Tuesday, 29 July 2014

ISR School: Gregorian Feed 1

Recently, we reported on the line feed of the iconic Arecibo Incoherent Scatter Radar. The Arecibo Observatory hosted this year's Joint US-EISCAT ISR School. This post is dedicated to the Gregorian feed, i.e. the "ball shaped" one on the right. Both feeds can move along the curved rail from the centre of the structure all the way to the edge. This is equivalent of steering the beam 15° off zenith (from 90° to 75° elevation). In addition, the entire curved rail can be rotated horizontally in a full circle providing said 15° steering in all directions.

Everything about the Arecibo antenna is hard to grasp, esp. the sizes. Even when standing right next to it, on the visitors' viewing platform, one cannot really guess how large the feeds are. The Gregorian feed is 80 feet across, i.e. 24.4 metres! In the photo above, you can see a set of two access doors on the left of the Gregorian dome. These are regular double doors, i.e. about 2 m in height! By the way, the line feed in the first photo is 36 m long, i.e. 50% longer than the diameter of the Gregorian.

Here's what it looks like from underneath. This photo was taken from the bottom of the sink hole, in which the main reflector is suspended during our recent trips to the Underworld. There's a hole in the reflector down there, from where it is possible to look straight up and into the Gregorian feed. The mesh-like structure protruding within the Gregorian from the bottom right (about 4 o'clock) direction is the underside of the tertiary reflector.

To be continue tomorrow...

Photos: Thomas Ulich.

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