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How much more light does a 25-centimetre telescope collect than a 12.5-centimetre telescope?

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Telescope Aerial view of the Keck Observatory's twin domes, which are opened to reveal the telescopes. Keck II is on the left and Keck I on the right.

A 25-centimetre telescope collects four times as much light as a 12.5-centimetre telescope.

The light-gathering power of an optical telescope is determined by the diameter of its aperture. To compare the light-gathering capacity of different-sized telescopes, you calculate the ratio of the squares of their diameters. For these specific dimensions, the calculation is:

$(25 \times 25) \div (12.5 \times 12.5) = 4$

By collecting more light, a larger-aperture telescope allows astronomers to observe fainter stars, nebulae, and distant galaxies that would otherwise remain invisible.1