Telescope Basics

O, telescope, instrument of much knowledge, more precious than any sceptre, is not he who holds thee in his hand made king and lord of the works of God?
—Johannes Kepler

Some Telescope Terms Explained

Aperture & Light-Gathering Power

Aperture is the size of the telescope's light gathering element; whether it is an objective lens, as in a refractor, or its main mirror, as in a reflector. The bigger the lens or mirror, the more light the telescope will gather, and the brighter the image will be.


This is a measure of how sharp and well-defined an image the telescope can produce. A telescope's ability to resolve fine detail depends on a combination of both its aperture and the quality of the telescope's optics. In most cases, good resolution of an image is more important than magnification.

Focal Length

This is the length of the telescope's light path, from the main mirror or objective lens, to the focal point (the location of the eyepiece). The longer the focal length, the higher the magnification the telescope will produce for any given eyepiece. The telescope's focal length is usually marked on the telescope tube.

Focal Ratio

Focal ratio is an indication of the "speed" of the telescope's optical system, and is given by the focal length divided by the aperture. Longer focal lengths, over f/8, are "slower," while shorter, under f/6 are referred to as being "faster."


Magnification can be changed by simply switching eyepieces in the telescope. To find out how much magnifying power an eyepiece gives, divide the focal length of the eyepiece into the focal length of the telescope, using the same units, of course!

Types of Telescopes and Mounting Systems

Types of Telescopes:

  • Refractor - uses lenses only, very expensive in larger sizes!
  • Reflector - uses mirrors, there are two main types:
    • Newtonian (very popular)
    • Cassegrain (less common)
  • Catadioptric - uses a combination of lenses and mirrors; there are three common types:
    •  Schmidt Cassegrain
    • Maksutov Cassegrain
    • Maksutov Newtonian (much less common)

Mounting Systems

  • Alt-Azimuth - uses an up & down, back & forth motion; does not compensate for the motion of the sky; there are two main types: - fork type - Dobsonian type (a modified fork-type mount)
  • Equatorial - compensates for the motion of the sky; again there are two main types of equatorial mounts: - German equatorial - Fork mount on an equatorial wedge

Focal Length, Focal Ratio & Magnification

One very popular telescope type is the 6" (150 mm) f/8 Dobsonian. It's focal ratio is f/8, its aperture is 150mm, and its focal length is 1200mm. To calculate:

Focal Ratio = focal length/aperture = 1200mm/150mm = f/8

Focal Length = focal ratio x aperture = 8 x 150mm = 1200mm

Magnification: Lets put a 20mm eyepiece in the telescope. Magnification is calculated by dividing the telescope's focal length by the focal length of the eyepiece. So, 1200mm/20mm = 60 x power.

A 12mm eyepiece yields a higher magnification of 100 x. (1200mm/12mm = 100 x)

What's a Good Telescope for a Beginner?

Many people ask this question and the answer is surprisingly simple. The best telescope for a novice stargazer is a good pair of binoculars.

Binoculars? Yes, binoculars!

Why Binoculars? Binoculars are easy to use. They give a wide field of view and a bright, right-side-up image. They may not offer a lot of magnification but there are many, many interesting celestial objects that can be viewed with a simple pair of binoculars. Another advantage is that binoculars are relatively inexpensive and if the person decides not to pursue astronomy as a hobby then the binoculars have lots of other applications. A pair of binoculars accompanied by a good astronomy guide book is all you need to begin a friendship with the night sky.

A Note on Binocular Sizes: Binoculars all have numbers on them that look something like this: 7x35 or 10x50. The first number is the binocular magnification factor and the second number is the diameter of the objective (big end) lens in millimetres. The larger the size of the objective lens, the more light gathering power that the binoculars will have and therefore, the better they will be at detecting fainter stars. So a pair of 7x50 binoculars have 7x magnification and 50mm objective lenses. Recommended sizes would be in the range of 7x50 or 10x50. Binoculars with objective lenses bigger than 50mm tend to be large and too heavy to hold comfortably.

Inexpensive "Department Store"Telescopes may look attractive at first glance but they are usually more of a deterrent to an astronomy hobby than a help. First, they are inexpensive. This means that their optical components are cheap and of poor quality. Mechanically, they are also poor with unstable mountings that wiggle all over the place, making it impossible to keep a steady view. They offer too much magnification and a very narrow field of view. This makes it nearly impossible to find your target. The cheap lenses give fuzzy, poor quality images. All together, they are disappointing and frustrating to use. There is no such thing as a good, inexpensive telescope. Often, these cheap telescopes turn people away from astronomy, making them feel as though astronomy is "too hard" for ordinary people to enjoy.

Nothing could be further from the truth! Astronomy is not too hard - it is a very enjoyable hobby that can lead to a lifetime of exploration, learning and fun. Start with a good pair of binoculars and a good guide book. When you have learned some constellations and learned how to navigate around the sky, then you will be ready for, and very confident in your ability to use, a good astronomical telescope.

Two good places to start are your local library and your local astronomy club. Your library should have a list of clubs and organizations to help you locate the nearest astronomy club. In Canada there is The Royal Astronomical Society of Canada (RASC) with branches in many Canadian cities. Check the RASC web page ( for links to the RASC Centre nearest you.

When you are ready for the big step of buying a good starter telescope (expect prices to begin between $500 to $1000), contact a reputable dealer. Who are these reputable dealers? Your local astronomy club can steer you in the right direction - they have had the experience of doing business with many different dealers and their advice will be sound!

Telescope Readiness Test

If you can do the following, then you are ready to make full and rewarding use of a telescope:

  • Find and name four circumpolar constellations
  • Find and name three constellations seen in the southern sky in each of the four seasons.
  • Find and name - (a) 4 double stars (b) 4 variable stars (c) 5 star clusters (d) 5 nebulae or galaxies
  • Have you seen all of the above objects with either the unaided eye or with binoculars?
  • Can you find all of the above objects by yourself (using binoculars if necessary)?
  • Are you familiar with at least one star atlas?
  • Are you sure that you want to spend time learning how to operate a telescope so that you can explore the universe on your own?
Last modified: 
Monday, October 29, 2018 - 1:18pm