What's the difference between a Refractor, Reflector and Catadioptric telescope?

Q. What's the difference between a Refractor, Reflector and Cassegrain telescope?

A. 

Refractor - 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. A refractor telescope is viewed through one end. The refractor is better for colour rendition.

Advantages:

  • Easy to use and consistent due to the simplicity of design.
  • Good for distant terrestrial viewing
  • Excellent for lunar, planetary and binary stargazing especially with larger apertures
  • Sealed tube protects optics and reduces image degrading air currents
  • Rugged, need little or no maintenance

Disadvantages:

  • Generally have small apertures, typically 3 to 5 inches
  • Less suited for viewing small and faint deep sky objects such as distant galaxies and nebulae
  • Heavier, longer and bulkier than equivalent aperture reflectors and catadioptrics
  • Limited practical usefulness

Good-quality refractors cost more per inch of aperture than any other kind of telescope.

Reflector - Reflecting telescopes use a huge concave parabolic mirror instead of a lens to gather and focus the light to a flat secondary mirror that in turn reflects the image out of an opening at the side of the main tube. You look through an eyepiece on the side of the tube up near the top.

Advantages:

  • Easy to use and construct
  • Excellent for faint deep sky objects such as remote galaxies, nebulae and star clusters because of their larger apertures for light gathering.
  • Low in optical irregularities and deliver very bright images
  • Reasonably compact and portable
  • A reflector costs the least per inch of aperture compared to refractors and catadioptrics since mirrors can be produced at less cost than lenses

Disadvantages:

  • Generally, not suited for terrestrial applications
  • Slight light loss due to secondary obstruction when compared with refractors
  • The tube is open to the air, which means dust on the optics even if the tube is kept under wraps

Reflectors may require a little more care and maintenance

Catadioptric - Catadioptric telescopes use a combination of mirrors and lenses to fold the optics and form an image. Catadioptrics are the most popular type of instrument, with the most modern design. There are two popular designs, the Schmidt-Cassegrain and the Maksutov-Cassegrain.

  • In the Schmidt-Cassegrain, light enters through a thin aspheric Schmidt correcting lens, then strikes the spherical primary mirror and is reflected back up the tube to be intercepted by a small secondary mirror. The mirror then reflects the light out an opening in the rear of the instrument where the image is formed at the eyepiece.

Advantages:

  • Most versatile type of telescope
  • Best near focus capability of any type telescope
  • First-rate for deep sky observing or astrophotography with fast films or CCD's
  • Excellent for lunar, planetary and binary star observing plus terrestrial viewing and photography
  • Closed tube design reduces image degrading air currents
  • Compact and durable

Disadvantages:

  • More expensive than reflectors of equal aperture
  • Its appearance may not be suited to everybody's taste
  • Slight light loss due to secondary mirror obstruction compared to refractors

The Maksutov-Cassegrain telescope design has basically the same advantages and disadvantages as the Schmidt. It uses a thick meniscus-correcting lens with a strong curvature and a secondary mirror that is usually an aluminised spot on the corrector. The Maksutov secondary mirror is typically smaller than the Schmidt's giving it slightly better resolution for planetary observing.

However, the Maksutov is heavier than the Schmidt and because of the thick correcting lens, it takes a long time to reach thermal stability at night in larger apertures. The Maksutov optical design typically is easier to make but requires more material for the corrector lens than the Schmidt Cassegrain.

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