Europe's First Wide Angle Coronagraph

A recently closed activity in GSTP Element 1 has developed Europe’s first wide-angle coronagraph.

This fundamental instrument for researching space weather, takes an image of the solar corona, by blocking the sun itself – similar to how the sun is seen during an eclipse. These images are vital for tracking Coronal Mass Ejections and understanding related space weather phenomena.

On Earth, it is challenging to image the corona reliably, especially further from the Sun where the signal diminishes, due to diffraction and attenuation of light by the atmosphere. As such, coronagraphs imaging the Thomson-scattered white light in space, where there is far less diffraction, can achieve the high signal to noise ratio that would be impossible through any kind of atmosphere.

Currently two ESA missions, Solar Orbiter and Proba 3, are flying with coronagraphs, but they respectively have a narrower field of view, or the concept requires two spacecraft flying in formation. While interesting for science these are less useful for operational space weather forecasting.

The newly developed Solar Coronagraph for OPErations (SCOPE) aims to provide imaging from very close to the Sun, where the corona is relatively bright, to around 30 solar radii, where the corona is very faint to track Coronal Mass Ejections (CMEs) using a single telescope.

The successful activity was conducted with RAL Space (GB), the Royal Observatory of Belgium (BE), the Centre Spatial Liège (BE), the University of Göttingen (DE) and Airbus DS GmbH (DE). The activity found that the only way to be certain the technology functioned was to build it through an iterative process of continuous testing and redevelopment.

The design consists of a white-light telescope with focusing optics, essentially a camera and an external occulter. This occulter is comprised of a stack of discs that blocks out the central sun itself from the image. The complexity for the design came from both reducing the size of the technology and in trying to prevent the structure holding the occulter disc and its housing from interfering with the image, through diffraction and reflection of the light. To reduce the stray light bouncing off the structure and the disc, it was important to get this exactly right. But the activity found that often the testing results didn’t meet the simulated ones. It was discovered that minute imperfections in the testing site apparatus were producing anomalies when the light source used to simulate the corona reflected back and forth off of the instrument and the mirror used for focus the light – causing an additional stray light source in the images. Repeatedly conducting real-life tests was the only way issues like this could be found, resolved and retested.

The technology is now at TRL 4, with the eventual intention to be fully verified for future ESA Space Weather missions as part of the Space Safety Programme (S2P).

G618-088EE closed in 2020.