The Powers of a TelescopeA telescope can aid our eyes in three ways. These are called light-gathering power, resolving power, and magnifying power. Most interesting celestial objects are faint sources of light, so we need a telescope that can gather large amounts of light to produce a bright image. Light-gathering power refers to the ability of a telescope to collect light. Catching light in a telescope is like catching rain in a bucket - the bigger the bucket, the more rain it catches. This is why astronomers use big telescopes and why they refer to telescopes by diameter. The second power, resolving power, refers to the ability of the telescope to reveal fine detail. Whenever light is focused into an image, a blurred fringe surrounds the image. Because this diffraction fringe surrounds every point of light in the image, we can never see any detail smaller than the fringe. There is nothing we can do to eliminate diffraction fringes; they are produced by the wave nature of light. If we use a large diameter telescope, however, the fringes are smaller and we can see smaller details. Thus the larger the telescope, the better its resolving power. Two other factors - optical quality and atmospheric conditions - limit the detail we can see. A telescope must contain high-quality optics to achieve its full potential resolving power. Even a large telescope shows us little detail if its optics are marred with imperfections. In addition, when we look through a telescope, we look through miles of turbulent air in Earth's atmosphere, which makes the image dance and blur, a condition called seeing. The third and least important power of a telescope is magnifying power, the ability to make the image bigger. Since the amount of detail we can see is limited by the seeing conditions and the resolving power, very high magnification does not necessarily show us more detail.
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