How does a refractor telescope work?

Refractors are what the average person instantly visualizes when he hears the word "telescope": a long, thin tube where light passes in a straight line from the front objective lens directly to the eyepiece at the opposite end of the tube.


Advantages of refractor optical design:

  • Easy to use and reliable due to the simplicity of design
  • Little or no maintenance
  • Excellent for lunar, planetary, and binary star observing especially in larger apertures
  • Good for distant terrestrial viewing
  • High contrast images with no secondary mirror or diagonal obstruction
  • Color correction is good in achromatic designs and excellent in apochromatic, fluorite, and ED designs
  • Sealed optical tube reduces image degrading air currents and protects optics
  • Objective lens is permanently mounted and aligned

Disadvantages of refractor optical design:
  • More expensive per inch of aperture than Newtonians or catadioptrics
  • Heavier, longer, and bulkier than Newtonians and catadioptrics of equivalent aperture
  • Cost and bulk factors limit the practical useful maximum size objective to small apertures
  • Less suited for viewing small and faint deep sky objects such as distant galaxies and nebulae because of practical aperture limitations
  • Focal ratios for traditional refractors are usually long (f/11 or slower) making photography of deep sky objects more difficult. Short focal-length (fast f/number) apochromats are now available that change this limitation.
  • Some color aberration in achromatic designs (doublet)
  • Poor reputation due to low-quality imported toy telescopes, which is unjustified when dealing with a quality refractor from a reputable telescope manufacturer

Updated 11/5/13