10 Most Influential Optical Telescopes in Astronomy

Galileo's Telescope
Galileo's first refracting telescope (1609), which has an aperture of 1.5 cm, and is currently housed at the Museo Galileo in Florence, Italy

With each successive generation of telescopes, more about the wonders and complexities of the Universe are revealed than could ever have been imagined. It is therefore natural to ask who invented the telescope? Nevertheless, the exact origin of such a revolutionary instrument as the optical telescope isn’t certain. A greater exploration of this fascinating topic can be found here in a piece entitled ‘Ancient Telescopes And Lenses‘.

What is known with certainty, however, is that while Galileo may not have invented the telescope, his use of this powerful instrument set astronomers on a course of discovery that is unmatched by any other scientific endeavor. The list that follows subsequently highlights some of the telescopes that have proved the most influential in the development and advancement of observational astronomy throughout history.

Galileo’s Refracting Telescope

Galileo (1564-1642) was almost certainly not the first person to point a “spy-glass” at the sky. While he may not have invented the telescope, Galileo was, however, the first observer to draw scientific conclusions from what the glass revealed about the Universe.

The first Galilean refracting telescope (lenses) could only magnify objects three times, but within a year of its construction, Galileo had built an instrument that magnified objects twenty times. This was then used to discover the four satellites of Jupiter that are now known as the Galilean moons. This instrument also revealed the phases of Venus, the rings of Saturn, sunspots, and a supernova. More importantly, though, this humble little telescope proved that Copernicus was right- the Earth and planets were indeed revolving around the Sun, although Galileo didn’t agree with Kepler that the planets’ orbits were elliptical, believing them to be circular, instead.

Newton’s Reflecting Telescope

Newton’s reflecting telescopeAlthough Isaac Newton (1642-1727) is almost universally regarded as the greatest mathematician and astronomer of all time, he never used the telescope he designed and built to make scientific discoveries, beyond improving on the design of Galileo’s telscope.

Galileo used two lenses in his telescope to refract light into a focused image. Newton, however, realized that much of the light was scattered and/or lost with this system, so he came up with the idea of reflecting light off a series of mirrors to produce an image. Because no light passed through the mirrors, all of the light gathered by the primary mirror could then be used to produce an image in the eyepiece, and thus was born the reflecting telescope.

Moreover, while the problem with chromatic aberration in the refracting telescopes of Galileo’s time has long been resolved, the mirrors in reflecting telescopes can be made to collect thousands of times more light than even the biggest refractors can. In practice, this means that large reflectors can “see” objects that remain invisible to refractors, which is without a doubt the greatest contribution Newton has made to the science of astronomy.

Newton’s reflecting telescope has survived to this day in its original condition, and is now in the care of the Royal Society of London, who sometimes allows it to be displayed as a part of travelling exhibitions.

William Herschel’s 40-foot Reflecting Telescope

William Herschel's 40-foot Reflecting TelescopeShown opposite is an engraving of William Herschel’s mammoth 40-foot reflecting telescope, which was built between 1785 and 1789 at Slough in England. The telescope had a 48-inch diameter primary mirror, and a focal length of 40 feet (12 m), hence the name the Great Forty-Foot, with the first astronomical object he used it to observe with it being the Orion nebula.

This telescope was the world’s largest instrument for 50 years, and although Herschel also used a variety of other, smaller telescopes during his life, there is some evidence to suggest that this particular instrument was used to discover the sixth and seventh moons of Saturn, called Enceladus, and Mimas.

It was mostly dismantled in 1840, and today only a small section of the tube and the original mirror survive.

The Leviathan of Parsonstown

The Leviathan of Parsonstown

The images opposite show the 3rd Earl of Rosses’ (William Parsons) 6-foot-aperture reflecting telescope, with which he used to identify the Whirlpool Galaxy (M51), the first “nebula” to be resolved into individual stars, in 1845.

Parsons was a wealthy “gentleman astronomer”, and he built this monstrous instrument with the express purpose of investigating the objects listed in both Messiers’ and Herschel’s’ famous catalogues. Parson’s was looking to resolve the question of whether the “nebulae” he and other astronomers were seeing were just unresolved clusters of stars, or regions of genuine nebulosity in space. As it turned out, his gigantic telescope not only clearly resolved individual stars in the galaxy now known as M51, but also showed the object to have a spiral structure, which prompted him to declare that there may exist as-yet undiscovered “dynamical laws” that determine the structure in this, and other spiral galaxies.

Apart from the major discovery that “nebulae” were in fact galaxies that contained stars, the Forty-Foot was used by J. L. E. Dreyer to compile his New General Catalogue of Nebulae and Clusters of Stars between 1874 and 1878. This telescope also had the distinction of being the largest in the world from 1845 until the 100-inch Hooker telescope entered service in 1917.

The Hooker Telescope

The Hooker telescopeThe image above shows the structure of the most important telescope of the 20th century, the 100-inch Hooker telescope at Mount Wilson Observatory, California. This is the telescope that Edwin Hubble (1889-1953) used to demonstrate that the Universe extends way beyond the Milky Way, and that the galaxies that comprise the Universe are moving away from us and each other; thus, that the Universe was expanding at a rapid rate. This discovery fundamentally changed the scientific view of the Universe.

However, other, equally important observations and discoveries were made using this telescope. Below are some details:

• Hubble and Milton Humason used the Hooker telescope in 1929 to measure both the dimensions of the then-known Universe, and the rate at which the Universe is expanding.
• Fritz Zwicky used the Hooker telescope in the 1930’s to find evidence of dark matter.
• In 1938, Dr. Seth Nickolson discovered two of Jupiter’s satellites, designated #10 and #11 at the time.
• During the 1940’s, Walter Baade used the Hooker telescope to discover distinctly different stellar populations as well as two distinctly different classes of Cepheid variables, which discovery effectively doubled the dimensions of the Universe calculated by Hubble. This discovery also resolved the problem of the Universe’s age, which according to Hubble’s calculations, was only half as old as it should be.

The Hale Telescope

The Hale TelescopeApart from the fact the 200-inch Hale telescope at Palomar Mountain, California, was twice as large as the next biggest instrument, the 100-inch Hooker telescope, the Hale telescope served as the test bed for many new technologies, including a vapor-deposited aluminium coating on its primary mirror, and the use of low-expansion thermal glass in all its optical elements.

However, the Hale telescopes’ birth was not painless. The chief optical designer, George Willis Ritchey wanted the primary mirror made according to the Ritchey–Chrétien design, which would have produced sharper images over a wider field of view than could be achieved by a normal parabolic mirror. Because the telescope’s construction had fallen behind schedule and was already over budget, Hale refused to allow the added delay of making a more complex mirror, and Ritchey left the project in a huff.

As a result of the falling out, the Hale telescope was the last world-leading telescope to use a conventional parabolic primary mirror. The Hale telescope is still in active service today and was the largest telescope in the world until 1975, when the BTA-6 telescope sort of entered service in Russia.

BTA-6 – a useful disaster

BTA-6 telescopeThe image opposite shows some detail of the BTA-6 6-meter (20 ft) aperture telescope in the Zelenchuksky District in southern Russia.

Sadly, due to poor design, even poorer workmanship, and an unfortunate location downwind of the Caucasus Mountains, this telescope was never to work at even remotely close to its theoretical limits. In fact, one of its replacement mirrors was so badly made that operators had to cover parts of the mirror with pieces of black cloth to be able to use the instrument at all, but worse still, the dome is oversized to the point where thermal currents within the dome render the instrument almost unusable.

Nonetheless, and despite its serious shortcomings, the BTA-6 remains a significant and important telescope, since it can somehow image objects down to magnitude 26. Moreover, the instrument’s light gathering ability makes it particular useful for spectroscopy and speckle interferometry, fields in which this telescope has made huge contributions to astronomy. Continuous upgrades, mirror replacements, and improvements to air flow in the dome are continuing, and it is expected that this major telescope will be fully functional sometime during the early 2020’s.

Keck 1 & 2- the most productive telescopes

Keck 1 & 2These images show the twin domes of the Keck 1 & 2 telescopes in Hawaii, as well as some detail of the segmented mirror of the Keck 1 instrument. Collectively, the Keck telescopes are the most scientifically productive telescopes in use in the world today.

With effective apertures of 10 meters (33 ft), the engineering challenges of using solid mirrors would have raised the price of these telescopes far beyond the $70 million (each) they had cost to build. Thus, their primary mirrors each consist of 36 hexagonal segments, that are each 1.8 meters (5.9 ft) wide, 7.5 cm (2.9 in) thick, and weigh 453 kg (1000 lb). In practice, each segment can be deformed by actuators to counter-act both the effects of gravity and atmospheric instabilities twice per second to an accuracy of four nanometers, or less, which means that the Keck instruments sometimes outperform the Hubble Space Telescope.

Hubble- the eye in the sky

Hubble telescopeAs of November 2022, the Hubble Space Telescope has exceeded its expected life span of 15 years by 12 years, and while it remains functional, the current view of its operators is that “for as long as it flies, we’ll keep on taking pictures.”

Hubble is known to just about everybody that was alive in 1990, and there is no denying the huge role it has played both in almost “rediscovering” the Universe, and its value as a public relations tool for NASA. However, it is as a scientific instrument that the Hubble telescope has made huge strides in affording astronomers a new understanding of how the Universe works. For instance, the Hubble Deep Field pictures revealed thousands of galaxies nobody knew existed; it revealed details in the structures of planetary nebula that nobody suspected, and it even placed new and very tight constraints on the rate at which the Universe is expanding.

Nonetheless, large ground-based telescopes are now routinely outperforming the Hubble telescope in observations made in optical wavelengths. Still, for as long as it can still observe in other frequencies, the Hubble telescope will remain one of the most important telescopes in service until its replacement, the James Webb space telescope, is placed into service.

The shape of things to come…

Extremely Large TelescopeThis image shows an artists’ impression of the Extremely Large Telescope (ELT), a monstrous optical telescope now under construction by the European Southern Observatory atop the Cerro Armazones Mountain in the Atacama Desert of northern Chile.

Everything about this telescope is superlative; from its 39.3-metre-diameter (126 ft) segmented primary mirror and 4.2-metre-diameter secondary mirror, to its 86-meter-diameter dome that will stand 74 meters tall from the ground and weigh 5,000 tons, but which will nevertheless rotate at a speed of 2 degrees per second. In practice, the ELT will collect 100 million times more light than the human eye, 256 times the light of the Hubble telescope, and produce images that will be at least 16 times sharper than the Hubble can ever produce.

Upon its completion in 2024, the ELT’s primary functions will be to search for planets around other stars, find the earliest galaxies in the Universe, investigate the environs of super massive black holes, and to detect among other things, water and organic molecules in protoplanetary disks around other stars. To extend the telescopes’ capabilities, it will be fitted with instrumentation that will allow it to see in near infrared frequencies as well.

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