A telescope is an instrument that collects and focuses light. The nature of the optical design determines how the light is focused. Some telescopes (known as refractors) use lenses and other telescopes, known as reflectors (Newtonians), use mirrors. Then, the Schmidt-Cassegrain telescope uses both mirrors and lenses. Each optical design is briefly discussed below:
Developed in the early 1600s, the refractor is the oldest telescope design. It derives its name from the method it uses to focus incoming light rays. The refractor uses a lens to bend or refract incoming light rays, hence the name (see Figure 3-1). Early designs used single element lenses. However, the single lens acts like a prism and breaks light down into the colors of the rainbow, a phenomenon known as chromatic aberration. To get around this problem, a two-element lens, known as an achromat, was introduced. Each element has a different index of refraction allowing two different wavelengths of light to be focused at the same point. Most two-element lenses, usually made of crown and flint glasses, are corrected for red and green light. Blue light may still be focused at a slightly different point.
Figure 3-1
A cutaway view of the light path of the Refractor optical design
A Newtonian reflector uses a single concave mirror as its primary mirror. Light enters the tube traveling to the mirror at the back end. There light is bent forward in the tube to a single point, its focal point. Since putting your head in front of the telescope to look at the image with an eyepiece would keep the reflector from working, a flat mirror called a diagonal intercepts the light and points it out the side of the tube at right angles to the tube. The eyepiece is placed there for easy viewing.
Newtonian Reflector telescopes replace heavy lenses with mirrors to collect and focus the light, providing much more light-gathering power for the dollar. Because the light path is intercepted and reflected out to the side, you can have focal lengths up to 1000mm and still enjoy a telescope that is relatively compact and portable. A Newtonian Reflector telescope offers such impressive light-gathering characteristics you can take a serious interest in deep space astronomy even on a modest budget. Newtonian Reflector telescopes do require more care and maintenance because the primary mirror is exposed to air and dust. However, this small drawback does not hamper this type of telescope’s popularity with those who want an economical telescope that can still resolve faint, distant objects.
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