Friday, 25 July 2014

ISR School: Arecibo Line Feed

The main reflector of the 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.

The line feed is indeed 36 m long, i.e. it is 4 m longer than the EISCAT UHF radar dishes are in diameter! Above another photo taken from the platform looking down and towards one of the masts from which the platform is suspended.

Here is a photo of the very first line feed used at Arecibo. It is cut in half and hidden in plain sight, disguised as some modern art piece right next to the Learning Centre, i.e. the building in which this year's US-EISCAT ISR School is housed every afternoon.

Some of us got the chance to visit the space under the 300-m reflector, the report of which we will leave for a future blog post. However, under the reflector we found these pieces, and Rich Behnke commented that "this is where line feeds go to die," meaning these are old line feeds not in use anymore.

Photos: Thomas Ulich.

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