“Science With the World’s Largest Telescopes” Seminar from Dr Katharine Johnston 05/11/2025
News articles occasionally focus on the newest, largest telescope. But why do we have big telescopes, and what are they actually used for? Why do they have to be so big? Dr Katharine Johnston explores the world’s largest telescopes in this seminar…. Light can be thought of as a particle (called a photon) with that photon travelling up and down in a wave motion, like a boat bobbing up and down on water. The more energy the light/photon has, the faster the frequency of this bobbing is. As such the length the photon travels with this higher energy/frequency, is shorter. This length is called the wavelength. So high energy photons have a higher frequency and so a smaller wavelength. This frequency is directly responsible for colour as well. Red light has less energy than blue light, so it has a lower frequency, which our eyes can see. There are many frequencies that our eye cannot see as well; frequencies lower than red become invisible to us, called infrared, and frequencies higher than violet become ultraviolet also invisible. The whole spectrum of light from lowest to highest frequency goes:
Radio -> Microwave -> Infrared -> Visible -> Ultraviolet -> X Ray -> Gamma Ray
So radio waves have the largest wavelength, but lowest frequency/energy and gamma rays have the highest frequency/energy but smallest wavelengths.
Telescopes follow a simple equation, which often governs their size. This equation states that the resolution (how sharp the image is) is proportional to the size wavelengths of the light that you’re trying to see. Meaning a telescope for radio waves (large wavelength) will have to be much bigger than a telescope for gamma radiation (small wavelength) in order to have the same amount of detail in their images. This equation also says that the resolution is dependant on the inverse of the size of the ‘light collection zone’ in the telescope, meaning if you are only collecting a little bit of light in a small area you would have worse resolution then if you collected loads of light in a huge area. This simple equations basically says that to get the best/smallest resolution possible (for any kind of light) you must have a really, really big telescope.
Some telescopes even combine multiple light collecting mirrors together for higher resolution, like the VLT (Very Large Telescope) which consists of four circular mirrors which are each 8.2 meters wide! As the VLT is an optical telescope its amazing to think of the scale of a perfect mirrored circle that is so large! Now dwarfing that we can talk about radio telescopes, which due to their much higher wavelength, need to be considerably bigger, reaching hundreds of meters in diameter. The FAST (Five-hundred-meter Aperture Spherical Telescope) is half a kilometre in diameter, which is just mind-boggling to think about!
Why do we look at different wavelengths of light, like radio waves then? The answer comes in the form of everyone’s favourite unused Christmas present in the corner of your room. DUST. As radio waves are so physically big, dust particles cannot absorb/block them, and so radio waves seem to pass straight through them, making the dust seemingly invisible. This is great as many areas of our universe are clouded in interstellar dust, which we can see through by using radio waves! We also know that space stuff often emits light at various wavelengths, which we can use telescopes to distinguish and learn more about those objects.
Dr Johnston focuses a lot of her work on the ALMA (Atacama Large Millimetre/submillimetre Array) telescope, which are a huge 66 antennae/dishes that work together to make one very powerful radio telescope. Her work looked into the galaxy’s centre at a light source that is 100,000x brighter than our sun! This light source is a star that hasn’t fully formed yet (a proto-star). With the ALMA she could spot a huge disk forming around it, along with the speed at which that disk is rotating. This give us an idea about the mass of this proto-star as well, at around 25x the mass of our sun, as well as many other interesting properties.
Many of the best telescopes in the world are arrays of antennae like the ALMA. These are often a special kind of telescope called ‘interferometers’, which have a property where the larger the longest distance between 2 of its antennae is, the better images it can get. This distance between antennae is called the ‘baseline’, with the longest distance being called the ‘max baseline’. Dr Johnston highlights the VLA (Very Large Array) with a max baseline of 36km, the SKA (Square Kilometre Array) at 150km and the e-MERLIN (Multi-Element Radio Linked Interferometer Network) in the UK at a massive 217km maximum baseline!
‘Very Long Baseline Interferometry’ (VBLI) is a dedicated field where the baseline gets absolutely massive. These are cutting edge with some even spanning the whole globe! Dr Johnston shows the VBLA (Very Long Baseline Array) at a max baseline of 8,611km; there’s also the e-VLBI Network (European VBLI) at a staggering 10,000km. One of the most famous and largest interferometers is the EHT (Event Horizon Telescope) at a staggering 10,700km, which spans much of the globe! The EHT combines many telescope facilities (including ALMA) to create the finest resolution images we have ever made with an often-quoted figure being ‘like seeing a bottle cap on the Moon’ [1]. In 2019 the EHT took the famous first ever image of a black hole, being M87*, and later releasing an image of Sagittarius A*, being the supermassive black hole at the centre of our galaxy!… These VBLIs are revolutionary, bringing our world closer to the future with unprecedented precision. I’ll end this blog post with a quote from Event Horizon Telescope Collaboration et al. (2019, pp. 9) stating “We have shown that direct studies of the event horizon shadow of supermassive black hole candidates are now possible , thus transforming this elusive boundary from a mathematical concept to a physical entity that can be studied and tested via repeated astronomical observations” [3].
References :
[1] https://eventhorizontelescope.org/blog/eht-makes-highest-resolution-black-hole-detections-earth
[2] https://science.nasa.gov/resource/first-image-of-a-black-hole/
[3] Event Horizon Telescope Collaboration, Akiyama, K., Alberdi, A., Alef, W., et al. (2019) ‘First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole’, The Astrophysical Journal Letters, 875(1), L1. doi: 10.3847/2041-8213/ab0ec7.
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Good start featuring the title of the seminar, the date and the name of the person who delivered it.
Good intro with relevant questions to readers.
Excellent descriptions of scientific processes to a lay audience
Watch your references as the first two are only links and double check the length of the report as it could be a bit long
Overall, a great report.
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This is a good piece of writing, it is understandable to a lay person and you have a good use of references. You have ticked lots of boxes on the mark scheme, well done!