Designs of Telescopes
Even among telescopes with the same aperture, some designs are more portable, others give sharper images while still others are more economical. There are three basic kinds of telescope to choose from depending on your specific requirements.
* Reflecting telescope,
* Refracting telescope, and
*Catadioptric telescope.
All these 3 telescopes have the same light-gathering properties, despite their differences in size and weight. They also have a similar purpose, to collect light and bring it to a point of focus so it can be magnified and examined with an eyepiece, but each does it differently. Consequently, each type of telescope has its pros and cons, which you can match with your observing needs.
1. The Refracting Telescope or Refractor
Refracting telescopes are the most common form of the 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
* 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.
2. The Reflecting Telescope or 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 even 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
3. Catadioptric Telescope
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, marketed throughout the world in 3 ?" and larger apertures. 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
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