Gerardo Capobianco

METIS: a novel coronagraph design for the Solar Orbiter mission

METIS (Multi Element Telescope for Imaging and Spectroscopy) METIS, the “Multi Element Telescope for Imaging and Spectroscopy”, is a coronagraph selected by the European Space Agency to be part of the payload of the Solar Orbiter mission to be launched in 2017. The mission profile will bring the Solar Orbiter spacecraft as close to the Sun as 0.3 A.U., and up to 35° out-of-ecliptic providing a unique platform for helio-synchronous observations of the Sun and its polar regions. METIS coronagraph is designed for multi-wavelength imaging and spectroscopy of the solar corona. This presentation gives an overview of the innovative design elements of the METIS coronagraph. These elements include: i) multi-wavelength, reflecting Gregorian-telescope; ii) multilayer coating optimized for the extreme UV (30.4 nm, HeII Lyman-α) with a reflecting cap-layer for the UV (121.6 nm, HI Lyman-α) and visible-light (590-650); iii) inverse external-occulter scheme for reduced thermal load at spacecraft peri-helion; iv) EUV/UV spectrograph using the telescope primary mirror to feed a 1st and 4th-order spherical varied line-spaced (SVLS) grating placed on a section of the secondary mirror; v) liquid crystals electro-optic polarimeter for observations of the visible-light K-corona. The expected performances are also presented.

Imaging polarimeters based on liquid crystal variable retarders:an emergent technology for space instrumentation

The use of Liquid Crystal Variable Retarders (LCVRs) as polarization modulators are envisaged as a promising novel technique for space instrumentation due to the inherent advantage of eliminating the need for conventional rotary polarizing optics hence the need of mechanisms. LCVRs is a mature technology for ground applications; they are wellknow, already used in polarimeters, and during the last ten years have undergone an important development, driven by the fast expansion of commercial Liquid Crystal Displays. In this work a brief review of the state of the art of imaging polarimeters based on LCVRs is presented. All of them are ground instruments, except the solar magnetograph IMaX which flew in 2009 onboard of a stratospheric balloon as part of the SUNRISE mission payload, since we have no knowledge about other spaceborne polarimeters using liquid crystal up to now. Also the main results of the activity, which was recently completed, with the objective to validate the LCVRs technology for the Solar Orbiter space mission are described. In the aforementioned mission, LCVRs will be utilized in the polarisation modulation package of the instruments SO/PHI (Polarimetric and Helioseismic Imager for Solar Orbiter) and METIS/COR (Multi Element Telescope for Imaging and Spectroscopy, Coronagraph).

Multi Element Telescope for Imaging and Spectroscopy (METIS) coronagraph for the Solar Orbiter mission

METIS, the “Multi Element Telescope for Imaging and Spectroscopy”, is a coronagraph selected by the European Space Agency to be part of the payload of the Solar Orbiter mission to be launched in 2017. The unique profile of this mission will allow 1) a close approach to the Sun (up to 0.28 A.U.) thus leading to a significant improvement in spatial resolution; 2) quasi co-rotation with the Sun, resulting in observations that nearly freeze for several days the large-scale outer corona in the plane of the sky and 3) unprecedented out-of-ecliptic view of the solar corona. This paper describes the experiment concept and the observational tools required to achieve the science drivers of METIS. METIS will be capable of obtaining for the first time: • simultaneous imaging of the full corona in polarized visible-light (590-650 nm) and narrow-band ultraviolet HI Lyman α (121.6 nm); • monochromatic imaging of the full corona in the extreme ultraviolet He II Lyman α (30.4 nm); • spectrographic observations of the HI and He II Ly α in corona. These measurements will allow a complete characterization of the three most important plasma components of the corona and the solar wind, that is, electrons, hydrogen, and helium. This presentation gives an overview of the METIS imaging and spectroscopic observational capabilities to carry out such measurements.

KPol: liquid crystal polarimeter for K-corona observations from the SCORE coronagraph

We describe the design and first calibration tests of an imaging polarimeter based on Liquid Crystal Variable Retarders (LCVRs), for the study of the solar K-corona. This K-polarimeter (KPol) is part of the visible light path of the UltraViolet and Visible-light Coronal Imager (UVCI) of the Sounding-rocket Coronagraphic Experiment (SCORE). SCORE/UVCI is an externally occulted, off-axis Gregorian telescope, optimized for the narrow-band (i.e., λ/▵λ ~10) imaging of the HeII, λ 30.4 nm and HI λ 121.6 nm coronal emission. We present some preliminary results of the application of LCVR plates to measurements of linear polarized radiation. LCVR plates replace mechanically rotating retarders with electro-optical devices, without no moving parts. LCVR are variable waveplates, in which the change of the retardance is induced by a variable applied voltage. The retardance of a LCVR is a function of the wavelength. KPol observations of the visible coronal continuum of the Sun (K-corona) will be made over the 450-600 nm wavelength band. We have studied the LCVRs properties in this bandpass. We tested a LCVR plate assembled in a linear polarization rotator configuration to measure the polarization plane rotation of input radiation as a function of wavelength. We estimated the LCVRs chromatic response in the KPol wavelength bandpass. The preliminary results show reasonable achromatic behaviour at high regimes of the driving voltage, Vd (i.e., Vd>3 volt).

Novel space coronagraphs: METIS, a flexible optical design for multi-wavelength imaging and spectroscopy

This presentation outlines a general optical design for coronagraphs working in both the visible-light (VL) and UV/EUV wavelength ranges by combining the use of reflective, multilayer-coated or interference-coated optics with Lyot stops. This design has been successfully applied to a sub-orbital coronagraph. Another version of this novel design for visiblelight/ EUV coronagraphs uses an inverted-occultation design in order to minimize the solar flux entering the instrument. This design has been used for the coronagraph – METIS - on board the ESA Solar Orbital mission. The current optical configuration of METIS adopted for the Solar Orbiter mission includes Visible-light and UV imaging. However, the innovative inverted-occultation concept is flexible enough that it can also accommodate a EUV spectrograph maintaining the same basic optical layout. The paper also describes the potential capabilities of the inverted-occulter coronagraph as a VL/UV imager and EUV spectrograph for future solar missions.

Design status of ASPIICS, an externally occulted coronagraph for 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 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.

Improved stray light suppression performance for the solar orbiter/METIS inverted external occulter

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.

Reflective and transmissive broadband coating polarizers in a spectral range centered at 121.6 nm

Polarimetry is a powerful tool for the interpretation of the role of the coronal plasma in the energy transfer processes from the inner parts of the Sun to the outer space. One of the key lines for observations is H I Lyman α (121.6 nm) among few spectral lines in the far ultraviolet (FUV), and hence efficient linear polarizers at this line are demanded. New designs based on (Al/MgF2)n multilayer coatings have been developed to obtain the smallest possible reflectance in the parallel plane of polarization (Rpar) with a simultaneous high reflectance in the perpendicular plane of polarization (Rper). Samples stored in nitrogen for ~8–17 months resulted in efficient polarizers at 121.6 nm, with Rpar ~ 0.01–0.017 and Rper ~ 0.69–0.725. The designs with a number n = 3–4 bilayers of Al/MgF2 result in a wider spectral range of efficient linear polarizers, compared to what can be obtained with n = 2. Coatings following various designs with good polarizing performance in a 7–8 nm wide FUV range were prepared. For the first time, a transmissive coating polarizer has been developed for this range, which has the benefit that it involves no deviation of the beam; it is based on another design of (Al/MgF2)3 multilayer coating. The transmissive polarizer has a good transmittance ratio between the two polarization components and, even though its figure of merit is not as high as that of the reflective polarizers, it incorporates filtering properties to reject wavelengths both below and above 121.6 nm; this property might enable a polarimeter for solar physics with an improved global figure of merit if a filter to isolate the H I Lyman α line could be avoided.

Space-qualified liquid-crystal variable retarders for wide-field-of-view coronagraphs

Liquid-crystal variable retarders (LCVRs) are an emergent technology for space-based polarimeters, following its success as polarization modulators in ground-based polarimeters and ellipsometers. Wide-field double nematic LCVRs address the high angular sensitivity of nematic LCVRs at some voltage regimes. We present a work in which wide-field LCVRs were designed and built, which are suitable for wide-field-of-view instruments such as polarimetric coronagraphs. A detailed model of their angular acceptance was made, and we validated this technology for space environmental conditions, including a campaign studying the effects of gamma, proton irradiation, vibration and shock, thermo-vacuum and ultraviolet radiation.

Electro-optical polarimeters for ground-based and space-based observations of the solar K-corona

Polarimeters based on electro-optically tunable liquid crystals (LC) represent a new technology in the field of observational astrophysics. LC-based polarimeters are good candidates for replacing mechanically rotating polarimeters in most ground-based and space-based applications. During the 2006 total solar eclipse, we measured the visible-light polarized brightness (pB) of the solar K-corona with a LC-based polarimeter and imager (E-KPol). In this presentation, we describe the results obtained with the E-KPol, and we evaluate its performances in view of using a similar device for the pB imaging of the K-corona from space-based coronagraphs. Specifically, a broad-band LC polarimeter is planned for the METIS (Multi Element Telescope for Imaging and Spectroscopy) coronagraph for the Solar Orbiter mission to be launched in 2017. The METIS science driver of deriving the coronal electron density from pB images requires an accuracy of better than 1% in the measurement of linear polarization. We present the implications of this requirement on the METIS design to minimize the instrumental polarization of the broad-band visible-light (590-650 nm) polarimeter and of the other optics in the METIS visible-light path. Finally, we report preliminary ellipsometric measurements of the optical components of the METIS visible-light path.