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Understanding Astro Telescopes

Understanding Astro Telescopes

The first person that is often credited with the invention of the telescope is Hans Lippershey of Holland (c1570-c1619) as he made the first device that is widely known. The telescope was introduced to astronomy in 1609 by the scientist Galileo Galilei who became the first person to see the craters of the moon, discovered sunspots, the four moons of Jupiter and the rings of Saturn. The telescope used was a pair of opera glasses in that used an arrangement of glass lenses to magnify objects, that provided limited magnification of up to 30 times with a narrow field of view.

There are three types of telescopes, with refractors (the spyglass type) which use lenses to collect and focus light; reflectors, which collect light with a mirror; and the Cassegrain which use a combination of mirrors and lenses. The first such telescope was designed by Bernhard Schmidt in 1930, in which it used a primary mirror at the back of the telescope with a glass corrector in plate in front of the telescope, which is designed to remove spherical aberration. From this design is the most popular type of telescope called the Schmidt-Cassegrain which entails a secondary mirror that bounces light through a hole in the primary mirror to an eyepiece.

Today telescopes are not judged by their magnification but their aperture (the diameter of the main mirror lens) so when astronomers refer to the a ‘small’ telescope they mean one with a small aperture. The aperture governs how much light the telescopes collects and the more light it collects, the more you can see. Consequently it is best to get the largest telescope you can afford, whether a refractor, reflector or Cassegrain. The smallest telescopes (those with apertures under 75mm) are always refractors whilst telescopes with larger apertures are usually reflectors or Cassegrain’s. The smallest refractors (50-60mm aperture) will show the moon’s craters, Saturn’s rings Jupiter’s cloud belts along with various galaxies and nebulae. Small or medium sized reflectors (100mm aperture) will show fainter objects and finer detail than small refractors.

There are many mountings for the telescopes from the Altazimuth, Equatorial, Dobsonian and the GOTO. The Altazimuth is the simplest type of mounting used by many small refractors and reflectors, which allows the movement of the instrument simultaneously about both axes. The Equatorial is for the larger telescopes that often incorporate an equatorial mount which has four axes. This is more expensive and complex but has the advantage that objects can be kept within the field of view. The Dosonian has in recent years has become increasingly popular as a low-cost portable alternative to equatorials. It incorporates a modified altazimuth design, and is best suited to reflectors used with low power eyepieces for a wide-angle viewing of the sky. Some of these can be fitted with driving system and computer control. The GOTO is a most recent addition with many of these telescopes are becoming more affordable and available also are in effect computer controlled.

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