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Telescopes are cool, right? You've probably seen them before and know they help us see far away things. But have you ever wondered why far away things look small or dark? Well, that's where telescopes come in handy! They help us see those small and dark objects in the distance. So, let's learn more about how telescopes actually work. And, if you're into stargazing or want to explore the universe, telescopes are definitely something you should consider getting. They can help you see things you never thought were possible! So, let's dive in and discover the amazing world of telescopes!

The retina of the eye

The retina is a crucial part of the eye that receives light and turns it into signals that the brain uses to create visual images. Unfortunately, the retina is small and can only handle a limited amount of light. But don't worry, this is where telescopes come in! They help us see far away objects by bringing them closer to our eyes. So, if you're interested in learning more about the amazing world of optics and telescopes, check out resources like Physics of the Eye. It's fascinating to discover how we can use technology to enhance our vision and explore the world around us!

How does a telescope work?

A telescope can capture more light. In other words, it can bring an image closer to you and then magnify it by using magnifying glasses.

A telescope. Image: Pixabay

A telescope works on the principle of two convex lenses. Have a look at the ray diagram for an object very far away – like the moon, for instance. The objective lens has a focal length of f0, and the eyepiece has a focal length of fe. An objective lens would be bigger than the eyepiece as it has to capture maximum light.

A diagram outlining how a telescope works

The focal length of a lens is very important. An object has to be within the focal length to be magnified and seen clearly. If a body is not within the lens's focal length, it will appear blurred.

The magnifying glass magnifies the object which is inside its focal length

Telescopes are fascinating devices that use lenses to help us see faraway objects in the sky. The objective lens is the first convex lens in a telescope that receives light rays from a distant object. Since these light rays are parallel, they converge at the focal point of the lens, forming an inverted image of the object.

The eyepiece lens is the second convex lens in a telescope that magnifies the image formed by the objective lens. It has a shorter focal length than the objective lens, but it is positioned at a distance that aligns its focal length with that of the objective lens. As a result, the light rays passing through the eyepiece lens become parallel and enter the viewer's eye an large's amazing to think about how these simple lenses can help us observe objects that are millions of miles away in the universe. Understanding the physics of telescopes can deepen our appreciation for the wonders of the cosmos.

What is the magnifying power of a telescope?

The magnifying power of a telescope is determined by the ratio of the focal length of the objective lens to the focal length of the eyepiece. This ratio is often referred to as the telescope's "focal ratio" or "f-number".

A higher focal ratio means a higher magnifying power, but it also means a narrower field of view. Conversely, a lower focal ratio means a wider field of view, but a lower magnifying power. So, choosing the right focal ratio for your telescope depends on what you want to observe.

For example, if you want to observe planets in detail, you should choose a telescope with a higher focal ratio to get a higher magnifying power. However, if you want to observe galaxies or star clusters, you should choose a lower focal ratio to get a wider field of view.

In summary, understanding the magnifying power of a telescope is important to ensure that we choose the right combination of lenses to observe the objects we are interested in with the desired level of detail and field of view.

What are the different types of telescopes?

There are three different types of telescopes: the refracting telescope, the reflecting telescope, and the radio telescope.

Refracting telescope

A refracting telescope consists of two or more lenses, and their primary purpose is to gather as much light as possible and focus it to one point. The bigger the aperture, the longer the telescope has to be to focus the image at one point. Another reason why refracting telescopes are longer is that the light must flow in a straight line through the telescope tube. The tube of the refracting telescope has to be at least as long as the focal length.

A refracting telescope
A refracting telescope

Why does the refracting telescope need multiple lenses? This is because more lenses in a refracting telescope reduce the effect of chromatic aberration.

Chromatic aberration: A phenomenon in which a lens is unable to focus all colors at a single point, causing a dispersion instead.

Chromatic aberration occurs because the refractive index of the lens elements varies with the wavelength of the light.

An example of an image that has chromatic aberration
An example of an image that has chromatic aberration

Reflecting telescope

Also known as a Newtonian telescope, a reflecting telescope uses mirrors to converge light at a single point from distant objects. Why mirrors? We are not concerned about light flowing in a straight line through the reflecting telescope tube, which is why they are not as long as refracting telescopes. As reflecting telescopes are comprised of mirrors and not lenses, they are often cheaper than refracting telescopes, especially if both have large apertures. A Newtonian telescope is a couple of mirrors aligned so that light from the curved (concave) mirror reflects the light onto a second flat mirror directed to the eyepiece. This is referred to as collimation.

Collimation: The process of arranging all elements in a telescope to get the best focus out of light rays. Sir Isaac Newton invented this telescope for the first time in 1668. This telescope is primarily used for larger objects at a distance which is why most telescopes used in astronomy are reflecting telescopes.

A reflecting telescope with a ray diagram
A reflecting telescope with a ray diagram


These telescopes take advantage of both lenses and mirrors, which are more compact and portable than refractive and reflective telescopes with the same aperture. In figure 8, a corrector plate C focuses the light onto the primary mirror M1, bouncing the light to the secondary convex mirror M2. This light is then reflected through a hole in the primary mirror. There are many variations of catadioptrics, like the Schmidt-Cassegrain telescope, but the underlying principle is the same.

A Schmidt-Cassegrain telescope
A Schmidt-Cassegrain telescope

One disadvantage of catadioptrics is the possibility of a spherical aberration, which depends on the shape of the primary mirror. Why does this happen? This happens because the shape of the primary mirror may focus light rays at slightly different points, causing a blurry image. Spherical aberration: when an image is blurred from its edges.

Monocular telescope

A monocular telescope is a type of telescope that uses mirrors and lenses to magnify distant objects. You can only view objects with a monocular telescope using one eye as it has only one eyepiece. It has a prism lens accompanied by converging lenses or mirrors that bends light and then magnify the object. Monoculars are lightweight, compact, portable, and cost less than binoculars.

A monocular telescope
A monocular telescope

Radio telescopes

telescopes are amazing tools that help us study the universe in a completely different way. Unlike optical telescopes, which capture visible light, radio telescopes capture radio waves emitted by astronomical objects like stars, black holes, and galaxies. These radio waves are often very weak, so radio telescopes have to be very sensitive to detect them.

Radio telescopes work by using a large dish or array of dishes to collect radio waves from space. These radio waves are then focused onto a receiver that amplifies the signals and converts them into an electrical signal that can be analyzed by astronomers.

The shape of the radio telescope dish is very important. A parabolic dish is the most common shape because it allows the maximum number of waves to focus on one point, which gives the highest possible sensitivity. However, this shape can also cause interference from other sources of radio waves, so radio telescopes are usually located in remote areas far away from human activity.

Overall, radio telescopes are amazing tools that have helped us make incredible discoveries about the universe. They have allowed us to study objects that would be invisible to the naked eye and have helped us uncover new mysteries about the cosmos.

Magnifying glass
Magnifying glass

Figure 5:

Refracting telescope of the Strasbourg observatory

Figure 6:

Warning Symbol – Chromatic Aberration
Warning Symbol – Chromatic Aberration

Figure 7:

Newtonian telescope
Newtonian telescope

Figure 8:

Diagram Reflector Schmidt Cassegrain
Diagram Reflector Schmidt Cassegrain

Figure 9:


Figure 10:

Mount Pleasant Radio Telescope


What is a telescope?

A telescope is an optic device that can magnify distant objects to make them easier to see.  

What can you see through a telescope?

We can see distant objects with a telescope including stars, galaxies, planets, sun, moons, etc. 

How does a telescope work?

A telescope captures a large amount of light in its aperture which the normal retina cannot, and hence allows us to see objects that would be impossible with the naked eye.

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