Dr. James Pierce
Emeritus Professor of Astronomy
Minnesota State University, Mankato
What does a telescope do?
How does a telescope work?
What size telescope is best?
How big will a telescope make things appear?
What types of telescopes are available?
What is the best type of telescope?
What will I be able to see through a telescope?
What should I look for when I buy a telescope?
Do telescopes need electricity?
How much do telescopes cost?
Where can I get a telescope?
Can I take pictures through a telescope?
Can I see colors through a telescope?
How can I look at the sun through a telescope?
Can I use a telescope to look at objects on the Earth too?
A telescope is used to observe objects in the sky. Because these objects are very far away, they usually appear to be rather small and are often very faint. The telescope makes the objects appear brighter and, except for stars, also makes the objects appear larger.
The telescope contains an objective, a large lens or mirror that acts as a big light funnel, collecting light and bringing it to a focus. The telescope also contains an eyepiece, a small lens that acts as a magnifying glass to enlarge the image created by the objective.
There are different ways to rate the size of a telescope. The principal way is to state the aperture -- the diameter of the objective. This number is usually given in inches -- an 8-inch telescope will have a tube that is 8 inches in diameter (not 8 inches long). In general, the larger the aperture, the more light the objective will gather and the better the images will be. (Of course, larger aperture telescopes are usually more expensive.) Another 'size' of a telescope is the focal length of the objective -- the distance required to bring parallel light rays to a focus. Telescopes with longer focal lengths will usually have longer tubes and will provide higher magnification for a given eyepiece.
The magnifying power of a telescope is found by dividing the focal length of the objective by the focal length of the eyepiece. Thus, the magnification can be varied by changing eyepieces: a shorter focal length eyepiece will yield higher magnification. Contrary to popular opinion, most observing is done using relatively low power; this provides a wider field of view, making it easier to locate your target object.
A telescope with an objective lens is called a refractor. A telescope with an objective mirror is called a reflector. Types of reflectors include Newtonian, Cassegrain, and Schmidt-Cassegrain.
There is no 'best' type of telescope, just as there is no 'best' type of automobile. Different types each have their own advantages and disadvantages. Some are cheaper, some are simpler to operate, some are more portable, some magnify better, etc. The best telescope for you depends on what you want to use it for and what your observing capabilities are.
Most telescopes with apertures of four inches or more will be able to show you the craters on the moon, the rings of Saturn, moons around Jupiter, binary stars, and some of the brighter nebulae and galaxies. The appearance of each object will depend on your telescope -- specifically on the aperture and the magnifying power you are using. The summary below shows how aperture and magnifying power limit the size and brightness of objects that can be observed with a given telescope:
Note: Small, faint objects are far more numerous than big, bright objects.
For a first telescope you should be looking for some combination of low price and large aperture. Portability and ease of operation should probably be considered too. Don't worry too much about fancy gimmicks and electronics -- you can always invest more money later. In addition to choosing a telescope, you will also be choosing a mount, a set of axes that allow the telescope to point to all parts of the sky. The type of mount you choose will affect the ease of operation of your telescope, but as with telescopes, there is no one type that is best.
Visual observations require no electricity. However, because the Earth turns, the object being observed will slowly drift out of view unless the telescope is continually adjusted. A telescope with an equatorial mount may be equipped with an electric clock drive that will perform this adjustment automatically, but the mount must be set for your latitude and lined up with North in order for this to work. Many observers prefer an Alt-Az (altitude-azimuth) mount, which is much simpler to set up and operate but does not have a clock drive. Many telescopes with clock drives are designed to run on a 9-volt battery, eliminating the need for extension cords and making the scope more portable. Some come with built-in computers, which can direct the telescope to point at thousands of different interesting objects, with minimal help from you.
Telescopes vary in price, depending on the style, size, and quality. To get a telescope that will produce acceptable images of a range of celestial objects, you should expect to pay $300 or more. If you want to pay less to look at the sky, get a good pair of binoculars, rather than a cheap telescope. Binoculars are portable and easily adaptable to other uses, should you tire of astronomy.
There are very few telescope stores in most towns, although some camera stores also carry a few telescopes. Most telescopes are purchased by mail order, from dealers in California or on the East Coast. Look for ads in
Sky & Telescope or
Astronomy magazines, or check out the sites for
With a proper camera and photographic accessories many telescopes can be used for photography. Some telescopes are better for this purpose than others. The camera best suited to simple astrophotography is the Single-Lens-Reflex (SLR) camera with manual settings. For this work, the normal camera lens is removed, and the camera body is coupled directly to the telescope, which then functions as the camera lens. This type of photography requires a great deal of time and patience but can produce very rewarding results.
Color in any object is visible when the light level is high enough to operate the cone cells in the eye. By gathering more light for the eye, telescopes enhance color perception in faint objects such as stars. Unfortunately, telescopic images of objects in the sky usually lack the brilliant colors seen in photographs.
Direct observation of the sun is dangerous due to the intensity of the sun's rays. A telescope may be used to project an image of the sun on a white screen or paper for easy viewing. Or a solar filter may be obtained for your telescope, which will prevent the majority of the sun's rays from entering the instrument.
Yes, but the image may be upside-down. Special adapters are available that will turn the image right-side-up.
- Small aperture telescopes at low power are good for observing big, bright objects, such as the Sun, the Moon, and terrestrial objects.
[CAUTION: Observing the Sun directly through a telescope can cause permanent damage to the eye. Special filters or a projection technique are required for solar observations.]
- Small aperture telescopes at high power are good for observing small, bright objects, such as planets and binary stars.
- Large aperture telescopes at low power are good for observing big, faint objects, such as large nebulae and nearby galaxies.
- Large aperture telescopes at high power are good for observing small, faint objects, such as small nebulae and distant galaxies.
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Created October 10, 1997; last modified April 16, 2013
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