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 chase 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 from another plane. What takes the place of a special light source in this situation?