Astronomical Telescope Functions
Telescopes Need Light Gathering and Resolving Power
The three major functions of an astronomical telescope are in order of importance: to gather light, to resolve details, and to magnify images.
Magnifying Power
Despite the fact that it is the first thing people think of, magnification is the least important function of an astronomical telescope. Why? Magnifying a crummy image just gives a bigger crummy image. For magnification to be useful, there must first be an image with high enough quality to be worth magnifying. Google Earth provides a nice way to see this. Using this program try to focus on your home. If you live in an area that only has low resolution photos, you won't be able to see details no matter how much you magnify the image. If on the other hand you live in an area with high resolution photos, you will be able to see more detail. In order for magnifying power to be useful, the image must first resolve details, so resolving power is more important than magnification. As a rule of thumb under the best seeing conditions, any magnification more than about 50 power for every inch of diameter of the telescope is too much. The magnifying power is determined by the eyepiece, and changing the telescope eyepiece will change the magnification.
Resolving Power
The resolving power of the telescope is its ability to show or resolve details in the images. A telescope with better resolving power will reveal smaller surface features on the surface of the Moon or a planet. It will also reveal double stars that are closer together and so forth. If a telescope has good resolving power, then magnifying the image will reveal fine details. The resolving power of a telescope is directly related to the diameter of the main mirror or lens. However it is limited by the Earth's atmosphere, so really large telescopes don't have better resolving power than modest size telescopes. Images from the Hubble Space Telescope are so detailed because being located above the atmosphere it has better resolving power than ground based telescopes.
Light Gathering Power
Light gathering power is the most important function of an astronomical telescope and the reason astronomers seek to build ever larger telescopes at major astronomical observatories. Light gathering power is what allows telescopes to reveal faint stars, galaxies, and so forth. For an analogy, think about being lost in a desert far from the nearest water. It begins to rain, so you open your canteen to collect more water. A funnel would be real nice, and a larger area funnel would help you collect more water into the small opening of your canteen. Astronomical telescopes are basically light funnels that allow collecting more light into the opening of the human eye or into a camera, CCD, or other instrument. Therefore it is possible to see fainter objects. The light gathering power of a telescope is directly related to the area (or to the square of the diameter) of the main mirror or lens.
When selecting a telescope, don't worry about the power. Rather look for the largest diameter mirror or lens in your budget. That will give the best light gathering and resolving power.
The main types of telescopes are reflectors and refractors.
