The radio amplifiers, or receivers, connected to the antenna at the focus of the dish-shaped reflector have to he of extraordinary quality to match the effectiveness of the radio reflector. Internal noise generated in the radio amplifiers, and in the cables or other hardware that connect the antenna to the receiver, competes directly with the faint radio whispers from space. If the internal noise levels are too high, faint radio sources will not be detected. In the early days of radio astronomy vacuum tube receivers had internal noise temperatures equivalent of thousands of degrees. Then, in 1961, a device called a parametric amplifier became available. Its noise temperature was first 350 Kand then later development reduced it to around 200 KThis meant that the sensitivity of radio telescopes suddenly improved tenfold, because the sensitivity depends directly on the noise temperature of the receiver. Modern radio telescopes use extremely low noise, solid-state receivers to produce overall noise temperatures of 20 I'Hadio receiver technology is unlikely to go lower because of limitations that have little to do with engineering. The 2.7 Knicrowave background sets the absolute limit, and atmospheric radiation contributes another 6 Kbr telescopes on earth.
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