Refracting Telescope

We all very well know that a telescope is used to see distant astronomical objects and bodies. But have you ever wondered what it is made of which enables us to see the distant objects so clearly? Its working is based just on the simple principles of optics as is discussed below.

There are two main types of telescopes- a reflecting telescope and a refracting telescope. As the names suggest a reflecting telescope works on the principle of reflection using mirrors, whereas, a refracting telescope works using lenses which cause refraction of the light entering the telescope.

A refracting telescope contains two lenses, known as the objective and the other one as the eyepiece. Basically the objective lens produces an image of a distant object and the eyepiece lens magnifies this image. The objective lens is the one through which light enters the telescope. The final image is observed by the observer through the eyepiece. Both the lenses used are biconvex lenses with the objective lens having a larger focal length than that of the eyepiece. We know that a biconvex lens forms the image of an object placed at infinity on its focus. The distant celestial bodies can be considered at infinity for practical purposes. Thus, the objective lens which collects all the light from the astronomical bodies focuses it on its focus. Thus an inverted real image is formed at the focus of the objective lens.

Now comes the most important part of the refracting telescope. The tube length i.e. the distance between the objective and the eyepiece can be adjusted such that it is equal to the sum of the focal lengths of the two lenses. Because of this arrangement, the image formed by the objective lens on its focus also lies on the focus of the eyepiece. This image acts as a virtual object for the eyepiece which forms its image. This image can be clearly seen by the observer at the other focus of the eyepiece. The astronomical object is brought closer to the observer by this combination of the two lenses.

The size of an image produced by a lens is proportional to the focal length of the lens. The longer the focal length, the larger the image. The brightness of an image from a telescope depends partly on how much light is collected by the telescope. The light-gathering power of a telescope is directly proportional to the area of the objective lens. The larger the lens, the more light the telescope can gather. For example, doubling the diameter of the lens increases the light gathering power by a factor of 4. Brightness of images also depends on how big an area the image light is spread over. The smaller the area, the brighter the image.

The magnifying power of a telescope is the ratio of an object's angular diameter to its naked eye diameter. This depends on the focal length of both lenses.

                                       Focal length of objective lens

Magnification =    ---------------------------------------------

                                       Focal length of eyepiece lens.

However, there arise problems as well in viewing the distant objects using a telescope. One common problem that arises is that of chromatic aberration. White light contains coloured lights of different wavelengths. The refracting angle for refraction at the objective depends on the wavelength of the light. Due to this the image is not exactly focused at a point but the image is blurred and coloured like fuzzy rainbows. One can get special lenses that have a special coating to reduce the amount of chromatic aberration but they are expensive. Lenses present other optical problems including how difficult and expensive it is to make large lenses completely free of defects. Glass also absorbs most ultraviolet light, and visible light is substantially dimmed as it passes through a lens. In addition, lenses in telescopes can only be supported around the outside, so large lenses can sag and distort under their own weight. All of these problems affect image quality and clarity.

-         Article done by Maulik