Multilayer beamsplitter polarizers for key UV-FUV spectral lines of solar polarimetry.

Multilayer beamsplitter polarizers have been developed for improved solar polarimetry at key spectral lines. The advantage of beamsplitter polarizers is that a single device separates s from p polarization; this helps minimize attenuation and enables a more compact and lighter polarimeter, which is important for space instruments. Polarizers based on Al/AlF3 multilayers were prepared for both C IV (155 nm) and Mg II (280 nm) lines, and based on Al/MgF2 multilayers for H Lyman α line (121.6 nm). Polarizers were designed to mainly reflect (transmit) s (p) polarization. Beamsplitter performance and throughput are shown to compare advantageously with polarizers in the literature. Beamsplitter polarizers kept a valuable performance after several years of ageing.

Sky Brightness Evaluation at Concordia Station, Dome C, Antarctica, for Ground-Based Observations of the Solar Corona.

The evaluation of sky characteristics plays a fundamental role for many astrophysical experiments and ground-based observations. In solar physics, the main requirement for such observations is a very low sky brightness value, less than 10 - 6 of the solar disk brightness ( B ⊙ ). Few places match such a requirement for ground-based, out-of-eclipse coronagraphic measurements. One of these places is, for instance, the Mauna Loa Observatory ( ≈ 3400 m a.s.l.). Another candidate coronagraphic site is the Dome C plateau in Antarctica. In this article, we show the first results of the sky brightness measurements at Dome C with the Extreme Solar Coronagraphy Antarctic Program Experiment (ESCAPE) at the Italian-French Concordia Station, on Dome C, Antarctica ( ≈ 3300 m a.s.l.) during the 34th and 35th summer Campaigns of the Italian Piano Nazionale Ricerche Antartiche (PNRA). The sky brightness measurements were carried out with the internally occulted Antarctic coronagraph AntarctiCor. In optimal atmospheric conditions the sky brightness of Dome C has reached values of the order of 1.0 - 0.7 × 10 - 6 B ⊙ .

Scaled model guidelines for solar coronagraphs' external occulters with an optimized shape.

One of the major challenges faced by externally occulted solar coronagraphs is the suppression of the light diffracted by the occulter edge. It is a contribution to the stray light that overwhelms the coronal signal on the focal plane and must be reduced by modifying the geometrical shape of the occulter. There is a rich literature, mostly experimental, on the appropriate choice of the most suitable shape. The problem arises when huge coronagraphs, such as those in formation flight, shall be tested in a laboratory. A recent contribution [Opt. Lett.41, 757 (2016)OPLEDP0146-959210.1364/OL.41.000757] provides the guidelines for scaling the geometry and replicate in the laboratory the flight diffraction pattern as produced by the whole solar disk and a flight occulter but leaves the conclusion on the occulter scale law somehow unjustified. This paper provides the numerical support for validating that conclusion and presents the first-ever simulation of the diffraction behind an occulter with an optimized shape along the optical axis with the solar disk as a source. This paper, together with Opt. Lett.41, 757 (2016)OPLEDP0146-959210.1364/OL.41.000757, aims at constituting a complete guide for scaling the coronagraphs' geometry.

External occulter laboratory demonstrator for the forthcoming formation flying coronagraphs.

The design and optimization of the external occulter geometry is one of the most discussed topics among solar coronagraph designers. To improve the performance of future coronagraphs and to stretch their inner fields of view toward the solar limb, the new concept of coronagraphs in formation flight has been introduced in the scientific debate. Solar coronagraphs in formation flight require several mechanical and technological constraints to be met, mainly due to the large dimension of the occulter and to the spacecraft's reciprocal alignment. The occulter edge requires special attention to minimize diffraction while being compatible with the handling and integrating of large delicate space components. Moreover, it is practically impossible to set up a full-scale model for laboratory tests. This article describes the design and laboratory tests on a demonstrator for a coronagraph to be operated in formation flight. The demonstrator is based on the principle of the linear edge, thus the presented results cannot be directly extrapolated to the case of the flying circular occulter. Nevertheless, we are able to confirm the results of other authors investigating on smaller coronagraphs and provide further information on the geometry and tolerances of the optimization system. The described work is one of the results of the ESA STARTIGER program on formation flying coronagraphs ["The STARTIGER's demonstrators: toward a new generation of formation flying solar coronagraphs," in 2010 International Conference on Space Optics (ICSO) (2010), paper 39].

Stray-light analysis for the SCORE coronagraphs of HERSCHEL.

The HERSCHEL (Helium Resonance Scattering in the Corona and Heliosphere) suborbital mission scientific payload consists of two extreme ultraviolet (EUV) coronagraphs forming the SCORE (Sounding Coronagraph Experiment) and an EUV Sun disk imager. The mission will be of great importance for the investigation of solar wind dynamics by obtaining the first global image of the Sun (disk and corona) in the He ii30.4 nm line. The most stringent requirement for the optical design of a coronagraph is stray-light reduction. We summarize the stray-light analysis for the SCORE coronagraphs, which are characterized by an innovative optical design, optimized for stray-light reduction.

Optical design of a high-spatial-resolution extreme-ultraviolet spectroheliograph for the transition region.

A spectroheliograph dedicated to the observation of the solar disk in the extreme-ultraviolet OV spectral line at 62.97 nm is described. As demonstrated in the Skylab SO-82A spectroheliograph [Appl. Opt. 16, 870 (1977)], this line is uniquely suited to characterize solar plasma in the important 250, 000 K temperature regime. No multilayer coating or suitable filter is yet available to select this wavelength, so an optical design based on a double spectrograph with a spatial filter to remove the unwanted radiation has been developed. Analysis of the optical design shows that this instrument can obtain a 1 arcsec spatial resolution (two pixels) with a relatively high image-acquisition cadence. A preliminary tolerance analysis has been performed. A simple method of instrument alignment in visible light is also described.