Refractor Telescope is the first of its kind; a telescope used at the time of its invention. After its invention, it was Galileo who used it to view extra-terrestrial objects for the first time. The telescope uses lenses to capture and magnify the image. The lens towards the end facing the sky is bigger and is known as the objective lens while the other through which we view is smaller and is known as eyepiece. The principle behind this telescope is that light propagates and changes its direction after passing through a lens and meets at a point known as focus where the image of the object from which the light came is formed. This property of light is known as refraction and thus the telescope is also named so.
It entirely works on the principle of refraction. The objective lens helps in gathering more light than what our eye can gather. These lights then refract through the lens and pass through the tube of telescope converging into a focus where the actual image is formed but a small one. The lights then pass through the eyepiece lens which is near the focus and it again follows the principle of refraction and diffracts through the lens magnifying the image by the time the light falls on our retina.
The formula of Magnification is (focal length of objective lens) / (focal length of eyepiece). Here, focal length is the length between the centre of the lens and the focus. Longer the focal length of the objective lens more will be the magnification of the image and focal length is indirectly proportional to the thickness of the lens. Thus, lesser the thickness of the lens more will be the focal length, which in turn will give a bigger image. On the other hand, focal length of eyepiece is indirectly proportional to the magnification of the image, so a thick eyepiece will give a well-magnified image.
The brightness of the image depends both on the amount of light the objective lens can collect and the area on which the light spreads beyond the focus. Thus larger the objective lens, it can gather more light giving more brightness to the image and lesser the distance between the focus and the eyepiece, there will be more clarity in the image.
There are two types of Refractor Telescopes. They are as follows:
Achromatic refractors: These are the Refractor Telescopes which use two objective lenses with different properties. The lenses are; ‘Crown glass’, made up of alkali-lime silicates, which has low refractive index and dispersion factor, and the high lead content ‘Flint glass’, which has high refractive index but low dispersion. The process of combining these two lenses was patented by John Dolland in 1758 and it was then known as ‘achromatic doublet’. Some of the achromatic Refractor Telescopes are ‘Littrow’, which was made by the Austrian astronomer Joseph Von Littrow, ‘Fraunhofer’, developed by the German Joseph Von Fraunhofer and ‘Clark’ made by the American Alvan Clark and his sons.
Apochromatic refractors: This type of telescope has a different quality of lens which has been improved dramatically over a period of time and today, it is widely used. The lens is made up of a special material which has extra-low dispersion and thus, also known as ED glass (Extra-low Dispersion). The glass also has a fluoride content which helped in overcoming a major issue which Refractor Telescopes have been facing all these while.
The major disadvantage of a Refractor Telescope is the chromatic and spherical aberration that occurs at the time of the formation of an image. This phenomenon is actually the formation of colour smears around the image which also fails to provide a clear and detailed image due to the different wavelengths that each of the colour in the spectrum possess. Due to the variations in the wavelengths, each of the colours get focused in a different position after refraction, and thus fail to form an image in a single point. This disadvantage is overcome in the apochromatic refractors.
Some of the other disadvantages are – cost, which is very expensive and the objective lens is held by the edge which causes the glass to sag a lot.
Some of the major advantages for using a Refractor Telescope are:
It requires less maintenance.
It can be used for both terrestrial and celestial viewing.
The small size makes it more comfortable for the travellers.