradation), associated with an ultimate thermal control (no central obscuration resulting in limited thermal gradients and easier heat evacuation, focus control, stabilization). Design and anticipated performances will be detailed as well as the realization process under way. Tests on a representative breadboard will be performed in 2018 (CNES R&T). SUAVE is the main instrument of the SUITS/SWUSV (Solar Ultraviolet Influence on Troposphere/Stratosphere / Space Weather and Ultraviolet Solar Variability) microsatellite mission.
Within the past decade, satellites constellations have become possible and practical. One of the interest to use a satellites constellation is to measure the true Earth Radiation Imbalance, which is a crucial quantity for testing climate models and for predicting the future course of global warming. This measurement presents a high interest because the 2001-2010 decade has not shown the accelerating pace of global warming that most models predict, despite the fact that the greenhouse-gas radiative forcing continues to rise. All estimates (ocean heat content and top of atmosphere) show that over the past decade the Earth radiation imbalance ranges between 0.5 to 1W-2. Up to now, the Earth radiation imbalance has not been measured directly. The only way to measure the imbalance with sufficient accuracy is to measure both the incoming solar radiations (total solar irradiance) and the outgoing terrestrial radiations (top of atmosphere outgoing longwave radiations and shortwave radiations) onboard the same satellite, and ideally, with the same instrument. The incoming solar radiations and the outgoing terrestrial radiations are of nearly equal magnitude of the order of 340.5W-2. The objective is to measure these quantities over time by using differential Sun-Earth measurements (to counter calibration errors) with an accuracy better than 0.05Wm-2 at 1σ. It is also necessary to have redundant instruments to track aging in space in order to measure during a decade and to measure the global diurnal cycle with a dozen satellites. Solar irradiance and Earth Radiation Budget (SERB) is a potential first in orbit demonstration satellite. The SERB nano-satellite aims to measure on the same platform the different components of the Earth radiation budget and the total solar irradiance. Instrumental payloads (solar radiometer and Earth radiometers) can acquire the technical maturity for the future large missions (constellation that insure global measurement cover) by flying in a CubeSat. This paper is intended to demonstrate the ability to build a low-cost satellite with a high accuracy measurement in order to have constant flow of data from space.
This paper, “Investigation of the low flux servo-controlled limit of a co-phased interferometer," was presented as part of International Conference on Space Optics—ICSO 1997, held in Toulouse, France.
The PICARD microsatellite mission will provide 3 to 4 years simultaneous measurements of the solar diameter, differential rotation and solar constant to investigate the nature of their relations and variabilities. The major instrument, SODISM, is a whole Sun imaging telescope of Ø110 mm which will deliver an absolute measure (better than 4 mas) of the solar diameter and solar shape. Now in Phase B, PICARD is expected to be launched by 2005. We recall the scientific goals linked to the diameter measurement with interest for Earth Climate, Space Weather and Helioseismology, present the instrument optical concept and design, and give a brief overview of the program aspects.
Classical externally-occulted coronagraphs are presently limited in their performances by the distance between the external occulter and the front objective. The diffraction fringe from the occulter and the vignetted pupil which degrades the spatial resolution prevent useful observations of the white light corona inside typically 2-2.5 solar radii (Rsun). Formation flying offers and elegant solution to these limitations and allows conceiving giant, externally-occulted coronagraphs using a two-component space system with the external occulter on one spacecraft and the optical instrument on the other spacecraft at a distance of hundred meters [1, 2]. Such an instrument ASPIICS (Association de Satellites Pour l’Imagerie et l’Interférométrie de la Couronne Solaire) has been selected by the European Space Agency (ESA) to fly on its PROBA-3 mission of formation flying demonstration which is presently in phase B (Fig. 1). The classical design of an externally-occulted coronagraph is adapted to the formation flying configuration allowing the detection of the very inner corona as close as ∼0.04 solar radii from the solar limb. By tuning the position of the occulter spacecraft, it may even be possible to reach the chromosphere and the upper part of the spicules [3]. ASPIICS will perform (i) high spatial resolution imaging of the continuum K+F corona in photometric and polarimetric modes, (ii) high spatial resolution imaging of the E-corona in two coronal emission lines (CEL): Fe XIV and He I D3, and (iii) two-dimensional spectrophotometry of the Fe XIV emission line. ASPIICS will address the question of the coronal heating and the role of waves by characterizing propagating fluctuations (waves and turbulence) in the solar wind acceleration region and by looking for oscillations in the intensity and Doppler shift of spectral lines. The combined imaging and spectral diagnostics capabilities available with ASPIICS will allow mapping the velocity field of the corona both in the sky plane (directly on the images) and along the line-of-sight by measuring the Doppler shifts of emission lines in an effort to determine how the different components of the solar wind, slow and fast are accelerated. With a possible launch in 2014, ASPIICS will observe the corona during the maximum of solar activity, insuring the detection of many Coronal Mass Ejections (CMEs). By rapidly alternating high-resolution imaging and spectroscopy, CMEs will be thoroughly characterized.
Formation flying, with ESA’s mission PROBA-3, is providing the chance of creating a giant solar coronagraph in Space. The scientific payload, the solar coronagraph ASPIICS, has been selected in January 2009 [1]. The advantages of formation flying are: 1) larger dimensions for the coronagraph, which leads to better spatial resolution and lower straylight level and 2) possibility of continuous observations of the inner corona. The PROBA-3/ASPIICS mission is composed of two spacecrafts (S/Cs) at 150 meters distance, the Occulter-S/C (O-S/C) which holds the external occulter, and the Coronagraph-S/C (C-S/C) which holds the main instrument, i.e. the telescope. In addition of the scientific capabilities of the instrument, it will continuously monitor the exact position and pointing of both S/Cs in 3D space, via two additional metrology units: the Shadow Position Sensor (SPS) and the Occulter Position Sensor (OPS). In this paper we are presenting the metrology of this formation flying mission combining the outputs of the above mentioned sensors, SPS and OPS. This study has been conducted in the framework of an ESA “STARTIGER” initiative, a novel approach aimed at demonstrating the feasibility of a new and promising technology concept (in our case formation flying applied to solar coronagraphy, cf. [2, 3]) on a short time scale (six months study).
SOLAR/SOLSPEC, a spectroradiometer measuring solar spectral irradiance is an instruments of the SOLAR payload mounted on the zenith external platform of the European Columbus module of the International Space Station. Solar flux is received by the SOLAR instruments thanks to the Coarse Pointing Device (CPD). A complementary Sun position tracking module, the Position Sensitive Device (PSD), is integrated in SOLAR/SOLSPEC. The PSD module has been a useful tool to monitor for misalignments between the CPD and the SOLAR payload. It is used in SOLAR/SOLSPEC’s operations to follow the quality of the Sun tracking. The PSD module is also valuable to monitor for SOLAR/SOLSPEC’s three spectrometers (ultraviolet, visible, infrared) angular response in orbit. We first give a detailed description of the PSD’s functionalities. We then present the results of the PSD data analysis. We will show that the PSD module has, despite working in a severe space environment, preserved its full potential from 2008 up to 2017 thanks to its design and appropriate selection of components. We conclude that its robustness makes of the PSD module a simple, yet reliable, instrument useful for future long term space-based missions.
The space environment is considered hazardous to spacecraft, resulting in materials degradation. Understanding the degradation of space-based instruments is crucial in order to achieve the scientific objectives, which are derived from these instruments. This paper discusses the on-orbit performance degradation of recent spacebased solar instruments. We will focus on the instruments of three space-based missions such as the Project for On-Board Autonomy 2 (PROBA2) spacecraft, the Solar Monitoring Observatory (SOLAR) payload onboard the Columbus science Laboratory of the International Space Station (ISS) and the PICARD spacecraft. Finally, this paper intends to understand the degradation processes of these space-based solar instruments.
M. Meftah, T. Corbard, A. Hauchecorne, A. Irbah, P. Boumier, A. Chevalier, W. Schmutz, R. Ikhlef, F. Morand, C. Renaud, J.-F. Hochedez, G. Cessateur, S. Turck-Chièze, D. Salabert, M. Rouzé, M. van Ruymbeke, P. Zhu, S. Kholikov, S. Koller, C. Conscience, S. Dewitte, L. Damé, D. Djafer
KEYWORDS: Space telescopes, Telescopes, Space operations, Radiometry, Aerospace engineering, Sun, Observatories, Solar energy, Solar processes, Sensors
PICARD is a mission devoted to solar variability observations through imagery and radiometric measurements. The main goal is to provide data for scientific investigation first in the area of solar physics, and second in the assessment of the influence of the solar variability on the Earth climate variability. PICARD contains a double program with in-space and on-ground measurements. The PICARD spacecraft was launched on June 15, 2010, commissioned in-flight in October of the same year and was retired in April 2014. The PICARD ground-based observatory is operational since May 2011. We shall give a short overview of the PICARD instrumentation. New estimates of the absolute values of the total solar irradiance, of the solar spectral irradiance at typical wavelengths, and of the solar oblateness will be given. We will also report about helioseismic studies. Finally, we will present our current results about solar radius variations after six years of solar observation.
The scientific objectives of a space mission result into instrumental developments and specific satellite operations
to observe astronomical objects of interest. The payload in its space environment is however subject to important
thermal variations that affect observations. This is well observed when images of the Sun are recorded with the
constraint of keeping the solar rotational axis in a constant direction relatively to the camera reference frame.
Consequences are clearly observed on image positions that follow the thermal variations induced by the satellite
orbit. This is, in particular, the case for the space mission PICARD. This phenomenon is similar to defocus
and motions of images recorded with ground-based telescopes. We first present some simulations showing these
effects. We then compare our results with real data obtained from the space mission PICARD.
The Earth’s atmosphere introduces a spatial frequency filtering in the object images recorded with ground-based instruments. A solution is to observe with telescopes onboard satellites to avoid atmospheric effects and to obtain diffraction limited images. However, similar atmosphere problems encountered with ground-based instruments may subsist in space when we observe the Sun since thermal gradients at the front of the instrument affect the observations. We present in this paper some simulations showing how solar images recorded in a telescope focal plane are directly impacted by thermal gradients in its pupil plane. We then compare the results with real solar images recorded with the PICARD mission in space.
SUAVE (Solar Ultraviolet Advanced Variability Experiment) is a far ultraviolet (FUV) imaging solar telescope of novel design for ultimate thermal stability and long lasting performances. SUAVE is a 90 mm Ritchey- Chrétien telescope with SiC (Silicon Carbide) mirrors and no entrance window for long and uncompromised observations in the UV (no coatings of mirrors, flux limited to less than a solar constant on filters to avoid degradation), associated with an ultimate thermal control (heat evacuation, focus control, stabilization). Design of the telescope and early thermal modeling leading to a representative breadboard (a R and T program supported by CNES) will be presented. SUAVE is the main instrument of the SUITS (Solar Ultraviolet Influence on Troposphere/Stratosphere) microsatellite mission, a small-size mission proposed to CNES and ESA.
Since the launch of the first artificial satellite in 1957, more than 6,000 satellites have been sent into space. Despite technological advances, the space domain remains little accessible. However, with the miniaturization of electronic components, it has recently become possible to develop small satellites with which scientific goals can be addressed. Micro-satellites have demonstrated that these goals are achievable. However, completion times remain long. Today, we hope through the use of nano-satellites to reduce size, costs, time of development and accordingly to increase accessibility to space for scientific objectives. Nano-satellites have become important tools for space development and utilization, which may lead to new ways of space exploration. This paper is intended to present a future space mission enabled by the development of nano-satellites and the underlying technologies they employ. Our future mission expands observations of the Sun (total solar irradiance and solar spectral irradiance measurements) and of the Earth (outgoing long-wave radiation, short-wave radiation measurements and stratospheric ozone measurements). Constellations of nano-satellites providing simultaneous collection of data over a wide area of geo-space may be built later and present a great interest for Sun-Earth relationships.
Formation Flying is now considered to be the most promising and effective approach to deploy the forthcoming
generation of very large instruments in space. PROBA-3 is a technology mission devoted to the in-orbit demonstration of
formation flying techniques and technologies. PROBA-3 will implement a giant coronagraph (called ASPIICS) that will
both demonstrate and exploit the capabilities and performances of formation flying. ASPIICS is distributed on two
spacecrafts separated by 150m, one hosting the external occulting disk and the other the optical part of the coronagraph.
ASPIICS will incorporate metrology units which will allow determining both the absolute pointing and the relative
alignment of the formation. Photosensors located around the entrance pupil of the coronagraph will determine the
absolute positioning of the instrument by sensing the penumbra behind the occulting disk. Light sources located on the
rear-side of the occulting disk will allow verifying the alignment of the formation. We carried out a complete numerical
simulation of the metrology system and showed how corrections are derived from the measurements to be applied to
each spacecraft in case of misalignments. This simulation was validated by a scaled model of the coronagraph developed
at Laboratoire d'Astrophysique de Marseille. This study has been conducted in the framework of an ESA
"STARTIGER" Initiative, a novel approach aimed at demonstrating the feasibility of a new and promising technology on
a very short time scale (six months).
Sébastien Vives, Luc Damé, Philippe Lamy, A. Antonopoulos, W. Bon, G. Capobianco, G. Crescenzio, V. Da Deppo, M. Ellouzi, J. Garcia, C. Guillon, A. Mazzoli, T. Soilly, F. Stathopoulos, C. Tsiganos
Formation Flying opens the possibility to conceive and deploy giant solar coronagraphs in space permanently
reproducing the optimum conditions of a total eclipse of the Sun ("artificial" eclipse) thus giving access to the inner
corona with unprecedented spatial resolution and contrast (low stray light). The first opportunity to implement such a
coronagraph "ASPIICS" will be offered by the European Space Agency (ESA) PROBA-3 technology mission devoted to
the in-orbit demonstration of formation flying technologies. Two spacecrafts separated by about 150 m form a giant
externally-occulted coronagraph: the optical part hosted by one spacecraft remains entirely protected from direct sunlight
by remaining in the shadow of an external occulter hosted by the other spacecraft. We developed and tested a scale-model
'breadboard' (i.e., 30m) of the PROBA-3/ASPIICS Formation Flying coronagraph. The investigations focused on
two metrology systems capable of measuring both the absolute pointing of the coronagraph (by sensing the projected
shadow and penumbra produced by the external occulting disk) and the alignment of the formation (by re-imaging light
sources located on the rear-side of the occulting disk with the optical part of the coronagraph). In this contribution, we
will describe the demonstrator and report on our results on the crucial question of the alignment and pointing in space of
long instruments (> 100 m) with an accuracy of a few arcsec. This study has been conducted in the framework of an ESA
"STARTIGER" Initiative, a novel approach aimed at demonstrating the feasibility of a new and promising technology on
a very short time scale (six months).
This article describes the calibration and alignment procedures of a demonstrator for the ASPIICS coronagraph proposed
for the ESA technology mission PROBA-3 aimed at demonstrating the feasibility of a Formation Flying coronagraph.
ASPIICS is distributed on two spacecrafts separated by 150 m, one hosting the external occulting disk and the other the
optical part of the coronagraph. The purpose of the demonstrator is to reproduce on ground the metrology systems that
will equip the coronagraph in order to realize the alignment of the two spacecrafts and the absolute pointing to the center
of the Sun. The demonstrator is composed of a device that reproduces the solar umbra/penumbra created by the solar
occulter[1] and of a Three Mirror Anastigmatic (TMA) telescope mounted on a hexapod, a new-generation platform that
allows 6 degrees of freedom. A large plane folding mirror is used on ground to obtain a distance between the occulter
and the TMA up to 30 m. Photo sensors located around the entrance pupil of the TMA determine the absolute positioning
of the instrument by sensing the penumbra behind the occulting disk. Light sources (LEDs) located on the rear-side of
the occulting disk allow verifying the alignment of the formation. The paper describes the whole demonstrator, its
integration, its calibration, and the performance of the metrology systems of the coronagraph. This study has been
conducted in the framework of an ESA "STARTIGER" Initiative, a novel approach aimed at demonstrating the
feasibility of a new and promising technology on a very short time scale (six months).
Classical externally-occulted coronagraphs are presently limited in their performances by the distance between the
external occulter and the front objective. The diffraction fringe from the occulter and the vignetted pupil which degrades
the spatial resolution prevent useful observations of the white light corona inside typically 2-2.5 solar radii. Formation
flying offers an elegant solution to these limitations and allows conceiving giant, externally-occulted coronagraphs using
a two-component space system with the external occulter on one spacecraft and the optical instrument on the other
spacecraft at a distance of hundred meters. Such an instrument, ASPIICS (Association de Satellites Pour l'Imagerie et
l'Interférométrie de la Couronne Solaire), has just been selected by the European Space Agency (ESA) to fly (launch
expected in 2014) on its third PROBA (Project for On-Board Autonomy) mission of formation flying demonstration
which is presently in phase B. It will perform both high spatial resolution imaging of the solar corona as well as 2-
dimensional spectroscopy of several emission lines (in particular the forbidden line of FeXIV at 530.285 nm) from the
coronal base out to 3 solar radii. For this, it will use filters, polarisers and a solid Fabry-Perot interferometer ("étalon").
The classical design of an externally-occulted coronagraph is adapted to the formation flying configuration allowing the
detection of the very inner corona as close as 0.04-0.05 solar radii (40-50 arcsec) from the solar limb. By tuning the
position of the occulter spacecraft, it may even be possible to reach the chromosphere and the upper part of the spicules.
Formation flying opens new perspectives in solar physics, and allow to conceive giant, externally-occulted coronagraphs
using a two-component space system with the external occulter on one spacecraft and the optical
instrument on the other spacecraft at a distance of hundred meters. Conditions close to those of a solar total
eclipse can then be achieved offering the capability of imaging the solar corona down to the limb at very high
spatial resolution. ASPIICS (Association de Satellites Pour l'Imagerie et l'Interférométrie de la Couronne Solaire)
is a mission proposed to ESA in the framework of its PROBA-3 demonstration program of formation flying
which is presently in phase A. ASPIICS is a single coronagraph which will perform both high spatial resolution
imaging of the solar corona as well as 2-dimensional spectroscopy of several emission lines from the coronal base
out to 3 R&beye; using a Fabry-Pérot étalon interferometer. The classical design of an externally-occulted coronagraph
is adapted to the formation flying configuration allowing the detection of the very inner corona as close as
0.01 R&beye; from the solar limb. Super-ASPIICS is an even more ambitious instrument part of the scientific payload
of HiRise, the High Resolution Imaging and Spectroscopy Explorer proposed to ESA in the framework of its
Cosmic Vision program. With an increased inter-satellite distance of 280 m, an aperture of 300 mm, a spectral
domain extending from the ultraviolet to the near-infrared, and spectroscopic capabilities, Super-ASPIICS will
offer unprecedented diagnostic capabilities, including the measurement of coronal magnetic fields.
We present the Active Stabilization in Stellar Interferometry (ASSI) beam combining optical table which was installed on the 2- telescope interferometer (I2T) of the Observatoire de la Cote d'Azur in 1993. To achieve very high angular resolution, the 26- centimeter telescopes can be positioned along a 140-meter North- South baseline. The limiting magnitude of the instrument depends dramatically on its ability to stabilize the fringe pattern despite the atmospheric disturbances. The function of the ASSI table is to perform this task. Three adaptive mirrors are used. The first two are fine pointing mirrors which correct the fluctuations of the angle of arrival of the two wavefronts. The other corrects the optical path difference fluctuations between the two telescopes. These corrections, e.g. tip-tilt and piston phase, are required to obtain high precision visibility measurements. We present our first observing results obtained on bright stars that have allowed the evaluation of the ASSI table performance in image tracking.
Recent advances in electronics and fast computer control allow to envisage extremely high spatial resolution observations of the Sun through the use of a compact array of phased telescopes. Several space missions (SUN/SIMURIS, SUN-SV, MUST/SIMURIS) have been proposed in that respect and will be briefly presented. Prospects for use of the space techniques for a solar array on ground are also indicated. Independently from the different mission concepts, solar interferometric imaging presents a unique case in the domain of optical aperture synthesis since the field- of-view is extended (larger than the diffraction spot of a telescope) and because the high resolution structures are evolving very rapidly and are naturally complex (low fringe visibility). These severe constraints drive solar arrays' design towards `compact' configurations (i.e. in which the spatial frequencies plane is filled) and real-time `cophasing' (direct-- hardware--zeroing of phase fluctuations by fine delay lines). They also influence the choice of the focal instrumentation which is optimum when using a subtractive double monochromator tunable over a large spectral range and providing narrow band filtergrams (up to 0.1 angstrom). We review the concepts and design issues of a solar interferometer and present numerical simulations and laboratory experiments of the system required to cophase an array of telescopes on a complex and extended field-of-view. Aperture configurations and image reconstruction are also discussed as well as the specific real-time metrology aspects of a ground array (atmospheric constraints derived from the performances evaluation of the ASSI Program).
The Solar Ultraviolet Network (SUN) is an instrument based on interferometric concepts, and capable of observations with a spatial resolution of 0.013' (10 km) on the Sun, in the UV and visible wavelength ranges. In this paper we present results on fringe pattern acquisition and stabilization as performed on a Mach-Zehnder set up representative of the interferometer cophasing system. The system algorithm is based on 'white light' fringe tracking controlled in a reference interferometer by a synchronous detection. This servo-system drives a two-stages delay line for real-time compensation of the optical path delays. Acquisition capabilities and stability possibilities are investigated as a function of flux and noise levels. Being stabilized, actively cophased, and in a 'compact' configuration, the SUN interferometer possesses remarkable imaging capabilities allowing high resolution diffraction-limited imaging on an extended field of view of 6 X 6 arcsec2. The dynamics of reconstructed images is superior to 400 for phase stabilities >= (lambda) /6 and photon flux of approximately 10,000 ph s-1 pixel-1 (on average). The SUN instrument is part of the Solar Interferometric Mission for Ultrahigh Resolution Imaging and Spectroscopy (SIMURIS) which was proposed to ESA in the framework of the Next Medium Size Mission (M2) in November 1989, and which completed a First Phase of Study in the context of the Space Station in August 1991.
We briefly recall the characteristics and particularities of the optical recombination of the Solar
Ultraviolet Network (SUN) experiment which consists in a non-redundant << compact >> array of 4
telescopes of 20 cm diameter aligned on a 2 meters linear baseline. We proceed to the evaluation of the
geometrical and optical aberrations of the recombination outlining the difficulties inherent to
interferometric systems (in particular, and forgotten up to now, the << cophasing condition >>).We
conclude on the advantages linked to a compact system like SUN.
We report preliminary results on fringe pattern acquisition and stabilization as performed on a Mach-
Zehnder set up representative of Stellar Interferometry needs. The system algorithm is based on "white
light" fringe tracking controlled from a reference interferometer synchronous detection (central fringe
locked interferometry). This servo-system drives a simple two-stages delay line for real time
compensation of the optical path delays either due to atmospheric or instrumental errors. Contrasts (nonoptimized)
of 70 % on the stabilized fringe pattern were measured, strongly encouraging to pursue the
development of the technique.
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