Focus On Universe

Astronomical images not only look beautiful, they also provide a wealth of information. What's behind it and what distinguishes four prominent telescopes? An overview.

To the point

• One telescope, one purpose: The Hubble and James Webb space telescopes focus on detailed observations of individual objects, while the Euclid space telescope and the Vera C. Rubin Observatory in Chile capture large sections of the sky in the shortest possible time. The latter are therefore considered survey telescopes.
• Highly complex observatories: Modern telescopes are highly complex and benefit from sensitive camera systems in combination with sophisticated optics and telescopes. The Max Planck Society has been involved in the development of many observatories, some of which took decades to complete.
• The universe in three dimensions: A two-dimensional image from a telescope often contains different astronomical objects, such as galaxies, which are at different distances from Earth. This allows computer models of the three-dimensional universe to be compared with real observational data.
• The explosive universe: Speed is of the essence at the Vera C. Rubin Survey Telescope. By photographing the entire southern sky and all the objects it contains very frequently in succession, it also captures explosive and fast-moving events in the universe.

Since 1990, the Hubble Space Telescope has delivered magnificent snapshots of distant galaxies and colourful gas nebulae in our home galaxy, the Milky Way. More than 30 years later, two new space observatories were launched: James Webb and Euclid have been supplying astronomical data centres with images since 2021 and 2023, respectively - in many ways surpassing Hubble. The James Webb Space Telescope specialises in sharp, detailed images of individual objects, while Euclid is designed to capture as many galaxies and objects in the sky as possible - while maintaining high image quality that also reveals the shape of a large number of distant galaxies. Euclid's mission will also receive ground-based support starting in 2025. The Vera C. Rubin Observatory, a brand-new telescope in the Chilean Andes, aims to capture large numbers of galaxies in the southern sky with its 8-metre mirror as part of the Legacy Survey of Space and Time (LSST).The Max Planck Society has contributed components and cameras both to the James Webb Space Telescope and Euclid as well as software to the Vera C. Rubin Observatory. And it has already secured data rights for the latter observatory.

In just a few years, there has been a succession of "first lights", the moment when light falls on new cameras through new telescopes for the first time. Each time, these new telescopes have revealed something unseen before. Time for an overview: What distinguishes the Hubble, James Webb, and Euclid space telescopes, as well as the ground-based Vera Rubin Observatory? What are each telescope's specialities? Why are their images not only beautiful to look at, but also highly relevant for research? This article answers key questions about four modern telescopes that observe the universe in visible or infrared light. Many of the basic characteristics of these telescopes also apply to other observatories that explore the universe in a different light, such as X-ray or radio waves. The Max Planck Society is also deeply involved in such telescopes, including the eROSITA space telescope and various radio telescopes, most notably the 100-metre telescope in Effelsberg, Germany. Astronomers from the Max Planck Society also collaborate closely with the 30-metre radio telescope in the Spanish Sierra Nevada and Noema, a cluster of several 15-metre antennas in the French Alps, both operated by the Institut de Radioastronomie Millimétrique (IRAM).

Telescopes on the ground, telescopes in space

Which brings us to the question: Why are some telescopes on the ground while others are in space? A telescope on the ground sees stars and galaxies through the planet's atmosphere, which can affect image quality - if it sees anything at all, since the atmosphere only allows infrared light to pass through at certain wavelengths and blocks ultraviolet and X-ray light entirely. For optical observatories, clear skies are of course essential. This is why the mountains of Chile - with over 300 clear nights a year - are among the best places in the world to build telescopes.

The atmosphere is spread out in a relatively thin layer several that extends several tens to hundreds of kilometres above the Earth's surface. Many of us have noticed that stars appear to twinkle on warm summer nights - this is because we are viewing them through the air above us, which is moving rapidly due to thermal activity. At the Vera C. Rubin Observatory, this effect is mitigated because the mountains in Chile typically have gentle slopes, allowing winds from the Pacific to flow over the observatories with minimal turbulence. Other telescopes, such as the European Southern Observatory's Very Large Telescope, also in Chile, measure atmospheric turbulence and adjust the shape of their mirrors in real time to ensure the clearest possible image - as if there were no atmosphere. This technique, called adaptive optics, brings images close to the sharpness of those captured by space telescopes. By contrast, the Vera C. Rubin telescope is only equipped with active aptics, which adjusts for changes in the mirror surface when tilting the telescope, for example.

Despite its drawbacks, a ground-based location offers big advantages: large mirrors can be used and transported to the target site. Also, one can build large cameras of immense complexity. If the Vera C. Rubin Observatory including its massive camera, were to be launched into orbit around the Earth, it would require a rocket of unrealistic size and thrust - not to mention numerous additional challenges.

The Hubble Space Telescope with its 2.4-metre mirror could only be launched because the cargo bay of the Space Shuttle offered enough space. Even if NASA's shuttle fleet were still in operation, it is doubtful whether it could have carried the James Webb Space Telescope. Instead, James Webb had to fit into the tip of a rocket and unfold its 6.5-metre mirror in space - like a Transformer. If even one mechanism had failed, the telescope would have been unusable. The planning process was accordingly long and complex, and the advantages of such a large mirror in space came at a high cost. Whether in space or on the ground, each telescope has its own strengths.

These strengths are defined years - often decades - in advance in a list of requirements. "A telescope isn't something you can buy off the shelf", says Eckhard Sturm, astrophysicist at the Max Planck Institute for Extraterrestrial Physics. He is the project manager for a house-sized camera being developed for the Extremely Large Telescope, which is set to become the world's largest telescope. "We have to develop technology that doesn't exist yet." According to Sturm, the complexity of today's ground-based telescopes and their instruments now rivals that of space telescopes.

A deep look into space

There are two core characteristics that define every telescope: the diameter of its mirror, which guides light from space into its optics, and the field of view in the sky that the optics project onto the camera. And the larger the mirror and the longer the camera's exposure time, the fainter the light that the camera can still detect. Many photography enthusiasts are familiar with this effect. The polar lights that reached Europe in 2024 and 2025 were barely visible to the naked eye, but appeared colourful in photographs. Similarly, a photo of the Milky Way can be taken even near a city, as long as the camera is exposed long enough.

This is because the intensity of a star's light decreases with the square of the distance. This means that the larger the telescope mirror and the more sensitive the camera, the further away a star can still be imaged. If scientists want to observe distant galaxies beyond our Milky Way that are more than hundreds of millions of light years away, they benefit from the fact that galaxies shine as brightly as all their hundreds of billions of stars combined.

Hubble and James Webb are two space telescopes designed to examine small sections of the sky in great detail, including distant galaxies. The same goes for ground-based telescopes like the Very Large Telescope of the European Southern Observatory in Chile. While binoculars can easily capture the entire full moon, Hubble's field of view only covers a single lunar crater - but an incredible resolution of around 80 metres. All in all, Hubble therefore took a total of eleven days to scan a very small quadrant of the sky over and over again: the Hubble Ultra Deep Field. It contains light from very distant galaxies, around 10,000 in number. The edge length of this image corresponds to only one tenth of the diameter of the full moon. If Hubble had photographed the entire sky with this accuracy, it would have taken half a million years.

At a glance

The Euclid space telescope combines the capabilities of an observatory such as Hubble and a survey telescope. It was not built just to look deep into space, with a field of view 240 times larger than Hubble, it is expected to survey a third of the entire sky for galaxies, including distant ones, in just six years. "Euclid simply has the better ratio of sky area to depth", says Frank Grupp, Euclid project manager in Germany. How is that possible? After all, its mirror surface, which means its ability to collect light, is only a quarter of that of Hubble. "However, Euclid's cameras have improved since Hubble", says Grupp. Euclid's goal is to study dark matter, which is invisible but influences the movement of galaxies solely through its gravity. To do this, Euclid will observe as many galaxies as possible in as many directions as possible. After a bit more than a year of operation, researchers have already discovered 26 million galaxies.

The alternative: a larger mirror. The 8.5-metre mirror of the Vera C. Rubin Observatory collects more light per unit time than the smaller mirror of the Euclid telescope in space. This is the only way astronomers can collect enough light in a section of the sky in a comparatively short amount of time, so they can quickly move on to the next section. The goal is to scan large areas of space. Eduardo Bañados from the Max Planck Institute for Astronomy calls this "cosmic cinema". Astronomers will take photos of every region of the southern sky in quick succession to capture the explosive and fast-moving parts of the universe. The huge amount of the data collected will also make distant galaxies visible. "The Vera Rubin Telescope will be transformational for my research in cosmology", says Esra Bulbul, astronomer at the Max Planck Institute for Extraterrestrial Physics. "With its wide field of view and depth, it will observe billions of galaxies across vast distances, allowing me to study the large-scale structure of the Universe and its evolution over time."

Eduardo Bañados is also on the lookout for galaxies that formed shortly after the Big Bang and - due to the universe's expansion - are now extremely far away from Earth. Thanks to the telescope's high frame rate, he is particularly interested in detecting "flickering" in the centres of distant galaxies. This is because a supermassive black hole at the centre of such a galaxy consumes gas equivalent to the mass of our Sun every year. The result is a series of extremely powerful and rapid bursts of brightness, occurring within a few days.

Where does the depth in the image come from?

An image is more than just a picture; it contains a wealth of information. On one hand, there is the shape of the galaxies. On the other, their distances. It turns out that the individual stars in the image are part of our Milky Way, while the galaxies are significantly further away. Each image captured by a surveying observatory like Euclid or Vera C. Rubin therefore contains information about a three-dimensional universe that can be directly compared with computer models created by cosmologists to describe how the universe has evolved since the Big Bang.

There are various methods to determine distances in astronomical images. If, for example, the luminosity of a star - its actual brightness - is known, astronomers can calculate how far away it is based on how much light reaches the camera. For galaxies at cosmological distances, astronomers take advantage of the fact that the universe is expanding. Cosmologists can calculate how fast two points in the observable universe us would have to move away from each other if they were at a certain distance from each other. Thanks to observatories such as Hubble and James Webb and the tools of spectroscopy, it is also possible to measure how fast an imaged galaxy and our location are actually moving away from each other. This reveals the galaxy's distance.

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