Occultation systems in space-borne telescopes dedicated to the observation of the solar corona

1. Occultation systems in space-borne telescopes dedicated to the observation of the solar corona Federico Landinia, Marco Romolib, SilvanoFineschic, John D. Mosesd a b d c

2. A simple disk is not enough! • Main focus: space borne externally occulted coronagraphs in the visible band. • A simple disk generates a diffraction pattern that jeopardizes the whole observation. • The occulter has to be optimized/apodized in order to reduce the stray light. Newkirk & Eddy, S&T, 1962

3. Apodization, softening, optimization? • " ... Diffraction from the external occulting disk is the main source of this stray light. The light diffracted into the objective may be reduced by removing the occulting disk to a great distance, as is the moon at total eclipse, or by "apodizing" or "softening" the edge of the disk. (Although "apodization" commonly refers to the alteration of the Fraunhoferdiffraction pattern of an objective lens by means of a radially graded filter, it is here used to describe the modification of the Fresnel diffraction by an opaque disk.) The serrated occulting disk developed by our colleagues at the Naval Research Laboratory (Purcell and Koomen, paper presented at the Spring Meeting of the Optical Society of America, 14-17 March, 1962) is one form of apodization. ...” From: Newkirk and Bohlin, "CoronascopeII: observation of the white light corona from a stratospheric balloon”, International Astronomical Union. Symposium n. 23, p. 287, 1965 • Two main principles: • Light scattering • Occultation

4. Light scattering: serrated edge • Light shall be scattered in order to miss the telescope entrance aperture • Teeth geometry depends upon the desired FOV and the occulter-pupil distance

5. Serrated edge principle • Critical issue: the teeth have to be sharp and must respect the geometry. The hollow between two teeth must be sharp as well, and this is hard to manufacture. One of the Spartan-201 occulters. Identical occulters were used by SOHO/UVCS. Two occulters were overlapped and de-phased by half a tooth.

6. Serrated edge heritage • Tousey (“Observations of the white light corona by rocket,” Ann. d’Astrophys. 28, 600–604, 1965) • Spartan-201 and SOHO/UVCS (From the original paper, 1965) (Original blue prints)

7. Occultation: multiple disks • A second disk in the shadow of the first one blocks the light diffracted by the first disk edge. A third disk in the shadow of the second…

8. Multiple disks principle • Truncated cone or barrel profile can be chosen for the occulter (and for the infinite disks case) • There is experimental evidence that the barrel profile (for compact coronagraphs) performs better (Thernisien et al., Proc. SPIE 9501, 2005) r1-2 r2-3 Critical edge rFOV R1 A2 A1 A3 Pupil d z

9. Multiple disks heritage • Three disks • Newkirk-BohlinCoronascope II • OSO-7 White light coronagraph • P78-1 SOLWIND • LASCO C3 • HERSCHEL/SCORE, HERSCHEL/HeCor • STEREO Cor2 • Multiple diaphragms • STEREO HI • Solar Orbiter SoloHI • Solar Probe WISPR • Large to infinite number of disks • LASCO C2 • Solar Orbiter METIS • ProbaIII ASPIICS Courtesy of R. Howard

10. Performance comparison • 5 milestone papers in chronological order: • G. Newkirk, Jr. and D. Bohlin, Appl. Opt. 2, 131–140 (1963): starts the whole story • B. Fort, C. Morel, and G. Spaak, Astron. Astrophys. 63, 243–246 (1978): first simulations, serrated edge • A. V. Lenskii, Sov. Astron. 25, 366–372 (1981): simulations, toothed, 2 disks  3 disks • S. Koutchmy, Space Sci. Rev. 47, 95–143 (1988): comprehensive comparison • M. Bout, P. Lamy, A. Maucherat, C. Colin, and A. Llebaria, Appl. Opt. 39, 3955–3962 (2000): comparison and introduction of polished cone

11. Simulations • With the whole Sun as a source, simulations have been performed only for the serrated edge occulter (Lenskii, Sov. Astr. 1981; Verroi et al., JOSAA 2008; Aime, A&A 2013) and for a two disks system (on axis only, Lenskii, 1981). • One of the main challenges for commercial softwares is the simulation of the whole Sun as a source. • A promising software (currently under evaluation) may be VirtualLab by Lighttrans.

12. Experimental activity • So far, the most reliable way of comparing the performance of different kind of optimization technique is to measure them. • Each optimization must be thought within the constraints of the mission and tuned on the objectives of the instrument. • A couple of cases: • Proba3/ASPIICS • Solar Orbiter/METIS

13. Proba3/ASPIICS (Lamy & Damé, 2010) ∅ ~1.5 m Occulter S/C ~150 m Telescope S/C Two satellites, the most “Moon-like” occulter ever! Great resolution, low stray light.

14. Proba3/ASPIICS: multiple disks? Landini et al., Applied Optics, 2011

15. Proba3/ASPIICS: multiple disks? Landini et al., Applied Optics, 2011

16. Proba3/ASPIICS: serrated? Landini et al., Applied Optics, 2011

17. Proba3/ASPIICS lab set-up • Laboratory demonstrator • A scan is performed in the solid angle subtended by the telescope entrance aperture Landini et al., Applied Optics, 2011

18. Proba3/ASPIICS: results Landini et al., Applied Optics, 2011

19. Solar Orbiter/METIS Solar Orbiter Perihelion: 0.28 AUs Orbit inclination over the ecliptic: >30o

20. Solar Orbiter/METIS Thermal shield METIS

21. Solar Orbiter/METIS M2 M1 IEO M0

22. Solar Orbiter/METIS: lab setup M0 support and alignment tool Solar simulator exit aperture Motorized translation stage with calibrated photodiode on the plane of M1

23. Solar Orbiter METIS: serrated edge • Classical case: light is scattered in order to miss the telescope entrance pupil. METIS case: practically impossible to achieve. • For METIS, in principle a serrated edge should not improve the simple edge stray light suppression performance. Scattered light Teeth Scattered light Inverted External Occulter (METIS case) and entrance aperture Entrance aperture Scattered light Solar disk light rejection mirror External occulter, classical case

24. Solar Orbiter METIS: serrated edge

25. Solar Orbiter/METIS: inverted cone Marginal ray from solar disk FOV Solar disk rejection mirror FOV Marginal ray from solar disk Entrance aperture Cone angle Classical cone optimization Cone length Inverted cone optimization

26. Solar Orbiter/METIS: inverted cone