While there are plenty of ways to enjoy astronomy without one, a telescope is the most important tool for the serious astronomy hobbyist! There are many types of telescopes all with their own purposes and quirks. In this guide we will discuss the different types and uses for some of the most popular scopes.

The basic purpose of a telescope is to collect light and focus the large amount of that light into your eye.

There are three main types of telescope designs:

• Refractor
• Reflector
• Catadioptric or Compound

And two types of mounts:

• Alt/Az
• Equatorial

Telescope Designs

First though we need to talk about two terms, Aperture and Magnification. Magnification refers to the size of the object in the eyepiece. Changing eyepieces increases or decreases the magnification. However with telescopes the magnification is not the most important factor. That would be the Aperture.

The Aperture is the size of the primary, or largest, lens or mirror in the scope. The larger the aperture the more light the scope can gather, the more light gathered, the brighter the object and the more detail that the scope can resolve. When you hear someone say the have a 10-inch scope they do not mean the length of the scope but the size of the aperture! When is comes to a telescope you want the largest Aperture you can get!

Refractors

Refractors are what most people think of when the think of a telescope. The simplest of designs, this scope consists of a single lens in the front and a place for an eyepiece in the back. These scopes usually have smaller apertures. Typically they range from 60mm to 100mm in aperture, occasionally you will see a 4 or 5 inch refractor but they are rare for visual observers. This because the larger the lens in the front gets the longer and more expensive the scope gets. A refractor with a 4 inch lens will usually be about 4 feet long, meaning you will be laying on the ground to view through the eyepiece. Also the larger and heavier the scope gets the bigger the mount (tripod) will have to be which will increase cost.

Double-Click image to re-play. Video from Sky and Telescope

Pros:

• Simple to setup
• Cheaper at smaller apertures
• Good planetary, solar and lunar scopes
• Can be computer controlled “go-to” scopes
• Make good finder scopes to help point larger scopes
• Can be used for terrestrial viewing as well and astronomical viewing

Cons

• Unwieldy at larger apertures
• Expensive at larger apertures
• Some smaller scopes are of low quality
• “Go-To’ functionality will require power

Reflectors

Reflectors, also known as Newtonian telescopes, use a set of mirrors to gather and focus light. The primary mirror is concave and sits at the bottom of the telescope tube. This light is reflected toward a flat diagonal mirror at the front of the scope that reflects the light path to the eyepiece on the side of the scope. These scopes range in sizes from 4-inches to 36-inches for private scopes but there are many larger scopes at observatories around the world and even one in space, you might know it as the Hubble Space Telescope.
The most popular Reflector is known as a Dobsonian. Developed by John Dobson, this is a reflector on a simple but sturdy wooden mount. This design is popular for its ease of use and because the cheaper mount lets you throw more of your money into the aperture!

Double-Click image to re-play. Video from Sky and Telescope

Pros:

• Largest aperture for your dollar
• Great for seeing the “Dim Fuzzies” such as galaxies and nebulae
• Easy to setup and use
• More comfortable to use. You can use standing up or sitting in a chair.

Cons

• Mirrors have to be occasionally aligned (a process called collimation)
• Usually not “go-to” capable, or not as accurate and as easy to accomplish
• Cannot be used for terrestrial viewing as the use of mirrors flips the image upside-down and left to right.
• Not ideal for astrophotography

Catadioptric (or Compound)

Catadioptric, or compound telescopes use a lens and a set of mirrors. This design combines refractor and reflector designs into a more compact telescope. Light passes through a slightly curved lens (also called a corrector plate) in the front and is focused to a concave primary mirror at the back of the scope tube. The primary mirror focuses the light a little more and sends the light to a small flat secondary mirror on the back of the corrector plate. The secondary mirror reflects the light out the back of the scope through a hole in the primary mirror to the eyepiece. Catadioptric scopes usually range in apertures from 5 to 16 inches.
Sounds more complicated than it is. Perhaps this diagram will help!
Astrophotographers love these scopes! With the use the use of a lens and mirrors, this design has more error correcting abilities and will produce better images, while not as noticeable visually this makes a huge difference with digital cameras.

Double-Click image to re-play. Video from Sky and Telescope

Pros:

• Great for Astrophotogaphy
• Great for seeing the “Dim Fuzzies” such as galaxies and nebulae
• Compact design compared to similar sized Reflectors
• Most are “Go-to” by design

Cons

• Expensive for the same aperture compared to other scopes (basically you are buying the aperture twice, once for the lens and once for the primary mirror)
• Requires a very heavy duty and sturdy mount (adds to the cost)
• “Go-To” functionality will require power
• Longer initial setup time

Telescope mounts

Telescope mounts support the telescope and aid in the accurate pointing of the scope. There are two main types of telescope mounts, Equatorial and Alt/Az. Each of the scope designs above can be used on either type of mount.

Alt/Az Mounts

Alt/AZ, Altazimuth, or altitude-azimuth allows the telescope to be moved in an up/down, or Altitude, direction as well and a left-right, or Azimuth, direction. The scope can be moved in both directions at the same time or individually. These types of mounts are usually cheaper and easier to learn that Equatorial mounts but can be more difficult or less accurate to use to track objects as the tracking has to make adjustments in two directions at the same time.

Equatorial Mounts

Equatorial mounts are designed to align the telescope to the east-west motion on the earth as it rotates on its axis. This allows you to track (either manually or with the aid of a computer) objects by moving (also called Slewing) the telescope in the direction counter to the earth’s rotation. To use this mount accurately you must align the mount’s north-south axis to be parallel to the Earth’s axis. This is known as polar alignment and it not as difficult as it sounds. Basically you have to point the north-south axis of the mount toward Polaris, the North Star. These mounts make tracking an object easier that Alt/AZ mounts but can be harder to learn how to use.

Computerized and tracking scopes

Many scopes today come with computers and electronics to help you point the scope toward objects. These scopes can be “go-to” or “push-to”. Once aligned “push-to” scopes will find the objects in the database, locate where they should be in the sky and tell you where to point the scope but you have to physically move the scope yourself. “Push-to” scopes will not track the objects. You will see these usually in larger Dobsonian designs. “Go-to” scopes, on the other hand, will move or slew, the scope to the objects and track the object as Earth rotates.

To take advantage of this functionality you must first align the telescope so that the computer knows where the scope is on the earth (by nearest city or by GPS coordinates), the date and time, and where the objects are in the sky. There are a few different ways telescopes can be aligned but usually you will have to enter the location, date, and time information and then point the telescope at two or three bright stars that you can identify so that it can calculate its position and the position of objects in the sky. There is no such thing and a “go-to” telescope you can setup turn on and start automatically moving to objects, all scopes have to be star-aligned, so you will have to know and be able to locate a few bright stars for each season of the year.

The “go-to” and tracking feature is nice and does make for a great observing experience, however it should be seen as a useful observing tool and not as a replacement for knowledge of the night sky.