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


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?

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?