Blog Report on the Seminar ‘Science with the World’s Largest Telescopes’ by Dr Katharine Johnston
An ongoing mission of our species has always been to find the best way to accurately observe and explain our surrounding universe, but it can be difficult to observe things that lie so far beyond our own atmosphere in adequate detail. Over time we have developed increasingly better telescopes and observation techniques to help us view these far-away systems in the greatest detail we can achieve. As it turns out, the larger the telescope, the better the resolution (how much detail we can observe). This was discussed in the seminar ‘Science with the world’s largest telescopes’ delivered by Dr Katharine Johnston on 10th December 2025.
The reason behind why larger telescopes produce finer detailed images is due to a few different factors. First of all, the larger the telescope, the larger the surface area and so the more photons (light) it can collect, allowing it to produce a better image. It also has something to do with the size of its mirrors [1]. Also known as resolving power, the larger your mirrors, the greater your resolution. When light passes through the opening of a telescope, it will spread out, or diffract. The smaller the opening, the more it diffracts, and so the more blurred the image will be. The wavelength of the light observed also affects your image, as the longer the wavelength, the more it will diffract. A general rule is that the longer the wavelength of radiation, the larger the telescope required to view it. There are different types of telescope depending on the wavelength of light you want to collect, the main ones being optical and radio telescopes. As the names suggest, the difference between these two types is that one collects visible light data, and the other collects radio. The wavelength of visible light is significantly smaller (about 1000x) than that of radio waves, meaning that to capture images of the same resolution using a radio telescope, it would need to be around 1000x larger than an optical one.
Recent technology allows us to construct telescopes of this size, allowing for accurate observations of both optical and radio wavelengths. But why is it important to observe in different wavelengths in the first place? Why can’t we just stick to optical telescopes, which do not require to be as large to observe high resolution images? There are actually many advantages to being able to observe far-away objects at different wavelengths. First of all, interstellar dust - conglomerations of largely silicon and carbon based molecules that are generally less than a micron in size - can block out visible light in telescope observations, concealing parts of the image. Radio telescopes, however, can look past the interstellar dust due to the longer wavelengths of radio waves, allowing us to view parts of the image that would otherwise be concealed. Observing at longer wavelengths also allows you to apply Wein’s Law, which describes the relationship between the temperature of a body and the wavelength at which it emits the most light [2]. In general, the hotter the object, the shorter the wavelength. This means that bluer stars tend to be hotter, and redder stars tend to be cooler. By using this idea, we can work out the physical dynamics of an observed system. By looking at the Doppler Shift, which ‘describes the changes in frequency of any kind of sound or light wave produced by a moving source with respect to an observer.’ [3], we can look for a redshift (moving away from us), or blueshift (moving towards us). Often, when observing a planetary disc, one side of the disc will be red-shifted, and the other blue-shifted, and from this we can determine in which direction it is spinning.
One such telescope is known as the Atacama Large Millimeter/submillimeter Array, or ALMA. This interferometer is located in the Atacama desert in Chile and consists of 66 separate antennas, or dishes, combined to form one giant telescope. The further apart these apertures are situated from one another, the bigger the telescope and so the higher the resolution. The distance between any two apertures is known as the baseline distance. Dr Johnston has conducted observations using this particular telescope to observe the star AFGL 4176, which is 100,000x more luminous than our Sun and is situated around 14,000 light years away (where one light year is the distance light can travel in a year). Observations were around 1mm in wavelength, with a baseline of around 1km. Dr Johnston observed thermal emissions from the dust in the system, seeing dust emission from the surrounding disc. There was also observed to be emission from the molecule CH3CN [4]. Comparing observations to those of a low-mass protostar shows that there is evidence of a rotating disc around a low-mass forming star. Dr Johnston states that they wished to investigate further into whether disc formation of higher-mass stars shares similarities with that of low-mass stars. Further observations at a higher resolution, with a baseline of around 12km, suggested evidence of spiral-arm structures within the disc. This is an interesting feature as it can give us an insight as to how the star system was formed and the nature of its physical dynamics.
One of the largest interferometers on the planet is known as the Event Horizon Telescope (EHT). It is so large that its baseline is almost equal to the diameter of the Earth, with a maximum baseline of 10,700km. The EHT combines telescope facilities from all over the globe, including the ALMA. Observing at a wavelength of less than 1mm, it can achieve a resolution of around 19 microarcseconds - the equivalent of being able to resolve a tennis ball on the moon - the highest resolution ever achieved! In 2017, the EHT was used to observe the supermassive black hole at the centre of the galaxy M87 - a giant elliptical galaxy that is many light years away.
Fig. 1: Images of the black hole M87* taken by the Event Horizon Telescope in 2017 and 2018 [5]
These observations allowed us to resolve close to the radius of the event horizon, also known as the Schwarzschild radius, which is the radius that relates to the escape velocity of light from the black hole. Scientists were also able to observe lens rings around the black hole, and the emission from material orbiting the black hole, known as Synchrotron emission, which is caused by a relativistic acceleration of electrons in a magnetic field. These observations allowed us to learn more about black holes - including our own, at the centre of our galaxy, which, more recently, we have also been able to observe. Also known as Sagittarius A* (Srg A*), the supermassive black hole at the centre of our galaxy was also found to have lens rings - leading scientists to believe it to be a common feature of black holes in general. The observations of these black holes lined up well with predicted models, which also showed gravitational lensing from synchrotron emission. Models also showed that the black hole could be spinning clockwise, as viewed from Earth, which again lined up with the EHT observations.
But that is not the end of the story. As telescopes continue to achieve more detailed images at higher resolutions, revealing more information about far away celestial structures, we still have plenty left to observe within our own galaxy and beyond.
[1] Koberlein, B. (2020). How Interferometry Works, and Why it’s so Powerful for Astronomy. [online] Universe Today. Available at: https://www.universetoday.com/articles/how-interferometry-works-and-why-its-so-powerful-for-astronomy.
[2] Fritzsche, H. (2019). Wien’s law | physics. In: Encyclopaedia Britannica. [online] Available at: https://www.britannica.com/science/Wiens-law.
[3] Bettex, M. (2010). Explained: the Doppler Effect. [online] MIT News. Available at: https://news.mit.edu/2010/explained-doppler-0803.
[4] Johnston, K.G., Robitaille, T.P., Henrik Beuther, Lines, H., Boley, P., Kuiper, R., Keto, E., Hoare, M.G. and Roy van Boekel (2015). A KEPLERIAN-LIKE DISK AROUND THE FORMING O-TYPE STAR AFGL 4176. The Astrophysical Journal Letters, 813(1), pp.L19-L19. doi:https://doi.org/10.1088/2041-8205/813/1/l19.
[5] eventhorizontelescope.org. (2024). M87* One Year Later: Proof of a Persistent Black Hole Shadow. [online] Available at: https://eventhorizontelescope.org/M87-one-year-later-proof-of-a-persistent-black-hole-shadow.
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