The beast is coming. It’s big, it’s powerful, and it changes everything. If that sounds like a slogan for Godzilla movie, then you mean a proper telescope, but I’m talking about a telescope…a very large telescope. It’s actually an Extremely Large Telescope (ELT) and only a few years away from the competition. If you’re excited about the images and results from the James Webb Space Telescope, you need to know about the ELT.
The first thing to understand about binoculars is that size is everything. Many people think that the purpose of a telescope is to magnify distant objects, but the first job is actually to gather light.
The further the light source is from us, the fainter it appears and its brightness decreases with the square of the distance. For example, if you move a light bulb ten times away, it will appear 100 times dimmer. To help us see distant light sources, telescopes act as a kind of “light bucket”. They collect as many photons (particles of light) as possible from distant objects to give us the all-important “signal” that can stand out against the background noise.
So why does size matter? The light-gathering power of a telescope is the square of its collecting surface. If we use mirrors – as all modern telescopes do – then a telescope built from a 10 meter mirror will collect 100 times more light than a one meter instrument. Astronomers therefore want a telescope with the largest possible mirror.
What is the largest possible mirror? That is the question that ELT will answer. For the last 30 years or so, large research-grade telescopes have had mirrors within 10 meters (that’s roughly 33 feet for those of us stuck in US units). These include two 10-meter Keck instruments atop Mauna Kea in Hawaii. Building a movable frame to hold a 10-meter mirror and then building an observatory dome around the whole thing was an amazing engineering feat. However, the ELT takes “awesome engineering” to a whole new level.
The mirrors on the ELT will be almost 40 meters in diameter, but they must be erected first. Then they have to be supported by a movable frame the size of a football field. Of course, a telescope is not just a primary mirror. Above it will also be a secondary mirror that collects the light reflecting off the giant primary before redirecting it down to the various instruments. And the entire structure has to fit inside the observatory’s dome to protect it from the elements – including wind, which could shake the whole thing into nothingness. Unsurprisingly, the ELT dome will be the largest ever built, measuring 74 meters high and 86 meters wide. Total weight of telescope and dome? 9000 tons!
Oh, and did I mention that the whole thing is being built in one of the most inhospitable environments on Earth? The ELT is designed to work in both optical and near-infrared wavelengths of light. Getting telescopes high above the atmosphere is always the #1 task for site planning, but since infrared light is absorbed by water vapor, you want the site to be high. and dry. That’s why this extreme engineering project is being carried out on Cerro Armazones, a mountain in Chile’s Atacama Desert, one of the driest places in the world. When you consider what is being built and where it is being built, the ELT is nothing short of an engineering marvel.
So, what comes out of all this? The answer is simple: a game-changing astronomical instrument. The ELT will have a light-gathering capability that is 100 million times better than the human eye and more than ten times better than the best telescopes available today. This capacity means that questions that are currently out of reach will get answers, or at least much better answers than we can get now. In my own field of astrobiology, the ELT will be able to directly detect light from Earth-like exoplanets. To achieve this, the telescope must be able to separate the light from the planet and its host star, a feat requiring a sensitivity of one part in a billion at distances on the sky of 0.1 arcsecond. By comparison, there are 180 degrees of arc from horizon to horizon and 3,600 seconds of arc in one degree. So 0.1 arcsecond is a small distance between the sky. With this kind of power, we will be able to characterize exoplanets and their properties – including perhaps signs of life – with unprecedented precision.
Planets are just one limit to which the ELT will push. Galaxies, cosmology and star formation will also get new answers. There is so much that will be possible with the ELT, which is why I am so excited to see the beast build on a mountain in the Atacama desert. First light is due around 2028, so this is a story to watch out for.