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We all have seen telescopes in our daily lives, and we all have a general idea of how they are used. A telescope makes objects in the distance visible, right? But before we dive into how a telescope does this, it is important to understand why far away objects seem small or dark.
The retina is the part of the eye that receives light and transforms it into neural signals so that the brain can convert them into visual information. It is small and can therefore receive only a limited amount of light. Take a look at Physics of the Eye for more information.
How can you bring things that are at a distance closer to the eye to make them visible? This is where telescopes are handy.
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.
Figure 2. A diagram outlining how a telescope works. Image: public domain.
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.
Figure 3. The magnifying glass magnifies the object which is inside its focal length. Image: AntanO, CC BY-SA 4.0 via Wikimedia Commons
The light rays reaching the objective lens would be parallel as they come from a fair distance. Any light passing through the optical centre of a convex lens would not deviate while the rest of the rays will deviate to the focal point f0. Hence, an image would be inverted for an object whose distance is smaller than the focal length of the lens.
Figure 4. A diagram showing three cases of light passing through it. Lightray 1 would pass through the principal focus, light ray 2 would deviate and be parallel to the principal axis, and lightray 3 would pass without any deviation. Image: public domain.
The eyepiece lens is the second convex lens which has a bigger width than the objective lens. It has a shorter focal length, but it is placed in the telescope at such a distance that its focal length aligns with the focal length of the objective lens.
Rays of light from the eyepiece lens to the eye would be parallel as the object is placed right at the principal focus and would be at an incident angle to the eye. As the new angle formed at the eye is β (as seen in Figure 2), a large image would be formed in the retina.
Have a look at figure 2 again, and let's take the angle subtended at the eye as β. The angle α is the subtended angle at the objective lens. The magnifying power of the telescope is:
= 
Where f0 is the focal length of the objective lens and fe is the eyepiece focal length.
Why do we need to measure the ratio above? It is this ratio that is going to decide how big an object is formed in the retina. To get the maximum magnification from our telescope, we need to have a very high focal length objective lens and a very low focal length for the eyepiece.
There are three different types of telescopes: the refracting telescope, the reflecting telescope, and the radio 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.
Figure 5. A refracting telescope. Image 1: public domain. Image 2: mclapics, CC BY-SA 2.0
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.
Figure 6. An example of an image that has chromatic aberration. Image: August Geyler, CC BY-SA 4.0
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.
Figure 7. A reflecting telescope with a ray diagram. Image 1: Kizar, CC BY-SA 3.0. Image 2: Bin im Garten, CC BY-SA 3.0
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.
Figure 8. A Schmidt-Cassegrain telescope. Image 1: HHahn, CC BY-SA 3.0. Image 2: WHS-2V, CC BY-SA 4.0
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.
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.
Figure 9. A monocular telescope. Image 1: Tamasflex, CC BY-SA 3.0 Image 2: Bradley Weber, CC BY 2.0
Just as optical telescopes capture light and magnify so that we can visualise distant objects clearly, radio telescopes capture weak radio light waves and amplify them for further analysis. We can use radio telescopes to examine radio waves from astronomical objects like stars, black holes, etc.
Radio telescopes are the biggest telescopes because they have to detect weak cosmic radio waves. Although radio telescopes are built in different sizes and shapes, all telescopes have a mounted antenna and a minimum of one receiver to capture the signals.
The working of a radio telescope is very much similar to a reflecting telescope where the radio waves are reflected from a metal plate onto an aerial without having to tackle the problem of spherical aberration. The aerial is placed at the focal point.
A parabolic antenna radio telescope is the most powerful as this shape allows the maximum number of waves to focus on one common point. One drawback of a radio telescope is that it can interfere with other radio waves from mobile phones, televisions, satellites, microwave ovens, etc. This interference can be minimised by placing radio telescopes away from densely populated areas.
Figure 10. A radio telescope. Image 1: public domain. Image 2: Noodle snacks, CC BY-SA 3.0
A telescope can bring a distant object closer to the eye and magnify it.
The primary function of a telescope is to capture as much light as possible through the aperture, which makes the objects that are invisible to the naked eye visible.
A typical telescope has an objective lens and an eyepiece.
The magnifying power of a telescope is the ratio of the focal length of the objective lens to the focal length of the eyepiece.
The more focal length an objective lens has, the more magnifying power a telescope generates.
There are many types of telescopes, but the three basic ones are refracting telescopes, reflecting telescopes, and radio telescopes.
ImagesFigure 3: Magnifying glass: https://commons.wikimedia.org/wiki/File:Convex_lens_(magnifying_glass)_and_upside-down_image.jpg
Figure 5: https://commons.wikimedia.org/wiki/File:Refracting_telescope_of_the_Strasbourg_observatory_4.pngFigure 6: https://commons.wikimedia.org/wiki/File:Warning_Symbol_%E2%80%93_Chromatic_Aberration.svg
Figure 7: https://commons.wikimedia.org/wiki/File:Newtonian_telescope.svg https://commons.wikimedia.org/wiki/File:Newtonian_telescope_Sofia_Bulv_Vitosha_2012_PD_2.jpgFigure 8: https://commons.wikimedia.org/wiki/File:Diagram_Reflector_SchmidtCassegrain.svg https://commons.wikimedia.org/wiki/File:35-cm-Schmidt-Cassegrain-Teleskop_der_WHS.jpgFigure 9: https://commons.wikimedia.org/wiki/File:Monocular.png https://commons.wikimedia.org/wiki/File:Eiffel_Tower_Monocular_(48117050598).jpgFigure 10: https://commons.wikimedia.org/wiki/File:Mount_Pleasant_Radio_Telescope.jpg
A telescope is an optic device that can magnify distant objects to make them easier to see.
We can see distant objects with a telescope including stars, galaxies, planets, sun, moons, etc.
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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