How is the camera positioned for these plane-to-plane Schlieren images of shock waves? These two popular press articles discuss the use of schlieren photography to image shock waves of jets during flight using a special schlieren camera on a third plane.


Gizmodo: NASA Captures First-Ever Images of Intersecting Shockwaves From Two Supersonic Jets
Ars Technica: NASA visualizes supersonic shockwaves in a new, awe-inspiring way


NASA's Glenn Research Center page on Schlieren photography is instructive but doesn't give a clue how this is done. Where is the camera relative to these target aircraft, and what's behind them?

This NASA page shows a diagram of traditional Schlieren photography but in that case a special light source is behind the plane, and the present photos are "in the wild" rather than in a laboratory setting.
Where is the camera relative to these target aircraft, and what's behind them?


Straight from NASA themselves:


  NASA flew a B-200, outfitted with an updated imaging system, at around 30,000 feet while the pair of T-38s were required to not only remain in formation, but to fly at supersonic speeds at the precise moment they were directly beneath the B-200


and (alternative link)