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.