Telescopes have been the most significant observational tool in astronomy since its invention by the Dutchman Hans Lippershey in 1608. It played a significant role in Galileo’s observation of the phases of Venus, Jupiter’s moons and craters on the Moon. Nowadays, we have the Hubble Space Telescope, and later in 2021 we will send the James Webb Telescope. They allow us to see the deepest corners of the universe. But how do they work?
The basic working principle of a telescope is magnifying an image, thus making it larger than it would normally be to the eye. The most basic type of telescope is the refractor, invented in 1608. It is made out of three main components, a long tube, a convex objective lens and a concave eyepiece. Light falls into the objective lens, which focuses it down on the back of the tube. The eyepiece magnifies the image and brings it to the eye.
Nonetheless, a better way telescopes use is implementing mirrors to magnify images. These are called reflectors and were created by Newton around 1680. They use a large curved concave mirrors to capture the light and reflect it to a smaller mirror, which then reflects the light into the observer’s eye. The first mirror flips the image, but this is fixed as the second mirror flips it into the correct orientation again. Large mirrors that do not distort images are easier to manufacture, therefore this is the way that many telescopes operate.
In the 20th century, there were massive breakthroughs in telescope technology. One example are radio telescopes developed in the 1930s and infrared telescopes developed in the 60s. As you can tell by the name, we do not have to observe the universe using only the visible parts of the electromagnetic spectrum. In fact, many of the distant objects are only observable using infrared light.
Radio telescopes are immense antennas and radio receivers. They have to be so huge, as signals from far away objects are very faint. This is why they are also located in remote and often stunning areas far away from major cities, to avoid radio interference. The biggest radio telescope is located in Guinzhou province in China and the dish diameter spans 500m.
The second prominent type of telescope is the infrared telescope. All objects in the universe with a temperature above absolute zero emit radiation and an infrared telescope can detect radiation that is too faint to be seen with visible light. This is a breakthrough in the search for planets and brown dwarfs. Furthermore, due to the longer wavelength of infrared radiation, it is less impacted by scattering in clouds of intergalactic dust, which allows us to see objects like the center of our galaxy. These telescopes need to be situated at high altitude, as Earth’s atmosphere absorbs a lot of infrared radiation. Therefore, they are often put on mountains, in planes and most effectively in space. The successor of the Hubble as NASA’s flagship observational project, the James Webb Space Telescope is an example of an infrared telescope.
With all types of telescopes, there are similarities in terms of what affects the quality of the image. For visible light telescopes, the most obvious limitation is a cloudy sky. Overall, the atmosphere creates all types of distortions, therefore it is crucial to have telescope up in space for the greatest image quality. Therefore, we should be very excited for the James Webb telescope replacing the now 30 year old Hubble. The name isn’t as catchy, but it might significantly change the paradigm in observational astrophysics.
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