How is the camera positioned for these plane-to-plane Schlieren images of shock waves?
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)