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 more distant objects that seem weaker from Earth.
This is the reason why we need to build ever larger 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 construction in Chile’s Atacama desert in 2017 and will be completed around 2024, will become the largest in the world.
Without the help of a telescope, with the naked eye, the farthest object in the Solar System 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 is 8.6 light years away or the furthest bright star that 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 weaker.
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 time. Depending on how far away the object we are looking at is, its light takes a certain time 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 at the same time we see images of different moments in the past.
And in addition, we must also consider that with larger telescopes we are not only able to see objects further away, but we also achieve 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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