Refractor or Dioptric
This type of telescope was the first to be designed. Its design consists of a long, thin tube with lenses that collect light, which is refracted to the eyepiece which is located at the opposite end of the tube.
Advantages
· Easy to use and reliable.
· Almost no maintenance needed.
· Great for terrestrial viewing.
· High contrast images.
· Due to its sealed optical tube, image degrading is reduced.
· Lenses do not need alignment.
Disadvantages
· More expensive in terms of inch/aperture than Relfectors or Catadioptric.
· Cheaper models usually suffer from chromatic aberration, which produces various colours around objects. For this reason a different type of reflector has been designed called apochromatic, which corrects the chromatic aberration.
Newtonian or Reflector
This type of telescope was the invention of Isaac Newton in 1668. It consists of two mirrors, one primary concave mirror and one diagonal secondary. The concave mirror is used to focus incoming light and reflect it on the secondary diagonal mirror and through the eyepiece, which is mounted on the side of the telescope.
Advantages
· They do not have chromatic aberration issues like refractor telescopes.
· Less expensive in terms of inch/aperture.
· Larger aperture which leads to excellent viewing of faint objects such as star clusters, nebula and galaxies.
· Reasonably compact and portable.
Disadvantages
· Usually not used for terrestrial observation.
· Suffer from collimation and might often need to align the mirrors.
· Sometimes viewing position can get difficult when used in equatorial mounts and might require a small ladder.
Catadioptric
Catadioptric telescopes use a combination of mirrors (catoptrics) and lenses (dioptrics) that combine both refraction and reflection. The light enters and is collected by the primary mirror in same way as in a reflector, then reflected to the secondary mirror which in turn is reflected in the rear of the telescope to the eyepiece. They are the most popular among all types of telescopes due to their modern and light design.
Advantages
· All purpose telescopes, both good for DSO (deep sky observation) and planetary observation.
· Excellent optics with sharp images over a wide field.
· Good for terrestrial viewing.
· Their closed tube reduces image degrading from air currents.
· Compact and portable.
· Very good price/aperture ratio and less expensive than equivalent refractors.
Disadvantages
· More expensive than Reflectors of equal aperture
· Slight light loss due to secondary mirror obstruction compared to refractors.