Rule 1: After reflection, a ray of light parallel to the principal axis appears to be coming from focus. Rule 2: A ray of light going towards centre of curvature is reflected back along the same path.

During the first light observations, several test targets were observed using the many different modes of SPHERE. These include one of the best images so far of the ring of dust around the nearby ...

However, a complete sphere would be invisible to Kepler as it would absorb all of the light from the star, so signs of this would have to come from something currently under construction.

Tohoku University. "Probing semiconductor crystals with a sphere of light." ScienceDaily. ScienceDaily, 8 June 2019. <www.sciencedaily.com / releases / 2019 / 06 / 190607110452.htm>.