I’m basically using the setup from here, with a used 700mm focal length telescope mirror. It’s a very clumsy setup and I’m very happy I got any results. Seeing it with your own eyes is kind of magical.
In this setup, light from a point source (LED covered in tin foil with a pinhole) is brought to a image half on a razor blade and the images are taken from behind the razor blade. Something that causes changes in air index of refraction (through heat from candles here) is in the light path. If there were no index of refraction changes in the air, one would see a uniform field of half intensity, but because the thing to be imaged changes the air’s index of refraction, you can see its effects.