Mission concept
PROBA-3 consists of two independent, three-axis-stabilized spacecraft: the Coronagraph Spacecraft (CSC) and the Occulter Spacecraft (OSC). The spacecraft will fly close to each other on a highly elliptical orbit around the Earth, with an apogee at 60,500 km altitude.

ESA said that by flying in tight formation about 150 metres apart, the Occulter will precisely cast its shadow onto the Coronagraph’s telescope, blocking the Sun’s direct light. This will allow the Coronagraph to image the faint solar corona in visible, ultraviolet and polarised light for many hours at a time.

Along the apogee arc, when the gravity gradient is significantly smaller, the two spacecraft will autonomously acquire a formation configuration, such that the CSC remains at a fixed position in the shadow cast by the OSC. The CSC hosts a coronagraph, which will then be able to observe the Sun's corona without being blinded by the intense light from the photosphere. Given the diameter of the occulter disk on the OSC and the intended corona observation regions, the CSC must be approximately 150 meters from the OSC and maintain this position with millimetric accuracy, both in range and laterally. The scientific objective is to observe the corona down to about 1.1 solar radius in the visible wavelength range.

Besides formation flying for coronagraphy, some demonstration manoeuvers (retargeting and resizing) will be attempted during the apogee phase of the orbit, as well as a space rendezvous experiment.

https://en.wikipedia.org/wiki/PROBA-3#Mission_concept

PROBA-3 is a dual-probe technological demonstration mission by the European Space Agency devoted to high-precision formation flying to achieve scientific coronagraphy. It is part of the series of PROBA satellites that are being used to validate new spacecraft technologies and concepts while also carrying scientific instruments. It lifted off aboard ISRO's PSLV-XL rocket from Satish Dhawan Space Center in Sriharikota, India.

https://en.wikipedia.org/wiki/PROBA-3