If you think about it, it was just a matter of time before the first telescope was invented. People have been fascinated by crystals for millenia. Many crystals – quartz for instance – are completely transparent. Others – rubies – absorb some frequencies of light and pass others. Shaping crystals into spheres can be done by cleaving, tumbling, and polishing – this removes sharp edges and rounds the surface. Dissecting a crystal begins with finding a flaw. Creating a half-sphere – or crystal segment – creates two different surfaces. Light is gathered by the convex frontface and projected toward a point of convergence by the planar backface. Because crystal segments have severe curves, the point of focus may be very close to the crystal itself. Due to short focal lengths, crystal segments make better microscopes than telescopes.
It wasn’t the crystal segment – but the lens of glass – that made modern telescopes possible. Convex lenses came out of glass ground in a way to correct far-sighted vision. Although both spectacles and crystal segments are convex, far-sighted lenses have less severe curves. Rays of light are only slightly bent from the parallel. Because of this, the point where the image takes form is much farther away from the lens. This creates image scale large enough for detailed human inspection.
The first use of lenses to augment sight can be traced back to the Middle East of the 11th century. An Arabian text (Opticae Thesaurus written by scientist-mathematician Al-hazen) notes that segments of crystal balls could be used to magnify small objects. In the late 13th century, an English monk (possibly referencing Roger Bacon’s Perspectiva of 1267) is said to have created the first practical near-focus spectacles to aid in reading the Bible. It wasn’t until 1440 when Nicholas of Cusa ground the first lens to correct near-sightedness -1. And it would be another four centuries before defects in lens shape itself (astigmatism) would be aided by a set of spectacles. (This was accomplished by the British astronomer George Airy in 1827 some 220 years after another – more famous astronomer – Johann Kepler first accurately described the effect of lenses on light.)
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The earliest telescopes took form just after spectacle grinding became well-established as a means to correct both myopia and presbyopia. Because far-sighted lenses are convex, they make good “collectors” of light. A convex lens takes parallel beams from the distance and bends them to a common point of focus. This creates a virtual image in space – one that can be inspected more closely using a second lens. The virtue of a collecting lens is twofold: It combines light together (increasing its intensity) – and amplifies image scale – both to a degree potentially far greater than the eye alone is capable of.
Concave lenses (used to correct near-sightedness) splay light outward and make things appear smaller to the eye. A concave lens can increase the focal length of the eye whenever the eye’s own system (fixed cornea and morphing lens) falls short of focusing an image on the retina. Concave lenses make good eyepieces because they enable the eye to more closely inspect the virtual image cast by a convex lens. This is possible because convergent rays from a collecting lens are refracted toward the parallel by a concave lens. The effect is to show a nearby virtual image as though at a great distance. A single concave lens allows the eye lens to relax as if focused on infinity.
Combining convex and concave lenses was just a matter of time. We can imagine the very first occasion occurring as children toyed with the lens-grinder’s toil of the day – or possibly when the optician felt called to inspect one lens using another. Such an experience must have seemed almost magical: A distant tower instantly looms as if approached at the end of a long stroll; unrecognizable figures are suddenly seen to be close friends; natural boundaries – such as canals or rivers – are leapt over as though Mercury’s own wings were attached to the heals…
Once the telescope came to be, two new optical problems presented themselves. Light collecting lenses create curved virtual images. That curve is slightly “bowl-shaped” with the bottom turned toward the observer. This of course is just the opposite of how the eye itself sees the world. For the eye sees things as though arrayed on a great sphere whose center lies on the retina. So something had to be done to draw perimeter rays back toward the eye. This problem was partially resolved by astronomer Christiaan Huygens in the 1650’s. He did this by combining several lenses together as a unit. The use of two lenses brought more of the peripheral rays from a collecting lens toward the parallel. Huygen’s new eyepiece effectively flattened the image and allowed the eye to achieve focus across a wider field of view. But that field would still induce claustrophobia in most observers of today!
The final problem was more intractable – refracting lenses bend light based on wavelength or frequency. The greater the frequency, the more a particular color of light is bent. For this reason, objects displaying light of various colors (polychromatic light) are not seen at the same point of focus across the electro-magnetic spectrum. Basically lenses act in ways similar to prisms – creating a spread of colors, each with its own unique focal point.
Galileo’s first telescope only solved the problem of getting an eye close enough to magnify the virtual image. His instrument was composed of two lenses separable by a controlled distance to set focus. The objective lens had a less severe curve to collect light and bring it to various points of focus depending on color-frequency. The smaller lens – possessed of a more severe curve of shorter focal length – allowed Galileo’s observing eye to get close enough to the image to see magnified detail.
But Galileo’s scope could only be brought to focus near the middle of the eyepiece field of view. And focus could only be set based on the dominant color emitted or reflected by whatever Galileo was viewing at the time. Galileo usually observed bright studies – like the Moon, Venus, and Jupiter – using an aperture stop and took some pride in having come up with the idea!
Christiaan Huygens created the first – Huygenian – eyepiece after the time of Galileo. This eyepiece consists of two plano-convex lenses facing the collecting lens – not a single concave lens. The focal plane of the two lenses lies between the objective and eye lens elements. The use of two lenses flattened the curve of the image – but only over a score or so degrees of apparent field of view. Since Huygen’s time, eyepieces have become much more sophisticated. Beginning with this original concept of multiplicity, today’s eyepieces can add another half-dozen or so optical elements rearranged in both shape and position. Amateur astronomers can now purchase eyepieces off the shelf giving reasonably flat fields exceeding 80 degrees in apparent diameter-2.
The third problem – that of chromatically tinged multi-color images – was not solved in telescopy until a working reflector telescope was designed and constructed by Sir Isaac Newton in the 1670’s. That telescope eliminated the collecting lens altogether – though it still required the use of a refractory eyepiece (which contributes far less to “false color” than the objective does).
Meanwhile early attempts to fix the refractor were to simply make them longer. Scopes to 140 feet in length were devised. None had especially exorbitant lens diameters. Such spindly dynasaurs required a truly adventurous observer to use – but did “tone down” the color problem.
Despite eliminating color error, early reflectors had problems too. Newton’s scope used a spherically ground speculum mirror. Compared to the aluminum coating of modern reflector mirrors, speculum is a weak performer. At roughly three-quarters the light gathering ability of aluminum, speculum loses about one magnitude in light grasp. Thus the six-inch instrument devised by Newton behaved more like a contemporary 4 inch model. But this is not what made Newton’s instrument hard to sell, it simply provided very poor image quality. And this was due to the use of that spherically ground primary mirror.
Newton’s mirror did not bring all rays of light to common focus. The fault didn’t lay with the speculum – it lay with the shape of the mirror which – if extended 360 degrees – would make a complete circle. Such a mirror is incapable of bringing central light beams to the same point of focus as those nearer the rim. It wasn’t until 1740 when Scotland’s John Short corrected this problem (for on-axis light) by parabolizing the mirror. Short accomplished this in a very practical manner: Since parallel rays nearer the center of a spherical mirror overshoot marginal rays, why not just deepen the center and rein them in?
It wasn’t until the 1850’s that silver replaced speculum as the mirror surface of choice. Of course the more than 1000 parabolic reflectors fabricated by John Short all had speculum mirrors. And silver, like speculum, loses reflectivity rather quickly over time to oxidation. By 1930, the first professional telescopes were being coated with more durable and reflective aluminum. Despite this improvement, small reflectors bring less light to focus than refractors of comparable aperture.
Meanwhile, refractors evolved too. During John Short’s time, opticians figured out something Newton had not – how to get red and green light to merge at a common point of focus by refraction. This was first accomplished by Chester Moor Hall in 1725 and rediscovered a quarter century later by John Dolland. Hall and Dolland combined two different lenses – one convex and other concave. Each consisted of a different glass type (crown and flint) refracting light differently (based on refractive indices). The convex lens of crown glass did the immediate task of collecting light of all colors. This bent photons inward. The negative lens splayed the converging beam slightly outward. Where the positive lens caused red light to overshoot focus, the negative lens caused red to undershoot. Red and green blended and the eye saw yellow. The result was the achromatic refractor telescope – a type favored by many amateur astronomers today for inexpensive, small aperture, wide-field, but – in shorter focal ratios – less than ideal image quality use.
It wasn’t until the mid-nineteenth century that opticians managed to get blue-violet to join red and green at focus. That development initially came out of the use of exotic materials (flourite) as an element in the doublet objectives of high-powered optical microscopes – not telescopes. Three element telescope designs using standard glass types – triplets – solved the problem as well some forty years later (just before the twentieth-century).
Today’s amateur astronomers can choose from a wide assortment of scope types and manufacturers. There is no one scope for all skies, eyes, and celestial studies. Issues of field flatness (particularly with fast Newtonian telescopes), and hefty optical tubes (associated with large refractors) have been addressed by new optical configurations developed in the 1930’s. Instrument types – such as the SCT (Schmidt-Cassegrain telescope) and MCT (Maksutov-Cassegrain telescope) plus newton-esque Schmidt and Maksutov variants and oblique reflectors – are now manufactured in the USA and throughout the world. Each scope type developed to address some valid concern or another related to scope size, bulk, field flatness, image quality, contrast, cost, and portability.
Meanwhile refractors have taken center stage among optophiles – folks wanting the highest possible image quality irrespective of other constraints. Fully apochromatic (color-corrected) refractors provide some of the most stunning images available for optical, photographic, and CCD imaging use. But alas, such models are limited to smaller apertures due to significantly higher costs of materials (exotic low-dispersion crystals & glass), manufacture (up to six optical surfaces must be shaped) and greater load bearing requirements (due to heavy disks of glass).
All of today’s variety in scope types began with the discovery that two lenses of unequal curvature could be held up to the eye to transport human perception over great distances. Like many great technological advances, the modern astronomical telescope emerged out of three fundamental ingredients: Necessity, imagination, and a growing understanding of the way energy and matter interact.
So where did the modern astronomical telescope come from? Certainly the telescope went through a long period of constant improvement. But perhaps, just perhaps, the telescope is at essence a gift of the Universe itself exulting in profound admiration through human eyes, hearts, and minds…
-1 Questions exist as to who first created spectacles correcting far- and near-sighted vsion. It is unlikely that Abu Ali al-Hasan Ibn al-Haitham or Roger Bacon ever used a lens in this way. Confusing the issue of provenance is the question of how spectacles were actually worn. It is likely that the first visual aid was simply held to the eye as a monocle – necessity taking over from there. But would such a primitive method be historically recounted as “the origin of the spectacle”?
-2 The ability of a particular eyepiece to compensate for a necessarily curved virtual image is limited fundamentally by effective focal ratio and scope archetecture. Thus telescopes whose focal length are many times their aperture present less of an instantaneous curve at the “image plane”. Meanwhile scopes that refract light initially (catadioptics as well as refractors) have the advantage of better handling off-axis light. Both factors increase the radius of curvature of the projected image and simplify the eyepiece’s task of presenting a flat field to the eye.
About The Author:
Inspired by the early 1900’s masterpiece: “The Sky Through Three, Four, and Five Inch Telescopes”, Jeff Barbour got a start in astronomy and space science at the age of seven. Currently Jeff devotes much of his time observing the heavens and maintaining the website Astro.Geekjoy.