The Physics of the Mirror: Parabolic Dish Antennas Quiz

If a surface is shaped like a parabola, then every incoming ray parallel to its axis will reflect toward the same point. If all these rays meet at one spot, then the energy of the signal is concentrated or "focused" for the receiver.

If the irregularities on the dish surface are much smaller than the wavelength of the incoming radio wave, then the wave "sees" the surface as a solid mirror. If the surface acts as a mirror, it can accurately reflect the wave toward the focus.

If parallel lines hit a parabolic curve, then they are all redirected to a single mathematical intersection. If this intersection is where the receiver is placed to catch the signal, then it is defined as the focal point.

If a radio wave has a wavelength of several centimeters, and if the holes in a metal mesh are only a few millimeters wide, then the wave cannot "fit" through the holes and will reflect off the mesh as if it were a solid sheet.

If a radio telescope needs to collect and process data, it needs a dish to reflect waves and a feed horn to catch them. If the signal is weak, it needs a cooling system to reduce electronic noise; however, it does not use a glass lens like a refracting telescope.

If the area of a circle is calculated as pi * (D/2)^2, and if a larger dish catches more photons over that larger area, then the signal power increases with the square of the dish diameter.

If a parabola can focus incoming parallel rays to a single point (reception), then by placing a transmitter at the focal point, the rays will reflect off the dish and travel out as a concentrated parallel beam (transmission).

If the dish acts as a giant funnel for radio waves, then there must be a specific device to "swallow" that energy and send it down a wire to the computer; this device is known as the feed horn.

If a dish is spherical, rays hitting the edges focus at different spots than rays hitting the center. If a dish is parabolic, all rays meet at exactly the same spot, creating a much sharper and stronger signal.

If a technology requires the collection of weak electromagnetic signals from a distance (like TV satellites or spacecraft), then it uses a parabolic dish; microwave ovens use the waves but not a focusing dish, and microscopes use lenses.

If the diffraction limit defines how much detail a telescope can see, and if this limit is calculated as 1.22 * (lambda / D), then the resolution is strictly determined by the wavelength used and the diameter of the dish.

If all rays from a distant star arrive at the focus at the exact same time, then they must have traveled the same total distance. If they stay in phase, the signals add together constructively to create a strong detection.

If a telescope is fixed in the ground, it can only see what passes directly overhead. If it is mounted on a motorized base, it can change its orientation to track stars across the sky, which requires the ability to rotate.

If resolution is wavelength / diameter, and if radio waves are millions of times longer than visible light waves, then the diameter of the dish must be millions of times larger to achieve the same level of image detail.

If the dish surface is not a perfect parabola, rays will miss the focus. If the feed horn is misplaced, it won't catch the concentrated energy. If rain is on the dish, it can absorb or scatter the radio waves.

If a spherical dish is used, the focus is blurry; if a complex set of secondary and tertiary reflectors is suspended above the dish to "fix" the path of the rays, then the telescope can behave like a perfect parabolic system.

If human technology generates radio "noise" that is much louder than the faint signals from space, and if the parabolic dish collects all waves in its path, then the telescope will be "blinded" by human interference unless it is in a protected area.

If the receiver is too heavy to hang at the primary focus, then a smaller convex mirror is used to redirect the waves to a more convenient location behind the main dish; this mirror is the sub-reflector.

If G represents how well the dish collects signal and T represents how much heat noise interferes, then the ratio G/T is the standard figure of merit for the overall performance of a radio telescope.

If a dish is larger, the focused area (beam) is narrower. If the beamwidth is the angular width of the main lobe of the radiation pattern, then it defines the size of the "pixel" the telescope sees in the sky.