There are various kinds of telescopes although the main three categories are refractor, reflector and compound telescopes. But what kind of telescope uses only lenses?
We will take a look at refracting telescopes and consider how they work including their limitations.
Refracting Telescopes
A refracting telescope is the only design that uses only lenses to form images. These are the earliest telescope design and first appeared in the Netherlands around 1608.
The idea was taken up by Galileo Galilei, who constructed his own version and applied it to making astronomical discoveries. Even though he use a telescope he did not invent the instrument. The inventor of the telescope is somewhat unknown.
The lenses that form refractions in telescopes are usually made of glass or plastic. A convex or converging lens takes parallel rays of light and bends them, this is called refraction.
The rays are then converged to a single point of light known as the focal point.
The distance between the lens and the focal point is known as the focal length of the lens.
Refracting telescopes use the combination of the objective lens and an eyepiece to gather more light than the human eye can collect on its own. The light is then focused and presents an image that is clearer, brighter and magnified.
The design used by Galileo is typically called a Galilean telescope which uses a convergent objective lens and a divergent eyepiece.
Improving on the design, Johannes Kepler used a convex lens as an eyepiece, thus converging the light rays and allowing for a wider field of view. But the image is inverted.
How Refraction Works?
Refraction is the bending of light when it passes from one transparent medium to another. Our eyes depend on this, too as without refraction, we would not be able to focus light onto our retina.
Light refracts when it travels at an angle into a substance with a different optical density, such as from air into water.
A change in speed causes a change of direction as the light slows down, which makes it travel in a different direction or at an altered angle.
The amount of bending of light rays depends on three factors; the change in speed, the angle of the incident ray and the light’s wavelength.
When a substance that the light passes through causes it to slow down or speed up more, the light will bend more. The greater the angle that the light enters a substance, the more noticeable the refraction.
Conversely, the straighter the entry of a light ray into a new medium, the less refraction will occur. If light enters a substance at 90 degrees it will still slow down but won’t change direction.
Different wavelengths of light are bent by different amounts. Blue light is bent more than red light. This is the reason why a prism creates a rainbow of color when white light is passed through it.
Types Of Lenses In A Refracting Telescope
The lens in a refracting telescope is made so that its surface is formed from segments of planes or spheres.
A convex lens causes light rays to gather at a single or focal point. A concave lens spreads out the light rays, which makes them appear as if they are originating from a point in front of the lens.
There are two main lenses used in a refracting telescope. In telescopes, the largest is the objective lens and the smaller is located in the eyepiece.
The objective lens is where the light rays enter and come to a focal point which is then viewed through the eyepiece lens.
Objective lenses are thicker in the center than they are at the edges (i.e. they are convex lenses). This bends the light more at the edges than through the center. This is how the light rays are bent into a single point where the image is then created.
The lens in the eyepiece of the refracting telescope then magnifies the image. The magnification of this kind of telescope is equal to the focal length of the objective lens divided by the focal length of the eyepiece.
Magnification
As we’ve seen, the image size that a lens produces is in proportion to the lens’s focal length. A longer focal length will produce a larger image.
How bright the image is will depend on the amount of light the telescope collects, and this depends on the size of its objective lens (also known as aperture). The bigger the lens, the greater the amount of light that can be gathered by the telescope. For more information, see our article Why Is It Advantageous To Use A Large-Diameter Objective Lens In A Telescope?
When the objective lens’ diameter is doubled, the amount of light the telescope can gather is increased four times. Image brightness depends on the size of the area that the light is dispersed over.
Therefore a less-magnified image will be brighter than one magnified to a higher degree.
A telescope’s magnification is the ratio of an object’s angular diameter to the naked eye diameter. It is a function of the focal length of the objective and eyepiece lenses.
Magnification may seem the most essential facet of a refracting telescope. However, it is more important to have a sharp, clear image which is determined by the light-gathering capability of the telescope.
Due to atmospheric effects, a blurred image magnified will still be a blurred image but larger. A telescope with good light-gathering capability will produce a sharper image that can then be magnified.
Chromatic aberration is the image distortion that is created when using lenses to view objects.
As light travels through a lens, the separate colors of the spectrum are bent at different angles, such as with a prism. This causes them to come into focus at separate points.
When viewing a star through a lens the star will be surrounded by a halo of colors. To correct this problem, a slimmer lens of different glass is added at the rear of the objective lens.
Glass is very difficult to make without any defects. It absorbs most ultraviolet light. In addition, visible light is dimmed substantially when passing through a glass lens.
In large telescopes, the lens can sag due to its weight and the effects of gravity, which causes problems with image quality.
Final Thoughts
We hope you have enjoyed this guide to the kind of telescope that uses only lenses and found it informative.
Hi, my name is Jason Anderson, and I am a Physics Professor. Ever since I was a kid, I’ve been fascinated with space, the universe, the moon, you name it. I spent hours and hours at the planetarium close to my hometown, wondering what else could be out in the universe.
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