Arecibo Incoherent Scatter Radar is not, like other radars, a parabolic dish, but a spherical dish. The reason for this is that the main reflector cannot be steered at all. Instead, the feed system can be moved into different positions under the platform suspended above the reflector. This allows for zenith angles up to 15° from vertical. A sphere, however, has a focal line rather than a focal point in the case of a paraboloid. Therefore, the feed must be along a line, which is placed such that it matches the focal line of the sphere.
This line feed is visible in the photo above, it is the long "stick" to the left of the Gregorian dome, pointing down from the curved slider along which it can move. It is hard to grasp the size of these structures from photographs. Even when standing at the edge of the radar, i.e. some 150 m away from the platform, it is hard to realise how big it is. If you look carefully you'll see some people on the gangway (called "catwalk") up to the platform.
US-EISCAT ISR School is housed every afternoon.
Photos: Thomas Ulich.