Properties of Refracting Telescopes


A telescope is an instrument that aids in the observation faraway objects by collecting electromagnetic radiation such as visible light. Telescopes "do their magic" by gathering and focusing light. Telescopes are usually used in astronomy to observe faraway, planets, moons, stars, etc. When we are looking at the stars in the sky with the naked eye,  they appear as points of light to us. The rays of light from the star shower down all over the landscape, which enters the pupils of the observer's eye. When the light rays enter the observer's eye, the light rays pass through the lens of the eye, it focuses them to a point on the retina, which lines the back half of the eye. Photoreceptor cells there translate the light into nerve impulses; the brain interprets, and "sees" the point like star in the sky. When watching a star through a telescope, you can see a clear, bright, magnified image of the star. 


There are two main types of telescopes, which are:

Reflecting Telescope
Refracting Telescope

The difference between refracting telescope and reflecting telescope is a refracting telescope uses a combination of lenses to produce a bright image of a distant object, while a reflecting telescope uses a combination of mirror and lens to view dim or dark faraway objects. 


A simple refracting telescope consists of two lenses, the Objective and the eyepiece. Basically the objective lens produces an image of a distant object at its focus and the eyepiece lens magnifies this image. We can represent this using a ray diagram which shows the path of imaginary rays passing through the telescope.

When light passes through the objective lens, it is bent or refracted. Light waves that enter on a parallel path converge, or meet together at a focal point. Light waves which enter at an angle converge on the focal plane. It is the combination of both which form an image that is further refracted by a secondary lens called the eyepiece.


The magnification of a telescope is how much bigger the image is compared to the object. The magnification of a refracting telescope is equal to the focal length of the objective divided by the focal length of the eyepiece.

Star Gazers usually have a selection of eyepieces at hand. You don't always want an incredibly high magnification, example: when studying the Moon. When you want to observe a star, you will often switch eyepieces. Less powerful to locate the star and more powerful to observe it closely. Eyepieces with smaller focal lengths are more powerful as they need to bend the light more. 


The images that are produced by the refracting telescope are upside down. In space, there is no up or down. Viewing stars upside down is no problem. But, terrestrial viewing will be an inconvenience. For example, viewing a tree upside down. The Galilean telescope was introduced by Galileo Galilei in 1609. 

Galilean telescope:
-Galileo improved the design of his telescope by using a convex objective lens and a concave eyepiece
-The improvement gave a much wider field of view and improved the comfort distance the eye must be away from the eyepiece to achieve focus and eye relief.
-However, the image is upright and it is best for terrestrial viewing.
-His best telescope was best capable of magnifying objects close to thirty times.

The first telescopes were refractors that used a convex lens to form an inverted image of a distant object, and used a concave lens to invert this image to an upright orientation as well as provide some additional magnification. Because this was the sort of device Galileo used to make the first important telescopic discoveries in astronomy, it is usually referred to as a Galilean telescope.

Keplerian telescope:                                                                                                                                                                        

-Invented by Johannes Kepler in 1611.

-The advantage of this arrangement is the rays of light emerging from the eyepiece are converging.

-This allows for a much wider field of view and greater eye relief but the image for the viewer is inverted. Considerably higher magnifications can be reached with this design.

Achromatic refractors:
-Invented by Chester Moore Hall in 1733
-The design overcame the need for very long focal lengths in refracting telescopes by using an objective made of two pieces of glass with different dispersion, "crown" and "flint glass", to limit the effects of chromatic and spherical aberration.
-Chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colours to the same convergence point.
-Spherical aberration is an optical effect observed in an optical device that occurs due to the increased refraction of light rays when they strike a lens or a reflection of light rays when they strike a mirror near its edge, in comparison with those that strike nearer the centre.

Apochromatic refractors:

-Apochromatic refractors have objectives built with special, extra-low dispersion materials.
-They are designed to bring three wavelengths (usually red, green, and blue) into focus in the same plane.
-Such telescopes contain elements fluorite or special, extra-low dispersion (ED) glass in the objective and produce a very crisp image that is virtually free of chromatic aberration.


Refracting telescopes find a wide range of applications in astronomy, though the applications include non-astronomical uses as in binoculars and cameras. Refracting telescopes make use of lenses. They usually focus the light waves with the help of these lenses.

Telescopes are one of the main ways that astronomers explore the universe. Refracting telescopes gather more light and better resolution. Resolution is the ability of a telescope to bring distant object close together: for an example the ability to clearly separate two stars that are very close together or the ability to see smaller craters on the moon.

Telescopes are used in stargazing and amateur astronomy. Lots of astronomical research is done on telescopes that look at other wavelengths of light. Each of these wavelengths ranges shows us something unique about universe and allow us to view events and objects that can't be seen with human eyes.

Astronomers study objects in space with the use of telescopes. This helped them to make observations to find amazing discoveries.