A Short Study on Telescope

Telescopes are an indispensable tool in the study of astronomy. It allows the user to magnify and observe distant celestial objects in the sky. The first-ever telescopes are believed to be invented in the Netherlands with a simple glass lens.  Galileo is often credited for the discovery of the telescope. He created several designs of the telescope using different lenses. 

The different types of telescopes used for detecting electromagnetic radiations and astronomical telescopes are:

• Optical telescope: It works on the principle of visible light

• Ultraviolet telescope: It works on wavelengths shorter than visible light

• X-ray telescope: It works on the principle of using light of a shorter wavelength than the ultraviolet and radio telescopes that work on a much longer wavelength of visible light. 

Different types of telescopes work on different wavelengths of light and offer different types of information about celestial bodies.

Construction and Working Principle of Telescope

A simple telescope can be described as a pair of lenses mounted inside a hollow tube. The first lens through which we see is the objective and it helps to focus on an object or the image. The lens behind the first lens is the eyepiece and it magnifies the image. 

A simple telescope is just a simulation of an astronomical telescope:  

• In some situations, light does not travel straight and when it hits a transparent object, it deviates from its original path. Just like when the sun’s rays hit the water, it bends and deviates.

• Glass lenses were used to create this light-bending phenomenon. The light should be made to hit the lens at a particular angle. The angle has to be smaller to make it bend much more sharply and produce a reflection.

• The rays falling on the lens reflect at an appropriate angle and make an image. Depending upon the aperture of the lens, the image is magnified. 

Astronomical Telescope

The above figure illustrates the working of an astronomical telescope:

• The objective lens is the first to receive the parallel beam of light from any celestial object such as stars, planets or meteors. A’B is the image formed by the objective lens, which is inverted but real. 

• The image A’B is an object for the eyepiece E. the position of the eyepiece is so adjusted that the image lies between the focus “fe” and at the centre of the C2 of the eyepiece.

• The eyepiece is ready to form the image as inverted but highly magnified at infinity. When the image is formed at infinity, the telescope is adjusted to a “normal setting”.

• Every viewer from the telescope should always remember that the image formed by the telescope is not erect and straight, rather it is inverted. Astronomical telescopes show an inverted image. 

Factors affecting the resolution of a telescope:

1. Apparatus for light-gathering: The light-gathering apparatus in a telescope depends on the diameter of the objective. For a refractive telescope, the objective is the first lens the light passes through. For a reflecting telescope, the objective is supposed to be the primary mirror. The resolving power of the telescope increases as the diameter of the objective increases.

2. Diffraction limit: The resolving power of a telescope is called the diffraction limit of the telescope. The smallest angular separation between two visible objects is defined as diffraction. The SI unit of diffraction is arcsecond. The relation between the diffraction and the diameter of the objective is inverse, which implies that as the diameter of the objective decreases, the diffraction limit increases.

3. Wavelength of light: With a higher wavelength of light the diffraction will increase. With a lower wavelength of light, the image will not be clearer. There can be different telescopes with changing wavelengths of light and at varying angles of the objective.

4. Atmosphere of the earth: The particles in the air and earth’s atmosphere cause light to get refracted. As the path of visible light changes, it deviates the image formation on the objective. This causes blurring of images. To avoid these, many viewers prefer mounting the telescopes on a mountain peak or high rise building to get a clearer view.

Conclusion

Telescopes have been said to be invented during the 17th century. Since then, there have been enormous refinements in the angle and viewing capabilities of telescopes. Different types of telescopes are used for viewing different aspects of the celestial system due to multiple wavelengths of light. The basic working of a telescope utilises two lenses, an objective and an eyepiece. Distant objects of the universe can be viewed with a telescope and the angle of diffraction can determine the magnification and clarity of the celestial bodies.