The Hale Telescope has a mirror of diameter 5.1 meters and a focal length of 16.9 meters. The Keck 1 Telescope has a mirror of diameter 10 meters and a focal length of 17.5 meters. Which telescope has a greater light-gathering power (i.e., can collect more photons per second)?

Answer 1

since the only light the telescope can gather has to come in through the lens, the larger diameter can gather more light.

The focal length is related to the magnification.

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