SwRI Solar Coronagraph – Prototype (SwSCOR-P): Technology Development for SwRI Coronagraphs, 19-R6373

Background

The solar corona is a major object of current study for the pure science of solar astrophysics, and for the applied science of space weather forecasting. The corona gives rise to the solar wind and to the space weather that affects terrestrial technology. The corona scatters visible light and can therefore be observed with conventional optics. Coronagraphs, telescopes that include a manufactured occulter to block out the direct rays of the Sun, are staples of modern solar physics and of space weather forecasting.

Coronagraph design is deceptively complicated because of the difficulties imposed by the high required level of stray light rejection: while the basics are well understood, the details of designing, manufacturing, and testing a real instrument are subtle and tricky. The front-end occulter must reject stray light at the 10-9 to 10-11 level, which challenges fundamental constraints from the quantum nature of light.

We sought to develop the technologies needed for fabrication, test, and qualification for suitability of a novel coronagraph front-end with improved occulter design; the technology is suitable for several anticipated opportunities at SwRI, including operational and scientific spaceborne and near-spaceborne coronagraphs.

Approach

The primary activity was to develop and demonstrate a complete engineering front-end/vestibule to advance design maturity for this class of instrument at SwRI. The design concept for a complete flight instrument (Figure 1) comprises a vestibule that controls stray light, followed by an optical system modeled on PUNCH/WFI, with additional features to further reduce stray light beyond the vestibule. Sunlight enters the primary aperture (A0), which has irregular/toothed shape to minimize Fresnel diffraction. Most sunlight passes directly back to the Heat Rejection Mirror (HRM) at the back of the aperture, to be focused and rejected back into space in front of the vestibule. The primary optical aperture (A1) is in the center of the HRM. Sunlight that would enter A1 directly is blocked by the occulter, which is held by a single pylon to minimize scattering by the support structure. The occulter is carefully structured to minimize diffraction into the A1 aperture. The entire vestibule is surrounded by circular baffles that prevent stray light from the side walls of the vestibule from entering into the A1 aperture. Every element of the vestibule has carefully controlled geometry relative to the other parts, to ensure maximum stray light reduction.

The novel aspects of this technology were the occulter itself, which is fabricated from a single piece of stainless steel with particular manufacturing process improvements to improve stray light and manufacturability, and the integration and alignment of the complete front end.

Accomplishments

We succeeded in improving occulter technology, developing a blunt-edged ogive-profile multi-vane occulter with significant performance improvements over existing occulter designs that use frustum or cone profiles and sharpened vanes. Using a pre-polished smooth occulter, and hogging out material to create vanes that are remnants of the original surface, is a SwRI development that greatly improves yield and mechanical tolerance of the vanes, with corresponding significant improvements in cost and performance. We developed a complete coronagraph front-end and developed and validated laboratory techniques to assemble, integrate, and align such instruments in-house (Figure 2). This activity culminated in a complete coronagraph assembly that survived GEVS shake test and performed well within requirements for a future instrument of this type (Figure 3).

Developing the laboratory test fixtures and procedures to align and test SwSCOR-P ensures rapid development and greatly reduced development risk for similar flight instruments in future programs both for scientific/research and operational/forecasting use cases.