Digital Signal Processing for the Event Horizon Telescope

The Event Horizon Telescope (EHT) [1] is an earthsize very long baseline interferometry (VLBI) array, operating at the shortest radio wavelengths of about 1 millimeter, corresponding to radio frequencies 230 GHz and higher. It has an extremely fine angular resolution of the order of 20 microarcseconds. For super massive black holes (SMBH) which are relatively nearby and sufficiently massive, this is the angular scale subtended by the event horizon. Relativistically lensed emission from the black hole's accretion disk and jet can be directly observed. Retrofitting wideband digital instruments and atomic clocks to existing lambda 1 mm radio telescopes last year led to the first image of the shadow of a black hole. I cover the scientific significance of the image and the technology that took the data.