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

Abstract

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.

Fresnel diffraction of multiple disks on axis

“…6. To take into account the effects of the full 3D structure, a small-scale model could be realized and its Fresnel diffraction studied in a montage that conserves the optical etendue, as described by Landini et al (2017).…”

“…In this section we seek to derive an optimal envelope for an ensemble of disks. To this aim, we follow the approach of Landini et al (2017), which is to replace the 3D structure by a series of 2D cuts. We assume that this remains valid within the approach of Fourier optics, an assumption that would require further investigation.…”

“…However, a numerical study of the Fresnel diffraction of a three-dimensional occulter is very difficult, because it is no longer in the field of classical Fourier optics, which must assume plane-to-plane propagations. The alternative may be the approach of Baccani et al (2016) and Landini et al (2017), which consisted of substituting planar cuts of the occulter to its true 3D shape, and computing the successive Fresnel diffractions numerically from one cut to another. This constitutes a first possible approach, and we adopt it here to derive an optimal shape for a 3D occulter.…”

Expected performances of the PROBA-3/ASPIICS solar coronagraph: Simulated data

“…Thus Landini et al (2010) and Landini et al (2011) compared the efficiency of linear segments of various occulters, and thick occulters were again found to be more efficient, with the advantage being about factor of two. Later, Landini et al (2016Landini et al ( , 2017a developed an analytical model and showed how an ASPIICS-size configuration can be rescaled to a laboratory size (linear scaling for distances, and quadratic scaling for thicknesses) so that the experiments would be representative. However, in this case, the intensity of the diffracted light can be measured only in the entrance aperture plane and not in the detector plane, as otherwise the remaining optical system would be scaled down to an unrealistic size.…”

“…In the entrance aperture plane the intensity of the diffracted light enhances very slightly in the umbra region with the radial distance from the axis; see Fig. 4 in Landini et al (2017a), Fig. 5 in RR17, and Fig.…”