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Proceedings Paper

Coating and surface finishing definition for the Solar Orbiter/METIS inverted external occulter
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Paper Abstract

The METIS coronagraph aboard the Solar Orbiter mission will undergo extreme environmental conditions (e.g., a thermal excursion of about 350 degrees throughout the various mission phases), due to the peculiar spacecraft trajectory that will reach a perihelion of 0.28 AUs. METIS is characterized by an innovative design for the occultation system that allows to halve the thermal load inside the instrument while guaranteeing the stray light reduction that is required for a solar coronagraph. The Inverted External Occulter (IEO) concept revolutionizes the classical scheme, by exchanging the usual positions of the entrance aperture (that is now the outermost element of the instrument facing the Sun) with the actual occulter (that is a spherical mirror inside the coronagraph boom). The chosen material for the IEO manufacturing is Titanium, as a trade o_ between light weight, strength and low thermal expansion coefficient. A 2 years long test campaign has been run to define the IEO geometry, and its results are addressed in previous dedicated papers. This work describes the results of a further campaign aimed at defining the IEO surface and edge finishing, the support flange geometry and the Titanium coating. Various edge finishing were installed on a prototype of the instrument occulting system and their performance in stray light reduction were compared. The support flange geometry was designed in order to reduce the overall weight, to control the thermal load and to accentuate its stray light suppression performance. The coating is a particularly delicate issue. A black coating is necessary in order to assess the stray light issues, typically critical for visible coronagraphs. Black coating of Titanium is not a standard process, thus several space qualified black coatings were experimented on Titanium and characterized. The impact of the IEO coatings was evaluated, the reflectivity and the BRDFs were measured and are addressed in the paper.

Paper Details

Date Published: 19 August 2014
PDF: 15 pages
Proc. SPIE 9151, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation, 91515H (19 August 2014); doi: 10.1117/12.2056331
Show Author Affiliations
Federico Landini, INAF - Osservatorio Astrofisico di Arcetri (Italy)
Marco Romoli, Univ. degli Studi di Firenze (Italy)
Sebastien Vives, Lab. d'Astrophysique de Marseille, CNRS, Aix Marseille Univ. (France)
Cristian Baccani, Univ. degli Studi di Firenze (Italy)
Clement Escolle, Lab. d'Astrophysique de Marseille, CNRS, Aix Marseille Univ. (France)
Maurizio Pancrazzi, INAF - Osservatorio Astrofisico di Arcetri (Italy)
Mauro Focardi, INAF - Osservatorio Astrofisico di Arcetri (Italy)
Vania Da Deppo, Istituto de Fotonica e Nanotecnologie, CNR (Italy)
John D. Moses, U.S. Naval Research Lab. (United States)
Silvano Fineschi, INAF - Osservatorio Astronomico di Torino (Italy)

Published in SPIE Proceedings Vol. 9151:
Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation
Ramón Navarro; Colin R. Cunningham; Allison A. Barto, Editor(s)

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