Refracting Telescopes

A refracting telescope is the type which most picture when the word "telescope" comes to mind. Visual memories of television pirates extending their brass refracting telescopes to see approaching warships or sea monsters are all part of our assumptions of what a telescope is, but now is as good a time as any to develop a more realistic and complete picture of what telescopes really are. A telescope is a device used to enlarge the image of an object for the viewer. In the image below, you will see the effect of a biconvex lens on light. The rays of light are bent by the double curvature of the lens, and focused on a point at a distance from the lens. This focal point distance is called the "Focal Length," and this value is dependent on the lens used. Lens can have long or short focal lengths depending on the extent of the curvature.

In the image to the left is a picture of what happens when two lenses are placed at a distance from each other ... much in the same manner as Galileo did when he constructed his first telescope in 1609. The objective lens is so-named because it is closest to the object we are looking at. This lens bends light down to a focal point, and the eyepiece is place just behind that point to straighten out the light rays for our eyes. We want an objective lens to have along focal length, but to view the image we want an eyepiece with a short focal length. The result is an enlarged image, which can be made increasingly larger by using a more strongly curved eyepiece. The image we see will be upside down and backward. This reversing of the image is due to the crossing over of the light rays as pictured below. This is a "problem" with refracting scopes, but devices have been constructed to right the image. The larger the objective lens, the more light can be captured. The smaller the eyepiece, the greater the magnification.

The image below shows a nice spotting telescope design which is a refracting telescope. The objecting lens is to the far right. The focal point inner a mirror at the back of the scope. The mirror redirects the light upward to an eyepiece. This allows greater comfort for the observer, and is the telescope design of choice for birdwatchers, surveyors, and casual star watchers.

Finally, the refracting telescopes are very nice for viewing planets and the moon, but the grinding of the lens to exact specifications is extremely difficult as you try to enlarge the lens. In order to achieve greater magnification, one must either make an extremely small eyepiece lens, or construct a very large objective lens. Large objective biconvex lens are very expensive and heavy, and almost prohibitively expensive. The largest objective lens is at Yerkes Observatory in Wisconsin. This telescope uses a 40 inch objective lens, but the focal length is incredibly long, meaning a very long tube must be constructed to place the eyepiece at the proper focal point.

As you can see in this photograph of the Yerkes Observatory Telescope, it is pretty long. In addition to the expense of grinding large objective lenses and lengthy tube assemblies, there is another problem with the way light is bent by the glass. When light is reflected through glass, shorter wavelengths like blue and indigo bend more than the longer wavelengths like red and orange. Blue light will come to focus on a point closer to the objective than red light. If we focus the eyepiece on the blue image, the red image is blurred, and vice versa. This color separation is called Chromatic Aberration. This problem can be corrected with additional glass lenses behind the objective, but this just adds more weight and cost to the project.

These scopes give great, clear images of planets, but are not useful for really deep space objects like nebulae, clusters, and galaxies. To see these things, we need a bigger objective lens, and to compensate for cost overrides, another design is necessary ... one conceived by Isaac Newton.

Please proceed now to Reflecting Telescopes to learn about how we can see farther out into space. Or, you can return to the Introduction to Light and Telescopes Page, the Syllabus, or the Home page.

 


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