Dr. Fineschi has more than 20 years of experience in the development and operation of space instrumentation for spectroscopy and polarimetry in the visible and UV wavelength bands. His scientific interests include the application of atomic spectroscopy and polarimetry to UV and visible-light remote-sensing techniques for the study of the solar wind and the coronal magnetic field. He has been involved in several solar space missions of NASA and ESA (SMM, Spartan/Space-Shuttle, SOHO, Solar Orbiter), sounding-rocket solar experiments (SCORE), and eclipse campaigns. He is currently the Project Scientist for the METIS coronagraph on the Solar Orbiter mission and the Principal Investigator of the sounding-rocket coronographic experiment (SCORE) for the sub-orbital re-flight of the HERSCHEL payload.
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Liquid crystal variable retarders (LCVRs) will be used in the polarization modulation packages (PMPs) of the instruments SO/PHI (Polarimetric and Helioseismic Imager) and METIS/COR (Multielement Telescope for Imaging and Spectroscopy, Coronagraph) of the Solar Orbiter Mission of the European Space Agency (ESA). Optical retarders are dependent on the angle of incidence (AOI). Since the optical retardances during the polarization modulations are optimized for a particular AOI, other angles increase the polarimetric measurement error. Coronagraphs, such as METIS, are characterized by having wide field-of-view (FoV), which involves large incidence angles through the entire instrument. METIS PMP will work with collimated beams and an AOI up to ±7.0 deg. For this reason, a double LCVR configuration with molecular tilts in opposite directions was selected for METIS PMP, which provides lower angular dependence. The polarimetric performance of the METIS PMP flight model was measured at different AOIs and compared to a single LCVR PMP. The results shown in this paper demonstrate that the functional concept used in METIS guarantees the polarimetric performances at the wide FoV expected in METIS coronagraph. Moreover, a detailed theoretical model is showed and compared to the experimental data, finding successful agreement, which can be very helpful for the design of instruments characterized by wide FoV.
The ASPIICS instrument on PROBA-3 looks at the solar corona through a refractive telescope, able to select 3 different spectral bands: Fe XIV line @ 530.4nm, He I D3 line @587.7nm, and the white-light spectral band [540;570nm]. The external occulter being located at ~ 150 meters from the instrument entrance, will allow ASPIICS to observe the corona really close to the solar limb, probably closer than any internally or externally occulted coronagraph ever observed.
This paper will present the straylight model and analyses carried out by CSL. A first specificity of the analysis is that the scene on the useful Field of View (FOV) is the solar corona which has a brightness dynamic range as high as 103 between the close corona, close to 1 solar radius (Rsun), and the “distant” corona around 3RSun. The specifications are very stringent for this type of instrument. A consensus was found and will be presented regarding the expected straylight within the FOV. It will also be shown that to achieve realistic estimations it is required to take into account the exact location of the created straylight as well as the entrance field.
The second specificity that had to be analyzed is that the diffraction from the solar disk by the external occulter enters the instrument un-obstructed until the internal occulter, and with a brightness 100 times higher than the close corona (~1RSun) brightness. The simulation of this diffraction as well as its propagation inside the ASPIICS telescope creating additional straylight, had to be carefully established in order to give realistic results of its impact on the performances while being actually possible to compute.
The entire alignment and verification phase has been performed by the Metis team in collaboration with Thales Alenia Space Torino and took place in ALTEC (Turin) at the Optical Payload System Facility using the Space Optics Calibration Chamber infrastructure, a vacuum chamber especially built and tested for the alignment and calibration of the Metis coronagraph, and suitable for tests of future payloads.
The goal of the alignment, integration, verification and calibration processes is to measure the parameters of the telescope, and the characteristics of the two Metis channels: visible and ultraviolet. They work in parallel thanks to the peculiar optical layout. The focusing and alignment performance of the two channels must be well understood, and the results need to be easily compared to the requirements. For this, a dedicated illumination method, with both channels fed by the same source, has been developed; and a procedure to perform a simultaneous through focus analysis has been adopted.
In this paper the final optical performance achieved by Metis is reported and commented.
The stray light calibration was performed in a clean environment in front of the OPSys solar disk divergence simulator (at ALTEC, in Torino, Italy), which is able to emulate different heliocentric distances. Ground calibrations were a unique opportunity to map the Metis stray light level thanks to a pure solar disk simulator without the solar corona. The stray light calibration was limited to the visible light case, being the most stringent. This work is focused on the description of the laboratory facility that was used to perform the stray light calibration and on the calibration results.
Metis features two channels to image the solar corona in two different spectral bands: in the HI Lyman ∝ at 121.6 nm, and in the polarized visible light band (580 – 640 nm). Metis is a solar coronagraph adopting an “inverted occulted” configuration. The inverted external occulter (IEO) is a circular aperture followed by a spherical mirror which back rejects the disk light. The reflected disk light exits the instrument through the IEO aperture itself, while the passing coronal light is collected by the Metis telescope. Common to both channels, the Gregorian on-axis telescope is centrally occulted and both the primary and the secondary mirror have annular shape.
Classic alignment methods adopted for on-axis telescope cannot be used, since the on-axis field is not available. A novel and ad hoc alignment set-up has been developed for the telescope alignment.
An auxiliary visible optical ground support equipment source has been conceived for the telescope alignment. It is made up by four collimated beams inclined and dimensioned to illuminate different sections of the annular primary mirror without being vignetted by other optical or mechanical elements of the instrument.
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.
METIS is an externally occulted coronagraph which adopts an “inverted occulted” configuration. The Inverted external occulter (IEO) is a small circular aperture at the METIS entrance; the Sun-disk light is rejected by a spherical mirror M0 through the same aperture, while the coronal light is collected by two annular mirrors M1-M2 realizing a Gregorian telescope. To allocate the spectroscopic part, one portion of the M2 is covered by a grating (i.e. approximately 1/8 of the solar corona will not be imaged).
This paper presents the error budget analysis for this new concept coronagraph configuration, which incorporates 3 different sub-channels: UV and EUV imaging sub-channel, in which the UV and EUV light paths have in common the detector and all of the optical elements but a filter, the polarimetric visible light sub-channel which, after the telescope optics, has a dedicated relay optics and a polarizing unit, and the spectroscopic sub-channel, which shares the filters and the detector with the UV-EUV imaging one, but includes a grating instead of the secondary mirror.
The tolerance analysis of such an instrument is quite complex: in fact not only the optical performance for the 3 sub-channels has to be maintained simultaneously, but also the positions of M0 and of the occulters (IEO, internal occulter and Lyot stop), which guarantee the optimal disk light suppression, have to be taken into account as tolerancing parameters.
In the aim of assuring the scientific requirements are optimally fulfilled for all the sub-channels, the preliminary results of manufacturing, alignment and stability tolerance analysis for the whole instrument will be described and discussed.
Since the coronal light is enormously fainter than the photospheric one, a very tough suppression is needed for the internal stray light, in particular the requirement for the stray light suppression is more stringent in the VL than in the UV, because the emission of the corona with respect to the disk emission is different in the two cases, and the requirements are a suppression of at least 10-9 times for the VL and a suppression of at least 10-7 times for the UV channel.
This paper presents the stray light analysis for this new coronographic configuration.
The complexity of the optomechanical design of METIS, combined with the faintness of the coronal light with respect to the solar disk noise, make a standard ray tracing approach not feasible because it is not sufficient to stop at the first generation of scattered rays in order to check the requirements. Also scattered rays down to the fourth generation must be treated as sources of new scattering light, to analyze the required level of accuracy. If used in a standard ray tracing scattering analysis, this approach is absolutely beyond the computational capabilities today available; therefore we opted for a scattering ray generation with a Montecarlo method in which after a father ray hits a surface, only one ray is generated, randomly selected according to the distribution of the transmitted energy. These rays bring with them all the energy that is otherwise distributed between all the rays of second generation, making the model more realistic and avoiding loss of energy due to the rays sampling. The stray light has been studied in function of the mechanical roughness of the surfaces and the obtained results indicate an instrument stray light blocking performance well within the requirements in both channels.
High performance polarizers can be obtained with optimized coatings. Interference coatings can tune polarizers at the spectral line(s) of interest for solar and stellar physics. Polarizing beamsplitters consist in polarizers that separate one polarization component by reflection and the other by transmission, which enables observing the two polarization components simultaneously with a single polarizer. They involve the benefit of a higher efficiency in collection of polarization data due to the use of a single polarizer for the two polarization components and they may also facilitate a simplified design for a space polarimeter. We present results on polarizing beamsplitters tuned either at 121.6 nm or at the pair of 155 and 280 nm spectral lines.
GOLD’s research is devoted to developing novel coatings with enhanced performance for space optics. Several deposition systems are available for the deposition of multilayer coatings. A deposition system was developed to deposit FUV coatings to satisfy space requirements. It consists of a 75-cm-diameter deposition chamber pumped with a cryo-pump and placed in an ISO-6 clean room. This chamber is available for deposition by evaporation of top-requirement coatings such as Al/ MgF2 mirrors or (Al/MgF2)n multilayer coatings for transmittance filters. A plan to add an Ion-Beam-Sputtering system in this chamber is under way.
In this and other chambers at GOLD the following FUV coatings can be prepared:
Transmittance filters based on (Al/MgF2)n multilayer coatings. These filters can be designed to have a peak at the FUV spectral line or band of interest and a high peak-to-visible transmittance ratio. Filters can be designed with a peak transmittance at a wavelength as short as 120 nm and with a transmittance in the visible smaller than 10-5.
Narrowband reflective coatings peaked close to H Lyman β (102.6 nm) with a reflectance at H Lyman α (121.6 nm) two orders of magnitude below the one at 102.6 nm. Other potential spectral lines at which these coatings could be peaked are the OVI doublet (103.2, 103.8 nm).
Narrowband reflective mirrors based on (MgF2/LaF3)n multilayers peaked at a wavelength as short as 120 nm. Target wavelengths include lines of high interest for space observations, such as H Lyman α (121.6 nm), OI (130.4 and 135.6 nm), CIV (154.8, 155.1 nm), among others.
Coating-based linear polarizers tuned at H Lyman α (121.6 nm) both based on reflectance or on transmittance. Reflective polarizers present a high efficiency. Transmissive polarizers have a more modest peak performance compared to reflective polarizers; however, they involve spectral filtering properties to reject the long FUV and even more the near UV to the IR, which turn them competitive compared to reflective polarizers.
In this communication we present a summary of our research on the above FUV coatings developed at GOLD.
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.
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