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What Size of Telescope to Get?

There is no way to completely cover the subject of telescopes in one book, much less one chapter. I hope to give some basic rules for acquiring and using a telescope. After going through this chapter, you will be able to intelligently read the advertisements in Sky and Telescope. Another good source of information is any store specializing telescopes. A reliable dealer will be glad to answer your questions. If the dealer is uncooperative, find another. Anyone buying a telescope is spending enough money to more than justify good service and a good instrument. Buying a telescope suitable for skygazing will set you back at least a few hundred dollars. If this seems too high, don't despair; it’s possible to build your own telescope for much less.

As we shall see, telescope have progressed quite a bit since a Dutch lens maker, Hans Lipperskey, in about 1608 looked at and was startled by a magnified image of church steeple through two lenses. No matter how impressive and complicated a telescope may appear to be at first glance, it is based on simple, easy to understand principles. This applies to the Hubble Space Telescope as well as to one you construct with your own hands.

The first question people ask after seeing my telescope is "How far can you see with it?" Since its possible to see billions and billions into space without any telescope, this question is not very relevant. It is based on the mistaken idea that the primary purpose of a telescope is to enlarge objects. The real purpose of a telescope is to gather light. George Gabor described a giant telescope as a "huge light bucket". Distant objects in space are not only small, but also extremely dim.

A telescope gathers the sparse incoming particles of light from these distant objects and focuses them on the eye's pupil. Telescopes are not rated by the overall size of the instrument buy by there "objective" which is the part of the telescope, which gathers and initially focuses the incoming light. An objective is either a glass lens or a mirror. The larger the objective, the more powerful the telescope. The objective of a pair of binoculars is the two large lenses in the front of the instrument. A pair of binoculars is merely a special form of telescope that both saves space and enables its user to utilize both eyes at once.

The size of an objective is measured by its diameter in inches while its ability to gather light is dependent on its area. By doubling the diameter of an objective, its area is quadrupled as is its light-gathering ability. A one-inch telescope gathers 16 times as much light as the quarter-inch pupil of the eye, while a 10inch telescope gathers 160 times more light than the eye.

Power Of Telescopes of Various-Sized Objectives

Objective Size (in inches) Faintest Star Seen (apparent magnitude)
1 ˝9.9
2 ˝11.0
3 ˝11.7
4 ˝12.3

Since there are many more stars of higher magnitudes then lower ones, increasing the objective size of the telescope greatly increases the number of stars that you can see.

Besides determining the amount of light to be gathered, the objective's size also fixes the limit of effective magnification. The effective magnification of an objective ranges from 4 times to 50 times its diameter size in inches. For instance, the range of magnifications for my four-and-a-half inch telescope is from 17 to 229 times the original size of the incoming image. In order to understand how one telescope can provide different magnifications, we have to understand what happens to light after it passes through the objective.

After passing through the objective, the incoming light forms a focused image somewhere within the telescope tube. The distance between the objective and the focused image within the tube is the telescope’s focal length and measured in millimeters. After the light passes through the point of focus (called the focal plane), it then passes through the lens of the eyepiece which once again focuses the light into an image, but this time outside of the telescope and onto the observer's eye. All telescopes work on this principle.

An objective, which may be a lens or a mirror, focuses an incoming image within some sort of a tube (the is actually optional and is used to block out incidental (interfering) light and then an eyepiece lens refocuses the image outside of the telescope for its user to see. The eyepiece lens also has a focal length. The ratio of the objective's focal length to the eyepiece’s focal length indicates the final magnification. My telescope has an effective focal length of 1000 mm (millimeters). I usually use a 20 mm eyepiece to obtain a magnification of 50 times. Occasionally, usually when Moon watching, I use my 6 mm eyepiece for 167 times magnification.

Why don't I get a 4.5 mm eyepiece to reach a maximum magnification of 222 times? The higher magnifications give a larger image but not always a better one. The image is dimmer and more difficult to see. High magnification, besides making the image larger, also amplifies any movement of the telescope. A light breeze will shake an image at high magnification. Even if your telescope is absolutely stead, the atmosphere isn’t and the image will float around quite a bit. It is best to use high magnification only on very bright objects such as the Moon, or on nights when the atmosphere is calm and steady.

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