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Refractor Telescopes
The refractor telescope was the original design used by Galileo and people of his era around 1610 AD. The original design included just two lenses, a plano-convex objective lens in front and a plano-concave secondary lens at the rear. Plano-convex means the lens has a plane shape on one side and a convex shape on the other, while plano-concave means the lens has a plane shape on one side and a concave shape on the other. Some Galilean designs may vary the plane sides of these lenses, but for our discussion we will concentrate on the design shown in Figure 1.
Figure 1
In this design, light from the observed object is bent as it passes through the objective lens to a focal plane inside the telescope barrel. We talk about a focal plane because light colors do not all focus at a common point. Different colors focus at different points on an imaginary plane inside the telescope tube. The light rays then continue beyond the focal plane to the secondary lens which slightly increases the bending light before entering our eyes. The bending from the secondary lens provides the magnification of the object being viewed. Since the light rays exchange places at the focal plane, top becomes bottom and bottom becomes top, the object being viewed appears upside down when observed through the eyepiece. The original refractor telescopes had problems with objects being out of focus at the peripheral ends of the field of view (spherical aberration), as well as blue and red colors being distorted (chromatic aberration). To correct these problems, telescope manufacturers developed a compound Objective Lens Assembly (OLA) by combining a bi-convex lens with a bi-concave lens and separating them with an air space. (See Figure 2). These lenses also have different qualities of glass which help in the bending of the colored light as it passes through them.
Figure 2
The first OLA's were named Achromatic lenses and helped reduce spherical as well as chromatic aberrations in the original refractor designs, however, they didn't remove all aberrations. So a more advanced Apochromatic OLA was developed, which introduced a plano-convex lens behind the bi-concave lens in the achromatic design. This addition further reduced both aberration problems and is featured in the more expensive refractors on the market today. Of course these lenses add significantly to the overall cost of the telescope. The design shown in Figure 2 not only shows the OLA but also a revised rear design for easier viewing of celestial objects when the telescope is mounted onto a tripod or other mount. This eyepiece adaptation is called the eyepiece prism and uses a flat mirror to redirect the image so the observer doesn't have to position themselves below the telescope to make observations. The refractor telescope design is also used in field scopes like binoculars, survey scopes, visual aids on long range hunting rifles and a host of other viewing apparatus. These field scopes introduce additional lenses or mirrors to flip the viewed object so it appears upright to the viewer. Celestial scopes, however, don't bother with righting the view since there is no upside down in space and the additional adjustments add cost and weight to the telescope. Cost of Refractor Scopes: The costs of refractor telescopes are very reasonable for the beginner models, but they rise at a phenomenal rate as the objective lens and the aperture increases in diameter. Beginner scopes with inexpensive lenses are available in aperture sizes of 50mm (2") to 76mm (3") in diameter, and new scopes can be purchased in the $100 to $150 price range. The price for one of these scopes can be drastically reduced if they are purchased second hand. These inexpensive scopes can be purchased from retail camera stores, some toy and hobby stores and on the internet. They are capable of observing the moon, some of the planets, large nebula like Orion, the large galaxy Andromeda, and large open clusters of stars like Pleiades. More expensive refractors also begin at around 50mm, but with the increased quality of the lenses, filters, equatorial mounts and host of other attachments, they can quickly approach $1,000 or more. Some low cost telescopes checked in March of 2009 are listed below: Orion Observer 60mm with Alt-Azimuth tripod; 2.3" Objective lens -- $100.00 Orion Astroview 90mm with Equatorial mount; 3.5" Objective lens -- $329.00 Celestron NexStar 102 SLT with Alt-Azimuth tripod; 4.0" Objective lens -- $500.00
Inexpensive 50mm (2") Refractor Telescope
Mounted on Alt-azimuth tripod |
