[![enter image description here][1]][1]

These two popular press articles discuss the use of [schlieren photography][2] to image shock waves of jets during flight using a special schlieren camera on a third plane.

 - [Gizmodo][3]: NASA Captures First-Ever Images of Intersecting Shockwaves From Two Supersonic Jets
 - [Ars Technica][4]: NASA visualizes supersonic shockwaves in a new, awe-inspiring way

NASA's Glenn Research Center [page on Schlieren photography][5] 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](https://www.grc.nasa.gov/www/k-12/airplane/tunvschlrn.html) 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.

  [1]: https://i.sstatic.net/LPs23.jpg
  [2]: https://en.wikipedia.org/wiki/Schlieren_photography
  [3]: https://gizmodo.com/nasa-captures-first-ever-images-of-intersecting-shockwa-1833103849
  [4]: https://arstechnica.com/science/2019/03/nasa-visualizes-supersonic-shockwaves-in-a-new-awe-inspiring-way/
  [6]: https://i.sstatic.net/AINvn.png