Image of the mass distribution over a patch of sky about one quarter of the area of the Full Moon. The left panel represents the kind of image which could be made by a low-frequency radio telescope with a diameter of 100 kilometers, using the gravitational distortion of radio waves. The right panel represents the kind of image which could be made for the same region of the sky using an optical telescope in space to measure the gravitational distortion of distant galaxy images (The contrast of the second image is three times that of the first to make small features stand out). Credit: Stefan Hilbert, Max Planck Institute for Astrophysics
