How do they aim the Arecibo radio telescope? It’s dish is built into a natural depression in the ground. Can they aim it by moving the receiver horn, or do they only study things directly above?
That’s the answer. They move the receiver suspended above the dish to aim the telescope at different parts of the sky. Of course, they are still somewhat limited in the amount of aiming they can manage.
http://www.naic.edu/about/ao/telefact.htm
The primary reflector on the ground is a spherical reflector, rather than a parabolic reflector. The dome shaped structure hanging in the air contains the secondary reflector and the tertiary reflector. The entire structure can be moved along a bow-shaped track. The track in turn can also rotate. All of this allows the telescope to focus on a particular part of the sky.
…curse my slow fingers…
A parabolic mirror will give you a perfect image (barring other distorting factors), but only on the direct line of the axis of the parabola. A spherical mirror will always give you some distortion, but it doesn’t get any worse as you go off-axis, at least until you run off the edge ofthe mirror.
Arecibo also has a rather limited area that it can view. If I recall correctly, it can only be aimed at objects within 30[sup]o[/sup] of the zenith.
30 degrees is about right.
The area of the sky covered by the SETI@Home project:
http://setiathome.ssl.berkeley.edu/images/skymap_wugenerated.gif
Seti@Home gets its data from Arecibo and it is pretty obvious from that picture how much (little?) of the sky it can see.
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Note that the constellation of Reticulum (the Grays home planet is around Zeta Reticuli) is not visible from Arcibo. Roswell! Roswell!
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