Newtonian Telescope: Reflections with Mirrors

Have you ever wanted a small, portable, and affordable telescope to look at faint objects like the moon or other planets? Newtonian telescopes are great for these purposes because they have a simple design, short focal length, and portable structure.

As the name suggests, the Newtonian telescope dates back to the 17th-century physicist Isaac Newton. In the mid-1600s, Newton noticed that chromatic aberrations occurred when light passed through the lens of a telescope. He theorized that aberrations appear when the light passes through the prism (i.e. lens), causing the colors to separate.

To remove these aberrations, Newton sought to create a telescope that used a mirror in place of a lens. Newton was not the first person to consider using mirrors, though. Galileo Galilei, Giovanni Francesco Sagredo, and others considered using a mirror, but Newton was the first to successfully complete the task. Thus, the telescope, which was completed in 1668, became known as the Newtonian telescope.

Even though Newton aimed to reduce the chromatic aberration from lens telescopes, all Newtonian telescopes come with a coma optical aberration. This aberration makes the stars appear wedge-shaped in short focal length scopes.

Additionally, Newton’s telescope was difficult to construct because the mirrors were difficult to polish into the correct shape. It was not until John Hadley improved Newton’s design by making the mirrors into a parabolic shape.

Newtonian Scopes Today

Today, Newtonian scopes are popular among amateur stargazers. Using Hadley’s design improvement, they are made with a primary concave mirror that is placed in the back of the tube. A secondary piece of the small diagonal mirror reflects the image from the side of the telescope to the eyepiece. Together, these pieces enlarge the image and provide a crisper image.

Since the Newtonian telescope reflects the image, the image is upside down. As a result, Newtonian scopes are mainly used for stargazing and other sky related purposes. More so, Newtonians often have a short focal length, which makes this scope type even more suited for stargazing.

The short focal length also makes Newtonians much smaller than other telescopes. The compact nature of these telescopes, along with their simple design, makes them much more affordable than other scopes. The affordability and compactness make this telescope ideal for amateur stargazers.