Since at least the early 17th century, humans have been using optical telescopes (telescopes collecting visible light) to look into space. These early telescopes were pioneered by the likes of Galileo (amongst others), but there is still plenty of science to be done. Today much larger telescopes take their place such as the Very Large Telescope (VLT) in Chile, or the Large Binocular Telescope (LBT) in Arizona, in the USA. The need for large telescopes was the subject of Dr Katharine Johnston’s lecture “Science with the world’s largest telescopes” on the 10th of December 2025, on which this blog was based.
The need for larger telescopes is motivated by the desire to study objects both in greater detail and further away. The larger the diameter of the telescope, the higher the resolution they can capture. One issue with optical telescopes, however, is dust. In the same way haze can reduce the visibility through the earth’s atmosphere, clouds of dust in space can obscure stars, planets, or any other structure behind. Light will only reflect off an object if it is large enough compared to the light’s wavelength so, to overcome the issue of small dust particles, radio telescopes look at light in the radio spectrum. Radio telescopes typically use frequencies of light with wavelengths of centimetres or metres (in contrast to optical telescopes which collect light with wavelengths of hundredths to millionths of a millimetre). This means that those small particles are invisible to the light used by these telescopes [1]. As the frequencies used are so much lower, the antennas of radio telescope require a much larger diameter to create an image of a similar resolution. The largest of these is the Five-hundred-metre Aperture Spherical Telescope (FAST) in China which has a diameter of (you guessed it) 500m. As well as letting scientists look further into space, radio telescopes also allow the structure of nebulas (gas clouds) to be seen. The very dense parts that block even radio waves can be identified alongside other things, such as young stars, that were previously obscured.
There are difficulties with building increasingly large telescopes though. The material and manpower cost of construction, as well as any operational and maintenance costs, increase incredibly fast [2]. If you also want the option of being able to aim the telescope at a particular point in space, there comes a point at which it is just no longer practical to construct an antenna of the desired size. To overcome this, the area of interferometry was created. An interferometer will compare the received signals of two or more antennas in real time to produce a resolution comparable to a single antenna with a much larger diameter. The ALMA interferometer in Chile consists of a total of 66 individual antennas that can be relocated to create a distance between antennas of up to 16km. This can be combined with the rotation of the earth to produce incredibly large diameters using only moderately sized ones (ALMA uses antennas with a maximum of 12m themselves). With the increasing speed of data transmission, it has even become possible to use antenna arrays in multiple countries simultaneously as a single, incredibly large, interferometer. This is called Very Large Baseline Interferometry (VLBI). One of these interferometers was used to capture the first image of a black hole, which you may remember being published in 2019. [3]
Large telescopes have been, and will continue to be, indispensable in a variety of areas of research including dark energy / matter, black holes (as mentioned), and the continued search for life. We do, however, likely find ourselves at a time in which the largest individual telescopes that will ever be built either already exist today or will come online within the next few years. The torch is now being passed to VLBI with expanding global networks of antennas, a nice reminder of the possibility of global cooperation within scientific research.
[1] IRAM, “Understanding radio-telescopes: What is radio astronomy at millimeter wavelengths?” Accessed: 2025. [Online]. Available: https://iram-institute.org/observatories/understanding-radio-telescopes/
[2] P. Plait, “Astronomy Is Facing an End of the Era of Monster Telescopes.” in Scientific American. Nov. 23, 2023. [Online]. Available: https://www.scientificamerican.com/article/astronomy-is-facing-an-end-of-the-era-of-monster-telescopes/
[3] NASA, “First Image of a Black Hole.” Accessed: 2025. [Online]. Available: https://science.nasa.gov/resource/first-image-of-a-black-hole/
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Excellent overall presentation with good grammar throughout and title, date and lecturer name included.
Accurate reporting of the seminar with definite take home message covered.
Accurate contextualisation as it touches upon difficulties in building them and how the different large telescopes link.
Extra sources have been used but could add a direct quote to specifically assess findings from other sources.
Writing style appropriate for a lay audience and easy to understand.
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Great introduction with lots of clear detail. Are there any references needed for the first paragraph?
Good description and explanation of wavelengths.
Overall, very clear and well written. Just ensure all parts are referenced clearly and correctly.