 Here's a unique way to detect the mass of a black hole. It uses the distortions the black hole creates in background images. Here's an illustration from Durham University. It highlights how the distortions created by using a black hole as a lens can determine the black hole's mass. We start with the distant galaxy, 4.7 billion light years away. The light from this object passes through the galaxy Able 1201, 2.1 billion light years away. The light passes within 3,000 light years of the supermassive black hole at the center of this lensing galaxy. We can't see the black hole, but we can see the distortions it creates. In this example, the lensing galaxy distorts the distant galaxy image into a wide arc. In addition, some of the light passes near to the central black hole. The black hole acts as a lens and forms a duplicate image of the distant galaxy. To find the mass of this black hole, astronomers, with the help of large computer models, simulated an image that a black hole would create. The output image depends on the mass of the black hole. Mass is an input to the algorithm. Masses too low or too high would not create the image observed. But the correct mass would. The best fit came when a mass of 33 billion suns was input. This mass makes it one of the most massive black holes ever detected.