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How Telescopes Work

We often think of a telescope as an instrument that magnifies distant images, seeming to bring them closer to us so we can see them more clearly. Magnification, however, is only a small part of a telescope's function, and not the most important one. The number one purpose of a telescope is to gather light. It does this by either bending it through a lens in the front of the telescope to a point at the viewing end, as with refractors, or by bouncing the light off of a curved mirror, again to a single point for viewing, as with reflecting telescopes. An eyepiece is then used to magnify the tiny point of light so that we may observe it.

The larger the telescope, the greater the light gathering ability. More detail is gathered in the small point of light, so greater magnification may be used. Other factors, however, effect the amount of magnification that can be used. If the atmosphere is unsteady, and it usually is, high magnification cannot be used. If the telescope optics are of poor quality, or the mounting is unsteady, magnification will only make the image worse. In general, magnification, rather than show more of the object clearly, simply magnifies problems that exist.

In other words, don't purchase a telescope based on magnification alone. Explore the different types of telescopes and equipment listed on the left to determine what is right for you and your budget.

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Focal Length: The distance from the mirror or lens to the point where the light is concentrated.

Focal Ratio: The ratio between the diameter of the mirror or lens and the focal length. Expressed as F-8 for a telescope that has a focal length 8 times the diameter of the mirror. A 6" mirror has a focal length of 48". An 8" F-6 telescope has the same focal length - 48".

Power: Magnification is found by dividing the focal length of the eyepiece into the focal length of the telescope.

Light Grasp: The pupil of the eye is only 7 mm in diameter. An 8-inch telescope is 200mm in diameter. It has 816 times as much surface area as the eye so it gathers 816 times as much light. Impressive.

  
 

Aperture Fever? Read on:

Resolution: The resolving power of a telescope is its ability to see detail. It is governed by diffraction and the steadyness of the atmosphere. Diffraction makes stars look like disks rather than pinpoints. The greater the aperture, the smaller the disks. Atmosphere can blur the disks, making them appear larger. The resolution then decreases because the detail blurs together. Under average conditions the largest usable size for a telescope is 8 to 16 inches. Under the best conditions telescopes larger than 20 inches show no more detail-just brighter blurs.