How Does A Refracting Telescope Work?

How Does A Refracting Telescope Work

These days you can find many types of telescopes in the market. But do you know what was the very first type? Yes, you guessed it right! It was the refracting telescope. Though being the simplest, it is still widely used for stargazing and other astronomical purposes.

In this article, we are going to dive deep and get a deep understanding of refracting telescopes. We’ll take a look at what are these actually, how refracting telescope works, advantages of refracting telescope and lastly, what are the limitations of it.

So, without further ado, let’s get started!

What Is A Refracting Telescope?

The main feature of a refracting telescope that distinguishes it from other types of telescope is its lens. Refracting telescopes, also known as dioptric scope, solely use a lens to create images.

A refracting telescope consists of 2 lenses. One is at the eyepiece and the other is the objective lens. The objective lens receives light ray from the source and focuses them into a single point. The image is formed at the focal point (the point where the light rays are focused).

After that, the eyepiece lens gathers the refracted lights and transmits them to the eye. The eyepiece lens is responsible for the magnification. It determines what the magnification will me. If you’ve read my how telescope eyepieces are measured post, you’ll know how changing eyepieces can change the magnification of a telescope.

Frankly, this is what a refracting telescope is. See how simple they are?

What Are The Parts Of A Refracting Telescope?

These are the structural components of a refracting telescope:

Objective Lens: Among the 2 lenses, the larger one is the objective lens. The objective lens sits at the front of the telescope and is responsible for gathering lights. The larger the objective lens is, the more light it can collect. As a result, the image will be crisper and sharper.

Also, a bigger objective lens allows us to see deeper and fainter objects in the night sky.

Eyepiece Lens: The eyepiece lens is responsible for the magnification of the telescope. With telescopes, you can easily change the magnification by changing the eyepiece.

With the same telescope, one eyepiece can offer 50x magnification whereas another can offer 100x. However, you shouldn’t get overboard with magnification. Refractive telescope has its limit.

In this post of mine, I have explained how magnification is measured, how to calculate the limit of your telescope and many other technical parameters that are fundamental for using any type of telescope.

How Does A Refracting Telescope Work?

how does a refractive telescope work

As I have mentioned before, refracting telescopes have 2 types of lenses: objective lens and the eyepiece lens.

The objective lens gathers light from the object we want to see (suppose the Moon). Then it bends the light rays and focuses them into a single point. This point is known as the focal point. The focal point is extremely important because this is the point where the image is formed. In this case, the image of the Moon will be formed at the focal point.

Then light rays from the focal point reach the eyepiece lens. The eyepiece lens magnifies the image formed at the focal point. Different eyepieces can offer different magnification of the same image formed at the focal point.

This is the simple working principle of a refracting telescope.

Quality Factors Of A Refracting Telescope

Does all refracting telescope produce the same quality image? Absolutely now. The quality of the image, as well as the sharpness, depends on some factors.

Here are the quality determining factors of a refracting telescope:

How Good The Lens Is?

When it comes to the refracting telescope, everything depends on the lens. If the lens has poor quality, there is no way you can expect a higher quality image. The lens needs to be top-notch for a good quality image.

A cheap lens can have imperfections that will produce a distorted image. And the only way to fix it will be to buy a better lens. There is no alternative to a good quality lens.

How Large The Lens Is?

I have mentioned that the objective lens gathers light. The larger the objective lens is, the more light it will gather. Consequently, the crispier and sharper image the objective lens will produce. That’s why we should always prefer a refracting telescope with a larger diameter objective lens.

I hope now you understand why a 12 centimeter lens is better than, let’s say, a 4 centimeter lens.

Do you know why we can’t read text from very far away? It is because the lens in our eyes (retina) is small and can’t gather enough light. If we could expand our retina size, reading a book from 10 meters away would be a piece of cake!

What Is The Magnification?

Magnification also plays an important role when determining the quality of a telescope. Every telescope has its magnification limit. Beyond that limit, the image gets distorted and blurred.

Magnification depends on many factors including the size of the image produced by the objective lens as well as the focal length.

Stay within your telescope’s magnification limit for a clear image.

Advantages Of Refracting Telescope

  • The revolving power is much greater than other types of telescopes
  • Produces steadier images
  • The light path doesn’t get interrupted
  • Requires minimum maintenance
  • Simpler structure

Limitations Of Refracting Telescope

Chromatic Aberration


The most common problem of refracting telescope is chromatic aberration. In fact, this is the main reason why reflecting telescopes were invented.

Before getting to chromatic aberration, let’s learn some basics.

The light as we see is not that simple. White light consists of 7 different constituent lights. We can test this by passing sunlight through a prism. Isaac Newton was the first to find this out.

Now that we know the light is made of several constituent lights or wavelengths of lights, let’s understand what chromatic aberration is.

We already know that the objective lens bends light into a single point. But as light is made of several wavelengths of other lights, each wavelength gets bent differently and focuses into a different point.

This results in a blurry and distorted image.

This effect is known as the Chromatic Aberration.

You can lessen the effect of chromatic aberration by using telescopes with higher focal ratio. To know the focal ratio, just divide the focal length with the aperture.

Dilemma With The Lens Size

If the lens size is small, the image quality will reduce. As I told earlier, the objective lens gathers light, which in terms determine the overall quality of the image. If the objective lens is small, then it can’t gather enough light. Consequently, the image will lose quality.

On the other hand, there is a problem with a larger objective lens too. Larger objective lens often faces lens sag. Lens sag generally occurs in the middle of the lens due to gravity. It leads to poor quality and distorted image.

My Favorite Refracting Telescope

If you are looking for a good quality refracting telescope at a cheaper price, I’ll highly recommend Orion AstroView refracting telescope. I haven’t found any other refracting telescope that offers so much at such a low price point.

  • Perfect for viewing Moon, planets as well as other deep sky objects
  • The equatorial mount makes it easy to track any object
  • Comes with an EQ-2 adjustable tripod
  • Includes everything you need for a successful stargazing experience

So, if you are interested to give it a try, check out the latest price on Amazon.

Final Words

So, this is all about refracting telescopes and how they work. I hope I’ve explained the working principle clearly to you. Though there are many more types of telescopes these days, refracting telescopes will always have their importance in astronomy and stargazing.



I have wondered how the Stars and Moons look like for many years. I’ve fallen in love with Cosmology since I was a boy. I am writing these articles to share my love for astronomy with you.

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