Introduction 

Several optical instruments have been designed using the concept of reflection, refraction, lenses, prisms and many more. Telescope is one example of such optical instruments. The first practical telescope was invented at the beginning of the 17th century, using glass lenses in the Netherlands. A basic telescope comprises an objective lens and an eyepiece lens.The objective lens focuses on a parallel beam of light, which then travels through an eyepiece to generate a magnified image.

 In general, there are two types of telescopes:

1. Terrestrial telescope

2. Astronomical telescope

Terrestrial Telescope:

It is an instrument that requires lenses and curved mirrors or even a combination of both to observe distant objects. The last image formed by the terrestrial telescope is non-inverted and erect.

Principle of Terrestrial Telescope:

The working principle of the terrestrial telescope uses three lenses. The first lens, L1, acts as the telescope’s objective lens, while L2 acts as the eyepiece. The third lens is placed between L1 and L2 such that the third length of short focal length is placed permanently. This lens is known as the erecting lens. The image formed by this telescope is reversed even if it is virtual.

Construction of Terrestrial Telescope :

It contains three lenses compared to the astronomical telescope, also called a spyglass. The terrestrial telescope creates an erected final image of the object. The third lens, shorter focal length f, is placed at 2f, which forms an inverted image. The image formed by this focal length serves as the object for the eyepiece. The lens responsible for erecting the image is the erecting lens and is placed at the center of the telescope. The two convex lenses will be coaxially mounted at the extreme of the two tubes such that one slides onto another. The distance between them can be changed by applying a screw arrangement.

Astronomical Telescope

An astronomical telescope is an instrument that works on the principle of optics used to view the enlarged image of heavenly bodies like stars and galaxies. The end image formed by an astronomical telescope is always virtual, inverted, and magnified. It is an excellent means for analyzing and collecting radiation from heavenly bodies situated far from the universe we live in. It helps scientists to observe subjects with a detailed view even if they are at an infinitely large distance.

Principle of Astronomical Telescope

The working of telescopes is based on the principle that when an object that is to be magnified is placed at a large distance from the objective lens of an astronomical telescope, magnified, virtual, and inverted images are formed at the least distance when held close to the eye piece.

Construction of Astronomical Telescope

An astronomical telescope’s construction consists of two convex lenses: An objective lens placed at the position O and an eyepiece placed at the position of E. The focal length of the astronomical telescope is fO, and the eyepiece’s focal length is fE. The aperture of the objective lens is taken large as compared to the eyepiece so that more light can be received by it from distant objects and a brighter image can be formed. The placement of both the objective lens and the eyepiece is done at the free ends of the two sliding tubes placed with some considerable distance in between.

Working Principle of Astronomical Telescope

When a parallel beam of light from distant objects like stars and planets falls on the telescope’s objective lens, a real, inverted, and diminished image is formed  of the object. The image of this kind will now act as an object for the eyepiece E by adjusting a proper position such that the image is formed between the focus  and the optical center of the eyepiece. Now when the image is formed, it is a virtual, inverted, and highly magnified image of the object at infinity, The final image is formed at the infinity and then the telescope is considered to be in ‘normal adjustment.

The final image of the object of heavenly bodies formed by the astronomical telescope is always inverted concerning the thing. However, it does not matter if an inverted image is an astronomical object because most heavenly bodies are generally spherical.

Types of Magnification lenses

Magnification lenses can be divided into two types:

Simple lenses

Compound lenses

Simple Lenses:

A simple lens is used to magnify objects. We also use a simple lens to read newspapers and magnify things in front of us when we want to see them up close. In addition to that, they have the least power (magnification strength), ranging from 2x (two times) to 6x (six times).

With these, the object appears twice as big as it really is. Another example of a simple lens is a magnifying glass or a pair of glasses (spectacles). They, however, produce low-quality images.

Compound lenses:

Even though compound and simple lenses both magnify objects, their magnifying powers differ. A compound lens can magnify objects with a higher degree of clarity and projects a clearer image than a simple lens.

A simple example can illustrate this. If you have someone move 5 feet closer to you, and he is standing 5 feet away from you, then this is simple magnification. It would be compound magnification, however, if the same person came closer six times.

Compound lenses are mainly used in microscopes and telescopes. They also focus by using multiple lenses. Apart from that, they can magnify objects four times, 10 times, 40 times, 100 times, and 400 times.

Formula of Magnification

Magnification is defined as the ratio between the height of the object and the height of the image. Additionally, the letter ‘m’ indicates the magnification of the object. Its formula is as follows:

Magnification (m) = h / h’

In this case, h is the height of the object, while h’ is the height of its image.

Further, it should also be considered in relation to the image distance and the object distance. As a result, it could be written as:                                          m = v / u

In this case, u represents the distance to the object, and v represents the distance to the image.

In this way, an expression of magnification would be:

                                   m = h’ / h = v / u

Magnifying power of telescope:

The ratio of the angle subtended at the eye by the image formed at least distance of distinct vision to the angle subtended at the eye by the object lying at infinity, the magnifying power of a telescope.

M=fo/fe

fo= the object’s focal length

fe= focal length of the eyepiece

Conclusion

The telescope is a device used to view remote gadgets clearly. It is composed of two parts, the Objective lens, and the Eyepiece. The telescope is used to observe objects very far from us. The telescope is used to offer angular magnification of remote gadgets. The Indian Astronomical Observatory (IAO) is located in Hanle, Ladakh, India, and is one of the world’s highest optical observatories. Cameras, spectrographs, and interferometers are three types of instruments used by the Hubble Space Telescope to investigate light from the universe.

Various Astronomy equipment, Telescopes, and radio dishes are used from the surface of the planet to review visible light, close to infrared light, and radio waves. connected to those telescopes are varied tools like specially created CCD cameras, a good type of filters, photometers, and spectrometers