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This is the Arecibo Observatory in Arecibo, Puerto Rico. Its reflector is spherical, measuring 1,001 ft. in diameter. It is considered the most sensitive radio telescope on Earth, but the fact that its reflector is spherical and not parabolic makes me wonder how much more sensitive it could be if the reflector were parabolic. What are the pros and cons of a sphere vs. parabolic?

Picture of the telescope

Plaque at Arecibo

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3 Answers 3

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It's spherical because the main dish cannot be steered; steering is done by moving the receiver (the big thing hanging over the center of the reflector). A parabolic reflector would produce varying errors when aimed in different directions; a spherical reflector has the same error for all directions. Presumably the receiver is designed to compensate for this.

Source: Wikipedia.

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  • $\begingroup$ Exactly. A spherical primary has no preferred axis. $\endgroup$
    – Florin Andrei
    Oct 27, 2011 at 14:27
  • $\begingroup$ OK, now since the spherical dish is stationary and the earth rotates, how are they able to stay focused on a specific point in the sky?.. Does the antenna adjusts its position in order to track the point, or is it like a snapshot? $\endgroup$
    – Frank Computer
    Nov 12, 2011 at 3:25
  • $\begingroup$ My question is seeking to find out if a parabolic dish would be better for reflecting incoming signals on to the antenna, versus a spherical dish which does not achieve the same capability to concentrate all signals on to the antenna, no matter where the signal hits like a parabolic dish does. $\endgroup$
    – Frank Computer
    Nov 12, 2011 at 3:32
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    $\begingroup$ A paraboloid would probably be better if the telescope were aimed only in one direction, straight up. Most radio telescopes steer by moving the entire dish until the axis and receiver are aligned directly with the target. But Arecibo's dish is immovable, and to aim at targets off the vertical, only the receiver is moved. A spherical shape gives uniform reception over the entire range to which it can point. (A 1000-foot steerable dish would probably be a paraboloid.) $\endgroup$
    – Keith Thompson
    Nov 12, 2011 at 6:03
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its always been my understanding that a parabolic dish focuses all reflected signals onto one specific point on the antenna

No. They only focus signals that come parallel to the axis on the focus. Signals that come into the dish from other directions are not focused on a single point. With a parabolic reflector the more off axis the signal is coming from the greater the aberration.

A spherical reflector induces spherical aberration but this is independent of the direction of the signal and hence can be compensated for.

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  • $\begingroup$ This is a response to this comment, and is not an answer to the question. $\endgroup$
    – HDE 226868
    Sep 30, 2015 at 1:32
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    $\begingroup$ But it actually does touch on the question, spherical vs parabolic... $\endgroup$
    – Jon Custer
    Sep 30, 2015 at 2:09
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There is a lot theory and practice around spherical vs parabolic aberrations, really too much to go into in one answer.

Also note the silver pod suspended above the dish, this contains several different surfaces to focus the energy onto the detector and in the process correct for the spherical primary.

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  • $\begingroup$ its always been my understanding that a parabolic dish focuses all reflected signals onto one specific point on the antenna, whereas a sphere cannot achieve that same capability!.. That's why, for example, satellite dishes are parabolic and not spherical! $\endgroup$
    – Frank Computer
    Nov 12, 2011 at 3:51

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