The technique of Very Long Baseline Interferometry (VLBI) is used to link the widely separated millimetre telescopes that are used for imaging the event horizon. By combining the telescopes, spread around the globe, they act as an interferometer: a virtual mm-telescope. The largest distance between the nodes of the VLBI network determines the resolution of the telescope – for the image of the black hole, i.e. to see the apple on the moon, we need an Earth-size telescope. In VLBI a signal from an astronomical radio source, such as a black hole, is collected at multiple radio telescopes on Earth. The distance between the radio telescopes and the time difference between the arrivals of the radio signal at the different telescopes are then used to combine the simultaneous observations of the object into one image.