When it comes to telescopes, size does matter. To realize this, it is enough to give a brief review of the evolution of the telescope since the Dutchman Hans Lippershey patented his telescope ” to see distant things as if they were close ” in 1608 and since Galileo Galilei built an improved version of Lippershey’s invention the following year, with which he discovered the four main moons of Jupiter and observed the surface of the Moon in detail. Galileo’s telescope would be considered rather a large spyglass by our standards and was only a few centimeters in diameter.
A century and a half later, William Herschel would build a telescope 12 meters long and 1.2 meters in diameter , with which he would discover, just a few days after completing it, Saturn’s moons Mimas and Enceladus, moons barely 400 and 500 kilometers in diameter respectively (about 10 times smaller in diameter than the Galilean moons of Jupiter and at a greater distance). Around 1845 Lord Rosse built a 1.8 meter diameter telescope in Ireland which was the largest in the world until 1917, when the 2.5 meter diameter Hooker telescope was built in California. All these telescopes were used to observe visible light.
At the end of the first half of the 20th century we also began to build radio telescopes , which could detect wavelengths longer than the visible one and soon surpassed these telescopes in size. In 1963 the Arecibo radio telescope was built, with a dish with a diameter of 300 meters . This radio telescope, the largest in the world by a good margin, ceased operations in December 2020. In 2007, the largest optical telescope in the world, the Gran Telescopio de Canarias, with an effective diameter of 10.4 meters , was inaugurated. An even larger telescope is currently being built, which should come into operation in the next few years, called Extremely Large Telescope and located in Chile , with a diameter of 39.3 meters . This instrument is expected to be able to produce images with 16 times the resolution of the Hubble Space Telescope .
All these advances are incredible and are taking astronomy and the study of the universe to new places, but inevitably they have a limit. Because the obsession with building ever larger telescopes is justified . A telescope, regardless of the quality of the sky it has and how extremely well polished it is, will have a physical limit on its resolution, given by the Rayleigh criterion . This basically tells us that the limit will be given by the wavelength of the light observed and the diameter of the main mirror. The larger the diameter and the shorter the wavelength (ie, more energetic light, pulling toward the ultraviolet), the higher the resolution . But we can’t keep increasing the size of our telescopes indefinitely. Or if?
Well yes, we can, but not in the way we have been trying historically. If we understand the mirror of a telescope as a surface to reflect and collect the light of a distant star to bring it to the detector, we can understand that we really do not need the entire disk to achieve this. In fact, the largest telescopes and even the James Webb Space Telescope are made up of many small mirrors joined together to form one large one.
Imagine then that we build a huge telescope, the size of the Earth, but instead of completing its entire diameter with small mirrors, we only put them in some strategic places. This is what the Event Horizon Telescope did, which managed to produce the first image of a black hole , published in April 2019. With telescopes located in places as remote as Hawaii, the South Pole, Spain or Chile , they achieved as much resolution as to observe a tennis ball on the surface of the Moon . Instead, they turned their telescopes to more interesting places, like the supermassive black hole at the center of our galaxy, known as Sagittarius A* , or at the center of the galaxy M87 .
This technique, known as Very Long Baseline Interferometry , has the advantage of achieving resolutions impossible with conventional telescopes , but the disadvantage that the amount of light captured is much less than that which would correspond to a full telescope. This makes sense, if we are only collecting the light on, say, 1% of the effective surface area of the telescope, we will see the image 100 times darker. For this reason, it has been used to date to observe especially luminous objects , such as nearby stars or the nucleus of some galaxies.
But again, if we use only telescopes on the ground, we will find on our planet a limit to the size of these networks of telescopes. For this reason, work has been carried out on the possibility of transferring this technique to space . We currently have the SVLBI network of telescopes , which can be combined to form an effective telescope two and a half times the size of the Earth , but in the future it is hoped that these distances can be extended to millions of kilometres. If at the same time we manage to improve the collection of the little light that these telescopes would capture, we could be able to observe distant corners of the universe in detail that is currently unimaginable .
REFERENCES:
Schuh et al, 2012, VLBI: A fascinating technique for geodesy and astrometry, Journal of Geodynamics. 61: 68–80, doi:10.1016/j.jog.2012.07.007
The EHT Collaboration, 2019, First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole, The Astrophysical Journal Letters. 875 (1), doi:10.3847/2041-8213/ab0ec7
