Retroreflector
This apple-sized reflector bulb is one of thousands embedded into the surface panels of the Green Bank Telescope. Engineers light the surface of the GBT and measure the brightnesses of each reflector to map the curvature of the dish. The shape of the dish, a perfect parabola, must be kept for the telescope to reflect radio waves accurately into its receivers.
Credit: NRAO/AUI/NSF
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