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You have a mirror of a certain size. How large is the reflected image according to distance to the imaging screen (the Earth in this case)? For a flat mirror?

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For a parabolic mirror?

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It would depend on the FL of the mirror and the relative positions of the source and the earth.

Usually, the transmitter is placed at the focal point of the reflector, which results in the reflection producing parallel rays going in the direction of the reflector's axis (which is usually pointed towards earth). Of course, they won't be perfectly parallel, so the beam will spread out somewhat, and given the distances involved it will probably be wider than the earth by the time it gets here. That is an advantage in that requires less pointing accuracy for the transmitter, and a disadvantage in that it requires more power to be bright enough to detect. Even with some spreading, you still save a lot of power compared to just having the light go out in all directions.

Jarad

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