Blockage distribution, B(x, y), for the Effelsberg telescope. Three effects can be seen (left-hand side): the center region is blocked by the subreflector, the cross-like lines originate from the four support struts, as do the triangle-shaped areas in the outskirts. The difference between the two support-leg-induced effects is whether a ray is first reflected from the dish and is then (eventually) blocked when it travels from the main dish to the secondary mirror (subreflector), or if it is blocked before it can even hit the main dish. The right-hand plot corresponds to the truncation of the subreflector only.