The capacity of a telescope depends on its size. When we talk about the size of a telescope, we refer to the diameter of what we call the primary mirror, which is the light collector, in the case of a reflecting telescope and lens in that of a refracting telescope. The larger the diameter of the mirror, the greater the distance we can reach because we will be able to collect more light and therefore observe objects farther away that seem weaker from the Earth
This is the reason why we need to build increasingly large telescopes, such as the ELT (Extremely Large Telescope or Extremely Large Telescope in Spanish) whose primary mirror has a diameter of 39 meters. The ELT, which began to be built in the Atacama desert, in Chile, in 2017 and will be finished around 2024, will become the largest in the world
Without the help of a telescope, with the naked eye, the most distant object in the System Solar that we can observe is Saturn, which is about 8.5 times the distance from the Earth to the Sun (this distance is 150 million kilometers). Outside the Solar System we can observe objects such as Sirius, which is the brightest star and which is 8.6 light years away or the furthest bright star which is Deneb, in the constellation of the Swan, which is 1,500 light years away. If we put those distances into kilometers, Sirius is 81 trillion kilometers and Deneb is about 14,000 trillion kilometers. The farthest object that we can observe with the naked eye is the Andromeda galaxy, which is 2.6 million light years from Earth.
Without the help of a telescope, with the naked eye, the farthest object in the Solar System that we can observe is Saturn
To give you an idea of what it means to use a telescope, a small one, about 20 centimeters in diameter, allows us to observe objects more than 2 billion light years away. And with the ELT we will be able to capture a hundred million times the light that our eye captures. That means that it will be able to detect objects whose light is millions of millions of times fainter.
Since looking far into space is simultaneously looking back in time, the ELT will allow us to get closer to the origins of the universe. This telescope will allow us to observe the first galaxies that formed in the cosmos, some 500 million years after the Big Bang.
And it is important not to forget that when we talk about the scope of a telescope we not only measure in distance, we also measure in weather. Depending on how far away the object we are looking at is, it takes a certain time for its light to reach us. For example, it takes 8 minutes for light from the Sun to travel from its surface to Earth. This means that we are seeing the Sun as it was 8 minutes ago and not as it is when we see it. The light from Sirius takes almost 9 years to be detected and that from Andromeda about 2.6 million years. For this reason, when we look at the sky, we do not see the cosmos as it is right now, but we see images of different moments in the past at the same time.
And furthermore, we must also consider that with larger telescopes we are not only capable of seeing more objects far away, yes but we reach higher resolution. That is, what we observe we see in more detail. If with the naked eye or with a small telescope we see some light, but we are not able to observe any structure, the larger the diameter of the telescope, the more details we are able to resolve
Ariadna Calcines Rosario is an expert in astrophysical instrumentation and works as a Senior Optical Engineer at the Center for Advanced Instrumentation at Durham University, UK.
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