Refracting Telescopes

Reflecting Telescopes

Catadioptric Telescopes

Telescope Mounts

Buying Hints

Calculating Magnification and f/ratios

Off Site Links about Telescopes

Star Parties with Northern Stars


Refracting Telescopes

Refractor Animation


  • Refracting Telescopes are what most people think of when they think of a telescope. It's the classic long tube with a lens at each end. Refractors are great telescopes, they are fairly portable, are fairly maintenance free and generally provide superb images of the moon, planets, star clusters and general sky gazing. They tend to be smaller in aperture than other types so they are not as good for viewing fainter sky objects such as galaxies and nebulae. Good quality refractors tend to be expensive. Beware of cheap refractors! However, they are also the most expensive type of telescope per inch of aperture. Remember that the most important thing a telescope can do is to gather light and make things brighter, small apertures don't do that very well.
    • Strengths: Great for viewing planets, the moon, star clusters, and splitting binary and multiple star systems. They have the classic telescope appearance.
    • Weaknesses: Because of their small aperture they are less well adapted for viewing very faint objects such as galaxies and nebulae. They are also expensive to get one with good optics and reasonable aperture.

Reflecting Telescopes
Reflector Animation
  • Reflecting Telescopes use a parabolic mirror to gather light and bring the image to the eyepiece. These telescopes are often referred to as "Newtonians" because they were first conceived of by Isaac Newton. Since the mirror reflects the image back towards the source, a small secondary mirror set at an angle reflects the light to the eyepiece is attached to the side of the telescope.
  • Reflectors provide good views of all types of sky objects such as planets, the moon, star clusters, galaxies, nebulae, and multiple star systems. They require some minor maintenance to keep the mirrors in proper alignment. They are the least expensive telescopes and provide the most light-gathering abilities for the dollar. A favorite choice of amateur astronomers everywhere.
  • Reflectors are commonly available with apertures ranging from 4 inches in diameter to 16 and even 20 inches in diameter. That's a lot of light gathering ability. You can't buy a refractor with that range of apertures.
    • Strengths: Great for viewing faint objects such as galaxies and nebulae, reflectors also provide good general viewing of clusters, planets, the moon and binaries. Affordable.
    • Weaknesses: They tend to be more bulky to move about and store. They also require periodic maintenance to keep the optics properly aligned.

Catadioptric Telescopes
Catadioptric Animation
  • Catadioptric Telescopes use a combination of both lenses and mirrors. The most common variety of "cats" would probably be the Schmidt-Cassegrains. They come in many sizes, but the 8" variety is by far the most popular. They give good views of all night sky objects. They have a vast array of accessories available. They are fairly compact and easy to use. The downside is that they are more expensive than reflectors.
  • Cats give great general viewing of all sky objects and are easily adapted for astrophotography. They have many peripheral devices available and can often be computerized quite easily. They are also more compact than reflectors, however, they are fairly expensive, usually at least $1,000.
    • Strengths: Great for viewing all objects in the sky. They provide good aperture and magnification. They are compact and relatively easy to move about. Abundant peripherals make it easy to dabble in astrophotography or computerization. Low maintenance.
    • Weaknesses: Relatively expensive.

Telescope Mounts:

Telescope DrawingThere are three basic types of mounts available. The main feature to look for in all telescope mounts is that it is sturdy and stable.

Alt-Azimuth Mount: The simplest by far to operate is the alt-azimuth mount. They are also the least expensive. The drawback is that they cannot be motorized to track the stars, so telescopes with this type of mount must be constantly adjusted. They are also difficult to use for astrophotography. They are the easiest for children to operate. Dobsonian telescopes use this type of inexpensive and easy to use mounting system.

Equatorial Mount: This type of mount is designed to make tracking the stars easier, however, set up is more complicated. To work properly, the mount must first be aligned with the earth's polar axis. They are often motorized, and make photography possible. They are more expensive, and somewhat difficult for children to operate.

Computerized Mounts: Many modern telescopes come with a computer guiding system, which often is aided by GPS technology. These computer systems offer huge databases of sky objects that they can find with the click of a button. It can easily get your scope pointed at objects that observers often spend hours seaching for! Yet, it also allows the sky watcher so much ease that there is less incentive to learn how to find many of those same objects on your own.

Telescope Buying Suggestions

Beware any Telescope Advertised by it's Magnifying Power!

Why? It is common to find telescopes for sale in department stores with labels making claims such as 500 X! When you see such things it should tell you one thing: "Buyer Beware!" Good telescopes are sold by their aperture not their magnifying ability.

The three main functions of a telescope are: 1. Magnification, 2. Resolution, and 3. Light Gathering Ability.

Magnification is by far the best known of these three functions, but it really is the least important. Most things in space are so far away that no matter how much you magnify them, they still look pretty much the same, the only difference is they get dimmer.

What really allows you to see more distant objects better is to make them brighter, this is done by light gathering. The larger the lens or mirror in diameter, the more light it's able to capture and the brighter the object appears. This enables you to see much more than you can by just magnifying. Any good telescope can change it's magnification by simply changing the eyepiece. The easiest way to understand light gathering is to think about the pupil in your eye. On a sunny day it gets small. In the dark it gets big, that lets more light into your eye so that you can see better in faint light, but your eyes can only get so big. When you use a telescope with an aperture of, say, 8 inches, it essentially makes your pupil 8 inches in diameter! This will allow you to see very faint objects.

Resolution is perhaps the most difficult to understand. At first it may seem to be the same as magnification, but it's not. Resolution is the ability to see fine detail. A telescope's resolution is determined by its aperture, not its magnifying power. For example when you view a double or binary star with an 8 inch telescope you can clearly see it is two stars at say 50 X, but if you look at the same star system with a 4 inch telescope at 50 X again, you may not be able to split the pair of stars, even though it's the same magnification. It requires more light to be able to split those close binary star systems. The larger the aperture the better the resolution and the finer the detail that you will be able to see.

Off Site Telescope and Observatory Links:

The World's Largest Telescopes.

Kitt Peak National Obervatory

Anglo-Australian Observatory

Hubble Space Telescope Pictures

Mauna Kea Observatories

European Southern Observatory

Hubble Heritage Project

Mount Palomar Observatory

Perkins Observatory

HST: What's New?

Lick Observatory

Mount Wilson Observatory

Space Telescope Science Institute.


Northern Stars Planetarium and Educational Services
Fairfield, ME 04937
(207) 453-7668
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