Telescopes And Observing

Major types of telescopes: Refractors, Reflectors and Compound

When astronomers talk about the size of a telescope they are talking about the diameter of the lens or mirror. The diameter is very important because it determines how much light the telescope can gather. Many objects aren't extremely small, but they are often very faint. The greater a telescopes diameter the more light it can collect, and the brighter objects will appear in the eyepiece.

Refractors have a lens in front and an eyepiece in the back. The lens bends (refracts) the light from a distant object to come to a focus near the eyepiece.

Reflectors have a special curved mirror in the bottom of the telescope and usually an eyepiece at the top, but sometimes at the bottom. The curved mirror reflects the light to a focus near the eyepiece. Some reflectors are easy and fun to build. Here are detailed plans on building a Dobsonian Reflector.

Compound telescopes use both a lens and mirrors to make the image, the eyepiece is usually at the back. Compound telescopes are usually very short in relation to their diameter. A compound telescope is a reflector with a lens in front that makes the stars at the edge of the field of view appear very crisp and sharp.

Each type of telescope has its advantages. Refractors can give very sharp images for their size (diameter), however, it is very difficult and expensive to make them very large. Reflectors are much easier to make and can be made very, very large. Compound telescopes fall in the middle, between refractors and reflectors. Here are several other important considerations:

Using An Astronomical Telescope

Magnification is important, but often the ability to gather lots of light is more important. The larger the diameter of lens or mirror, the more light the telescope can pass through the eyepiece.

Finding objects in the night sky takes some practice. A simple, non-magnifying finder, like a TELRAD, makes it easy to accurately point the main telescope. At the same time it helps to use a low power eyepiece in the main scope. After you find the object, then try more power.

Star charts often make finding objects easier. They don't need to cost a lot. One of your charts should show a wide part of the sky and the another should be very detailed, showing a small part of the sky near your object of interest.

Most astronomical telescopes don't show objects the way you see them through bino- culars. Telescopes often show the image upside down and some show the image reversed from left to right. To make the image like it is in binoculars you must place more lenses in the light path. Since lenses and mirrors are not 100% efficient at 'passing light, the fewer times you require the light to be changed, the more light you get out the eyepiece.

How to get better at observing: Keep practicing, come to star parties, and ask questions. The more observing you do the more you will see. Your brain learns to see more detail. If an object is very dim try centering your gaze just off to the side of the object.


NORTON'S 2000.0 STAR ATLAS AND REFERENCE HANDBOOK, edited by Ian Ridpath, excellent general astronomy reference source.

BURNHAM'S CELESTIAL HANDBOOK by Robert Burnham, Jr., 3 vol. set, a wealth of information about things outside the Solar System.

Copyright Rich Neuschaefer
Contact Mark Wagner via E-Mail