METIS- ESA Solar Orbiter Mission: internal straylight analysis E. Verroi, V. Da Deppo, G. Naletto, S. Fineschi, E. Antonucci University of Padova (Italy) CNR-Institute for Photonics and Nanotechnologies INAF-Osservatorio Astronomico di Torino International Conference on Space Optics 7-10 October 2014 Tenerife, Spain
SOLAR ORBITER ESA mission dedicated to solar and heliospheric investigation Mission launch: Jan 2017 High elliptic orbit (perihelion @ 0.28 AU) Quasi-heliosynchronous observation period investigation of low atmospheric structures (precluded from near-earth orbits) 10 instruments allocated Objectives: how the Sun creates and controls the heliosphere. What drives the solar wind and where does the coronal magnetic field originate from? How do solar transients drive heliospheric variability? How do solar eruptions produce energetic particle radiation that fills the heliosphere? How does the solar dynamo work and drive connections between the Sun and the heliosphere? 2
METIS telescope Inverted external occulter solar coronagraph for VIS and UV imaging: UV imaging channel HI 121.6 nm VIS imaging polarimetric channel 580 640 nm 3
METIS telescope exploded view 4
METIS UV optical path 5
METIS VIS optical path 6
Stray light requirements Coronal light is much smaller than photospheric light. The latter usually provides a high noise contribution that has to be minimized Coronal emissions in VIS band: 6+ orders of magnitude lower than the solar disk; in UV, the coronal emission is relatively more intense Critical orbit: perihelion @ 0.28 AU, Sun disk as large as 1.8 The average requirements on instrumental stray light rejection capability (I stray /I Sun ) are very tight: I stray (VIS) < 10-9 I sun I stray (UV) < 10-7 I sun 7
How to reduce stray light in a solar coronagraph (I) Blocking the sun disk light scattered by the edge of the external occulter by means of an internal occulter 8
How to reduce stray light in a solar coronagraph (II) Blocking the (second order) light scattered by the edge of M0, which is the first optical element, by means of the so-called Lyot stop. Unfortunately, this is not enough 9
Stray light estimation method Analysis on how spurious light propagates through an instrument to the detector and how it afflicts the quality of the signal: ray tracing simulation (with ASAP by BRO). 1. Geometrical model of the system 2. Mathematical description of the element physical properties 3. Radiation propagation simulated by rays hitting the surfaces of the geometrical structure and interacting with them as described by mathematical model 10
METIS CAD model 11
Element surfaces parametrization (I) When light hits a surface, its "transmitted" (propagated) energy depends on the physical properties of the elements: a Bidirectional Scatter Distribution Function is assigned to each element, depending on the material and on its mechanical characteristics. 12
Element surfaces parametrization (II) We simulated three different micro-roughness levels for UV and VIS channel mechanical surfaces. The parameter used for surface description is the average micro-roughness with L length of the sampling region and z height function (surface one-dimensional profile) R1: 0.1 m perfectly smooth surface (lapping process) R2: 1.0 m extra-fine grinding for machine tools R3: 3.0 m smooth grinding for machine tools The roughness simulation varies the local surface orientation. Scattering from: Flat surfaces Rough surface Random Gaussian distributed heights assigned to the surface. Two assumptions: surface isotropy, stationarity of profile autocorrelation length (the profile function does not change neither with the direction nor with the position on the surface). 13
Simulated sources VIS channel reference wavelength: λ= 600 nm. Square source in front of the entrance aperture: 5000 5000 rays square grid (about 19 M rays enter the instrument) UV channel wavelength: λ = 121.6 nm Square source in front of the entrance aperture: 4600 4600 rays square grid (about 16 M rays enter the instrument) Rays are emitted within an angular semi-aperture of 0.95, which corresponds to the Sun angular radius seen by METIS at perihelion. 14
Stray light estimation: brute force Rays hit a surface and create a new generation of scattered rays Each scattered ray is treated as a new ray; so following generations of scattered rays are created, and so on For each father ray, millions of son rays have to be simulated after the scattering on a single surface for a good sampling of the stray light effects on the detectors IN METIS: Huge intensity ratio between disk light and coronal light: also a tiny fraction of spurious light contaminates the vision of the coronal structures. Even third generation scattered rays can lead to relevant noise contributions. The number of so generated rays is too large to be suitably handled. NOT REALLY FEASIBLE 15
Stray light estimation: stochastic After a father ray hits a surface, only a few son rays are generated, randomly selected according to the distribution of the transmitted energy. Son rays are generated by using the normalized bidirectional BSDF as a probability distribution function. The total reflected energy is brought by the generated rays. This process continues for 3 rd, 4 th generation rays. 16
Ray propagation limitations Simulated sunlight: N rays ℇ: Total beam energy ℇ/N: energy carried by each first generation ray A threshold value of 10-15 ℇ has been set in the energy carried by a single multi-scattered ray. A limit of 6th scattering generation has been set. Beyond these thresholds the single ray is suppressed and is no longer taken into account in the simulation 17
Simulation results Only a very limited amount of rays survives and hit the detectors, because of the many METIS design constraints: Rejection by M0 Internal occultation of the IEO edge Lyot Stop suppression Field Stop cut off Absorption by the instrument structure The rays surviving the multiple scatters multiplied by their intensities are integrated over the detector surface and then compared with the rays of intensity 1 that would reach the detector if it were directly illuminated by the Sun disk radiation. This analysis was repeated in both channels for the three different values of mechanical surface roughnesses under consideration. 18
Visible channel results R1: 0.1 m perfectly smooth surface (lapping process) R2: 1.0 m extra-fine grinding for machine tools Position on the VIS detector of the rays surviving the multiple internal scatters R3: 3.0 m smooth grinding for machine tools 19
UV channel results Position on the UV detector of the rays surviving the multiple internal scatters R1: 0.1 m perfectly smooth surface (lapping process) R2: 1.0 m extra-fine grinding for machine tools R3: 3.0 m smooth grinding for machine tools 20
Summary Requirements: I stray (VL) < 10 9 I sun I stray (UV) < 10 7 I sun Assumed average roughness Total noise intensity on the detector (normalized to initial flux) R1: 0.1 m 8.30E 12 R2: 1.0 m 2.38E 11 R3: 3.0 m 3.07E 11 Assumed average roughness Total noise intensity on the detector (normalized to initial flux ) R1: 0.1 m 8.06E 10 R2: 1.0 m 8.82E 10 R3: 3.0 m 9.25E 10 The difference between the UV and VIS detectors is larger than one order of magnitude, and is due to the different detector positions inside the instrument, which provides a much larger scattering attenuation for the VIS detector. Also considering the different mechanical surface roughnesses, the simulated instrument performance is well within the requirements for both channels. 21