Information about Telescopes
We know from experience that it is too easy for the newcomer to be misled by unrealistic advertisements, such as one advertising a small refractor that offers 500x views of the planets and stars. The newcomer buys one of these telescopes and is expecting to see images that are comparable to those taken with large professional telescopes. However, when the newcomer looks through his new equipment, he sees only dim, fuzzy blobs that jump around or vibrate badly every time he touches the telescope. These scopes are frustrating to use and generally will end up in the back of a closet instead of out in the backyard at night. The price difference between a cheap toy telescope and a quality instrument of similar size is not as great as you may think.
Quality telescopes are measured by their aperture, i.e., the diameter of the lens or mirror that gathers the light. The diameter, usually stated in inches or millimeters, determines the telescope's light-gathering ability. Most of the reasonably priced quality telescopes are made by Celestron, Meade, and Orion. Companies such as Astrophysics, Takahashi, and Tele Vue manufacture very expensive, premium grade telescopes.
Types of Telescopes
Reflectors use a mirror to gather light. Only a single surface needs to be ground and polished, so the mirrors are relatively easy to make in large sizes. A mirror reflects all wavelengths of light identically; thus, there is no color distortion. The cost of a reflector is very reasonable.
Refractors use two or more lenses ("the objective") to gather light. Each lens must be ground and polished on both sides, making refractors more expensive to produce. A single lens is much like a prism, dividing light into its many colors like a rainbow. This effect is very detrimental to observing, so additional lenses, made from special glasses, must be added to keep all the colors focused together. A two-lens objective, called an Achromat, can focus reds and greens together but not blue. A bright star will have a blue halo around it when viewed with an Achromat. The blue halo is not as noticeable on dim and faint objects, so an Achromat does very well on these objects. Adding a third lens permits the focusing of all three colors. This type of lens, called an Apochromat, is usually made from expensive glass or fluorite crystal and is the ultimate refractor. An Apochromatic telescope is very expensive in contrast to the cost of an Achromat or a reflector telescope.
Catadioptrics use both mirrors and a "corrector" lens at the front of their tubes, thus achieving a long focal length telescope in a very compact tube. The most common of these types is the Schmidt Cassegrain, which is very popular with amateur astronomers. These telescopes are typically moderately expensive.
Eyepieces
Eyepieces are an integral part of the telescope system. What does an eyepiece do? It is simply a magnifying system, consisting usually of at least two small lenses arranged close together inside a metal mount. The eyepiece greatly magnifies the image formed by the telescope lens or mirror. By using eyepieces of different focal lengths, we can change the magnifying power of our telescope. Most observing is done at low magnifications of 50x to 150x. Higher powers only come into play when observing planets or very small objects. The maximum amount of power that can be used on a telescope is about 50x per inch of aperture. With larger telescopes, the maximum magnification that can be used is often less than this, since the unsteadiness of the air above the telescope will cause the image to shimmer and waver at high magnification. Astronomers call this the "seeing" limit of the atmosphere.
A telescope's magnification is calculated by dividing the focal length of the telescope by the focal length of the eyepiece. A telescope with a focal length of 1000mm using an eyepiece of 10mm focal length will have a magnification of 100 (1000/10=100). Astronomical eyepieces are available in a considerable range of focal lengths. The longest focal lengths are about 50mm, while the shortest may be as little as 3mm.
Eyepieces have a variety of designs with varying properties. One of these properties that you will see advertised is the "apparent field of view". This is the size of the field you see when looking in the eyepiece. The "true" field of view is the apparent field of view divided by the magnification of the telescope/eyepiece combination. If an eyepiece has a 50 deg apparent field of view and gives a magnification of 100x on your telescope, then the true field is .5 deg or about the size of the full moon.
Kellner is a simple 3-element eyepiece that gives good images at lower powers. The apparent field of view is about 40 deg.
Orthoscopic is a 4-element eyepiece that used to be popular. It has a narrower, apparent field of view (about 45 deg) than some of the more modern types but is noted to have very little distortion. Some observers consider it to be very good for planetary studies.
Plossl is a very popular 4-element eyepiece. It has an apparent field of view of about 50 deg., has good contrast, and has relatively little optical distortion.
Erfle is an older, wide angle eyepiece with 5 or 6 elements. It has an apparent field of view of 60 to 70 deg. At low magnifications it gives good images. At higher magnifications it suffers from lack of sharpness at the edges of the field.
Widefields are made by a variety of manufacturers and are refinements of the Erfle design. Typically, they have about 6 elements and a 65 deg apparent field of view. These eyepieces can be expensive.
Ultrawides are made by a variety of manufacturers and are a highly corrected eyepiece design with 6 to 8 elements. They have a very wide, apparent field of view, 82 deg or more. These eyepieces are quite expensive.
Several factors to be considered when purchasing eyepieces are: magnification desired, wide angle or normal field of view (45-50 deg), ability to resolve fine detail and, finally, cost. If you wear glasses, you will want to determine how much eye relief an eyepiece provides (eye relief is the distance between the eyepiece lens and the eye).
Which Telescope Is Best For You?
Decide what you want the telescope to do for you and then buy the best you can afford. You will never be satisfied if the telescope doesn't do what you want it to do. If you are thinking of purchasing a telescope for your children, why not spend a few extra dollars, buy a quality instrument, and enjoy the hobby yourself. Don't buy a toy and expect it to function well. You'll end up discarding it.
There are also weight and size issues. How much weight can you carry comfortably? A 6 to 8 inch telescope tube can weigh from 13 to 25 pounds, and then the mount can weigh an equal or greater amount. Can you get it into the trunk of your car? Some amateur astronomers have "aperture fever" (desire for large diameter telescopes with great light- gathering power). These telescopes often have to be transported in a van, trailer, or truck.
A mount that is easy to set up and use will be out in your backyard more often when spur-of-the-moment opportunities arise than one that is difficult to set up and align. If the telescope is for your children, will you have to set it up for them each time they want to observe, or is it simple enough for them to set up and operate by themselves?
The ideal telescope for casual viewing is the Dobsonian. It offers high quality optics, simple setup, and adequate aperture for viewing faint objects. The 6 and 8 inch models are favorites for casual use, but Dobs can range in size from 4 inches to over 20 inches in diameter. Expect to pay $200 to $500 for a new 6 or 8 inch Dob.
A used telescope can also make a good first telescope. Optical equipment, if properly cared for, is durable and will last for many decades.
And, Finally, Let the IAS Members Help You
The members of the IAS believe that if you are going to buy a telescope for yourself or as a gift, it should be an instrument that will meet your expectations. We encourage you to attend our star parties and meetings where you will have the opportunity to look at and observe through many different types and sizes of telescopes, using many different eyepieces. We enjoy observing and demonstrating our scopes to visitors and newcomers to the Society. By looking through several members' telescopes and asking questions, you will learn what type and size of telescope best meets your needs. You will then be in a better position to make an informed choice.
©2005 Indiana Astronomical Society