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TELESCOPES

Preface: Telescopes come in a variety of types and sizes. Which one is best for you? That depends on what you want to do among other things. Here you will find descriptions and diagrams of the most popular types along with their advantages and disadvantages. Hopefully this information will help you out.

The most common types are:

  1. Reflector (Newtonian)
  2. Achromatic Refractor
  3. Apochromatic Refractor
  4. Cassegrain
  5. Schmidt-Cassegrain
  6. Maksutov-Cassegrain
  7. Maksutov-Newtonian
top REFLECTOR (Newtonian) top
Description
  • Diagram
  • The Newtonian telescope uses a series of mirrors to gather light and send it to the eyepiece. The primary mirror should be parabolic but on smaller (< 6") cheap telescopes may be spherical. A spherical mirror has greater spherical aberrations (bad) so a parabolic mirror is preferred. The secondary mirror is mounted at the front of the scope and bends the light 90° to the eyepiece.
Advantages
  • Mirrors are cheaper than lenses to manufacture therefore this type of telescope has a low cost vs. aperture.
  • Dew (and frost) are not a problem because the scope tube acts as a large dewshield protecting the primary mirror.
  • Can be very portable in a truss-tube design.
Disadvantages
  • Needs occasional recollimation (though it is easy to do).
  • Open tube design permits dust and dirt accumulation.
  • The secondary mirror and vanes that hold it are obstructions to the incoming light, making the image not as sharp as a refractor.

 

top ACHROMATIC REFRACTOR top
Description
  • Diagram
  • An achromatic refractor uses lenses to focus light through the eyepiece. The main lens cell has two lenses.
Advantages
  • Sharp images.
  • Moderate price vs. aperture.
  • Fairly portable in small apertures.
  • Usually includes a dewshield.
  • Typically does not need collimated.
Disadvantages
  • Small apertures.
  • Can have chromatic aberration.
  • Typically cannot be collimated except at factory.

 

top APOCHROMATIC REFRACTOR top
Description
  • Diagram
  • An apochromatic refractor uses lenses to focus light through the eyepiece. The main lens cell has four lenses.
Advantages
  • Extra sharp images.
  • Fairly portable in small apertures.
  • Usually includes a dewshield.
  • Typically does not need collimated.
Disadvantages
  • Very high cost vs. aperture.
  • Typically cannot be collimated except at factory.

 

top CASSEGRAIN top
Description
  • Diagram
  • The Cassegrain telescope uses a series of mirrors and a corrector lens to gather light and send it to the eyepiece. The corrector lens (or plate) corrects various problems associated with reflector type telescopes. The primary mirror is parabolic. The secondary mirror is mounted in the corrector lens at the front of the scope and bends the light 90° to the eyepiece.
Advantages
  • No spider vanes holding the secondary mirror.
  • Easy to collimate.
Disadvantages
  • Needs occasional recollimation (though it is easy to do).
  • Corrector subject to dew (and frost). Needs a dewshield or dew heater.
  • The secondary mirror is an obstruction to the incoming light, making the image not as sharp as a refractor.
  • Corrector "steals" some incoming light.
  • Moderate to high price vs. aperture.
  • Sealed tube causes long acclimation time.

 

top SCHMIDT-CASSEGRAIN top
Description
  • Diagram
  • The Schmidt-Cassegrain telescope uses a series of mirrors and a corrector lens to gather light and send it to the eyepiece. The corrector lens (or plate) corrects various problems associated with reflector type telescopes. The primary mirror is parabolic and cored to allow light passage to the eyepiece in the back. The secondary mirror is mounted in the corrector lens at the front of the scope and sends the light through the primary mirror to the eyepiece. Focussing is usually done by moving the primary mirror.
Advantages
  • No spider vanes holding the secondary mirror.
  • Moderate cost vs. aperture.
  • Portability due to short tube.
  • Long focal lengths = larger eyepieces for high power = good eye relief (usually).
Disadvantages
  • Minimally collimatable.
  • Corrector subject to dew (and frost). Needs a dewshield or dew heater.
  • The secondary mirror is an obstruction to the incoming light, making the image not as sharp as a refractor.
  • Corrector "steals" some incoming light.
  • Sealed tube causes long acclimation time.
  • Primary mirror shift (makes it hard to focus).
  • Long focal lengths = hard to get low power and wide fields of view.

 

top MAKSUTOV-CASSEGRAIN top
Description
  • Diagram
  • The Maksutov-Cassegrain telescope uses a series of mirrors and a corrector lens to gather light and send it to the eyepiece. The corrector lens (or plate) corrects various problems associated with reflector type telescopes. The primary mirror is parabolic and cored to allow light passage to the eyepiece in the back. The secondary mirror is mounted in the corrector lens at the front of the scope and sends the light through the primary mirror to the eyepiece. Focussing is usually done by moving the primary mirror.
Advantages
  • Sharp images.
  • No spider vanes holding the secondary mirror.
  • Portability due to short tube.
  • Long focal lengths = larger eyepieces for high power = good eye relief (usually).
Disadvantages
  • Higher cost vs. aperture.
  • Minimally collimatable.
  • Corrector subject to dew (and frost). Needs a dewshield or dew heater.
  • The secondary mirror is an obstruction to the incoming light, making the image not as sharp as a refractor.
  • Corrector "steals" some incoming light.
  • Sealed tube causes long acclimation time.
  • Primary mirror shift (makes it hard to focus).
  • Long focal lengths = hard to get low power and wide fields of view.

 

top MAKSUTOV-NEWTONIAN top
Description
  • Diagram
  • The Maksutov-Newtonian telescope uses a series of mirrors and a corrector lens to gather light and send it to the eyepiece. The spherical corrector lens (or plate) corrects various problems associated with reflector type telescopes. The primary mirror is spherical also. The secondary mirror is mounted in the corrector lens at the front of the scope and bends the light 90° to the eyepiece.
Advantages
  • Extra sharp images.
  • Apochromatic refractor-like images at approx 1/3 the cost.
  • Usually includes a dewshield.
  • Very small secondary mirror.
  • No spider vanes holding the secondary mirror.
  • Easy to collimate.
Disadvantages
  • Corrector subject to dew (and frost). May need a dew heater.
  • The secondary mirror is an obstruction to the incoming light, making the image not as sharp as a refractor.
  • Corrector "steals" some incoming light.
  • Sealed tube causes long acclimation time.
  • Quite heavy and bulky.

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