Friday, June 5, 2009

Well, that's over.

Last "Gotta write this term paper in one night, lets do this, Whooo!" night of my undergrad career. I told myself I'd post the paper as motivation, so even though it's pretty dismal, here it is, the last term paper.

Telescopes have an incredibly rich history, having been influenced not only by leaps of the mind but those of technology as well. In the 401 year history of the telescope it has taken on many forms, and wildly different structures. It has also progressed from observing visible light to cover the other ends of the electromagnetic spectrum: radio, infrared, x-ray and gamma ray telescopes are now operational. All this started with a simple optical telescope, in 1608.

The refracting telescope appeared in a near mythic fashion in late 1608. Like Athena, it seemed to spring forth fully formed from the minds of a few men. In this year, three people presented documents to the effect that they had invented the telescope, or similar device for enlarging distant objects: Hans Lippershey, James Metius, and Zacharias Jensen (King, 30-32).

Lippershey is most commonly credited with the idea (King 30), and not all works acknowledge the other two men. It should be noted however that lenses for spectacles were being made for at least 80 years before this point (Willach 94). The theory behind a telescope was understood, and there was no shortage of scholars mucking about with lenses at the time. Thus, it is somewhat surprising that an otherwise unremarkable man should somehow come upon a means of making an effective telescope where so many others had failed.

His merit was twofold: he was an excellent lens maker, or was able to produce lenses rapidly enough to select only those of high quality. He not only produced the first telescope, but the first binoculars, which require pairs of identical lenses (Willach 95). In addition to his ability, he had an excellent insight: he used a diaphragm with a small hole in it in front of the primary lens (Willach 95). This seemingly trivial modification made all the difference in the world: by using only the very center of the lens, where there is less spherical aberration (distortion caused by the shape of the lens) and avoiding the edges of the lens, which tended to by less perfectly ground, the resolution of the telescope was vastly improved. This made it a useful instrument, and this was rapidly propagated. Less than a year later Galileo had constructed his own telescope and was making observations.

James Metius and Zacharias Jensen also put forth instruments, within weeks of Lippershey (King 31,32). Thiers were however of a much poorer quality (King 32), and were perhaps copies of Lippershey’s original. (Adding a diaphragm is an obvious alteration, and an easy one to make once it is known of (Willach 95).) It has also been noted that Jenson was a counterfeiter of coins (perhaps also telescopes), and ended up fleeing the country to avoid prosecution (King 32).

The refracting telescope uses lenses as its optical element. It was the state of the art in optics manufacture at the time, but several physical properties of the lenses limited the telescopes. First, lenses are subject to spherical aberration (fig. 1). As light enters the lens, it encounters the surface at a steeper angle as it approaches the edge. This causes the light at the edges to be focused in front of the light from the center, making it impossible to achieve focus using the entirety of the lens. As it noted earlier, a diaphragm that limits light to the center of the lens alleviates this problem, as does reducing the curvature of the lens so that the variation in the incident angle of the light is less. However, reducing the curvature of the lens increases the focal length of the lens, requiring a much larger instrument. Chromatic aberration is another issue with lenses made of a single material: Light of different wavelengths bends at different rates in a given material. This has the effect of focusing different colors at different points, separating them out as a prism does (fig. 2). This effect would appear as a colored halo around the object being viewed, and interferes with good viewing. Again, the flatter a lens is (the longer the focal length) the less this effect is noticeable. This is why light passes through a window undisturbed, but a prism has the effect of separating light.

These limitations required that telescopes be very long relative to their diameter. Instruments that were capable of resolving the moons of Jupiter needed to be about 30 feet long (King 36) and ranged up to 150 feet in length (Hoskin). Johannes Helvius created his 150 foot telescope and suspended it using a mast and a collection of block and tackle to raise and lower it (Img. 1). One can readily see that it would be incredibly difficult to properly align this telescope, and to track objects with it. Furthermore, the lens diameter was still very small, less than a foot. By having such a small aperture, a very small amount of light could be received. Modern telescopes have an effective aperture that ranges from a few inches up to ten meters. A larger aperture allows you to see fainter objects, assuming that the quality of the rest of the telescope remains the same. This was simply not the case for refracting telescopes of this day, and despite Galileo’s forays into lens making, he was unable to significantly improve the design.

Despite these limitations, these telescopes opened up an entirely new world of observation. In his first year with the telescope Galileo made a number of key observations. He saw four moons around Jupiter, the craters of the moon, the motion of sunspots, and the millions of faint stars that form the Milky Way (King 37, 39). He also observed the rings of Saturn, but lacked the resolution to discern that it was a flat ring, and not a part of the planet itself (King 38). Galileo’s observations through the telescope reinforced his attacks on geocentricism, and certainly contributed to his disfavor with the papacy. The refracting telescope would remain the cornerstone of observation until 1663.

Mirrors a so commonplace now that we rarely give them any thought. Yet the bathroom mirror in your house is far superior to anything available to scientists in the 18th century. Mirrors were produced from polished metal, and so the reflectivity of the surface was limited to how well one could polish a given metal. Speculum metal, a alloy of copper, tin, zinc and arsenic was the standard for mirrors (Manly 3). This alloy had a high reflectivity and was stiff enough to hold its own weight. Unfortunately, it still only reflected 40% of the light that it receives, a dismal percentage (Manly 22). In certain formulations, such as the one that Newton devised, the reflectivity was as low as 16% (King 77). In comparison, the Gemni telescopes on Mauna Kea were recoated recently, and obtained a reflectivity ranging from 93-98% in the visible spectrum (http://www.gemini.edu/node/110). Speculum metal also tarnishes quickly, requiring a resurfacing every 2-3 months (Manly 22). In large reflecting telescopes, such as the 40ft reflector that William Herschel made, the mirrors could weigh over a ton, making removing them for polishing a harrowing experience (Manly 22). Despite these limitations reflecting telescopes had some great advantages over refracting telescopes.

Reflecting telescopes could be made much shorter for a given magnification: Newton’s first reflector was 16 inches long, but had a magnifying power greater than that of a four foot refractor (Watson 129). The Newtonian reflector used a simple spherical primary mirror, a flat secondary mirror, and used a convex lens in the eyepiece (Fig. 3). Newton felt that this gave a better view, as it was easier to make a flat mirror and focus using a lens. Many smaller commercial telescopes still use this design, as it is relatively straightforward. It also places the eyepiece towards the top of the telescope tube, which is advantageous for smaller (tabletop sized) telescopes as it alleviates the need to stoop down. A contemporary of Newton, Cassegrain, produced a design that placed the eyepiece at the rear of the telescope, using a second parabolic mirror to send the light through a hole in the primary mirror (King 75) (Fig 4).

This arrangement had the advantage that the concave and convex mirrors reduced the aberration experienced, but suffered from the fact that the parabolic mirrors needed could not be accurately ground at the time (Watson 131). A similar design, by James Gregory failed for the same reasons (King 71). It used a concave secondary mirror rather than the convex that Cassegrain utilized (King 71). This had the effect of requiring a longer telescope, as the secondary mirror had to be placed beyond the focal point of the primary. In contrast, the secondary mirror in Cassegrain’s design is placed roughly halfway to the focal point of the primary. In this way Cassegrain found a very elegant design, which has become the basis for almost all modern reflector telescopes (Watson 131).
Newton was a vocal opponent of Cassegrain’s design, and as such very little work was done on it until mirror technology had advanced sufficiently for high quality parabolic mirrors could be produced.
It would be an error to leave William Herschel out of any dialogue on reflector telescopes. If a patron saint of polymaths were to be elected, he would certainly be in the running. Self taught in astronomy at the age of 35, Herschel chose to survey the sky, and became and avid collector of stars and nebulas (Cambridge 235). He also designed and constructed his own reflecting telescopes, up to a 40 foot-long reflector with a four foot primary mirror. It differs from all other reflectors so far mentioned, in that it has no secondary lens. Rather, the primary lens is viewed from directly from an eyepiece set in the side of the telescope tube (Manly 22). By reducing the number of optical elements that the light encounters Herschel was able to achieve a much brighter image than would be seen in a comparable Newtonian (Manly 22). Due to the size of the 40 foot telescope, Herschel rarely used it, preferring to use a 20 foot telescope of the same design (King 133). However with these smaller instruments, Herschel was able to first observe Uranus, and speculate extensively on the composition and location of nebulas, discovering 2,500 (Cambridge 240). His sister was instrumental in assisting him in his efforts to categorize nebulas, but was also an astronomer in her own right. She used a wide aperture low magnification telescope to search for comets, of which she found eight (Cambridge 232, Watson 178). The telescope that she used would have been difficult to make as a refractor, as the lenses available would have limited her to a narrower aperture, a detriment when attempting to scan large sections of the sky.

Herschel’s work on nebulas was continued by William Parsons, who was equally seized with a fascination with large mirrors. He constructed a massive reflector, using a 6 foot primary mirror (Cambridge 253). This impressive instrument allowed him to see the spiral structure of M51. His sketch of the nebula as compared to modern photographs is incredible, and a great improvement over previous drawings of the same nebula (Cambridge 255). See Images 2 and 3.

It was about this time that the refractor began to advance again. Newton’s theory of light seemed to squash all hope of rendering a lens free of chromatic aberration. In a calculated move against public opinion, various people experimented with lenses, attempting to circumvent the issues which had plagued refractors in the past. Leonhard Euler provided a theoretical basis from which John Dollard experimented with achromatic lenses (King 147). Initially these attempts used water between two glass lenses, however these were very high in spherical aberration (King 148). He then hit upon flint glass, which is has a higher refractive index than ordinary glass or water (King 148). By combining a series of flint and crown glass lenses he was able to produce the first achromatic lenses (King 148). His son John went on to produce and market these lenses, but the general state of glass making limited the size of lenses to about 5 inches (King 150, 176). Pierre Guinand would next advance lens making, by increasing the ability to produce high quality glass.

Lenses above 5 inches generally contained imperfections which rendered then unsuitable for good viewing (King 176). Pierre Guinand’s method of creating high quality flint glass enabled the production of full 9.5 inch aperture refractors (King 178, 180). The Dorpat refractor constructed by Fraunhofer in 1824 is considered the first modern achromatic refractor, and is an impressive telescope. It was clever in all ways, being not only optically fine, but utilizing a equatorial mount and counter weights which allowed it to be moved quite easily despite its 14 foot length (King 182-183). In this way the Dorpat refractor serves to illustrate the resolution of all the issues that the refractors of Galileo’s period suffered. Being of a manageable length and mounted in a way that facilitates observation, and having a set of good achromatic lenses it could give reflectors a run for their money.
Fraunholfer’s contributions ran far deeper than his telescopes however. He invented the spectrometer, which may be the single most important piece of equipment that has been attached to a telescope. Not only does it allow for determining the composition of various luminous objects, the matter that we cannot see between us and said luminous objects, the composition of out own air, it also allows us to determine the speed of objects relative to us. Unfortunately he was not as interested in the spectroscope as in optics, and so he did not expand it to its fullest potential. He did however note the absorption spectrum of the Earths atmosphere. The lines that he noted and lettered (Img. 4) are still known by his name (Watson 238).

In 1864 an amateur astronomer named William Huggins, armed only with a modest refracting telescope and a spectroscope solved a question that had baffled even Herschel. Are nebulas amorphous collections of gas, or are they aggregates of stars that are too dim to resolve? The answer is yes. Huggins looked at the spectral lines for several nebulous objects and found that the diffuse objects had a defined, narrow, spectrum, indicating that they were illuminated clouds of gas (Watson 239). In comparison stars had a more complex emission pattern, which one cold see by taking the spectrum of various stars. By doing this Huggins also established that the elements present on Earth exist throughout the visible universe, thus proving the common chemistry of both (Watson 238). He is credited with starting astrophysics, and popularizing one of the most useful tools we have in conjunction with optical telescopes.

As refractors continued to improve, the theoretical limit for their size was eventually found: as lenses increase in size, they begin to sag. This is particularly an issue for lenses, as they have to be suspended by their edges which out of necessity leaves the center unsupported. It seems counterintuitive that glass should sag, but it certainly does. If you have the opportunity to manipulate a large plate glass window its flexibility will be readily apparent. This limits lenses to about 40 inches in diameter, a size which was achieved in 1897 with the Yerks refractor (Watson 244). A 48 inch refractor was completed for the Paris Exposition Universelle but it was decommissioned after the end of the exposition (Watson 245). The second largest refractor in the world today is the refractor at Lick, measuring 36” and completed in 1888 (Watson 243). The refractor at Lick is not only notable for its size but also its location: it was the first mountaintop observatory in the US (Watson 243). The shift to mountaintop observing reflects the increasingly institutional method of astronomy, with the scientific community forming and specialization with in it becoming the norm. The need for better seeing resulted in the use of mountaintop observatories, which were possible due to the institutionalization of astronomy. This is a notable departure from the majority of astronomical observation using telescopes: as can be seen with Herschel, Galileo, Huggins and Newton, it was the case that telescopes were built where the scientists were. Now it is quite the opposite!

Reflectors were certainly not left out of the turn of the century surge. A new method for producing mirrors was discovered: the chemical deposition of silver onto glass (King 262). This produced a mirror with much higher reflectivity, into the 90% range, making it more than a 100% improvement over speculum metal. The glass used is easier to work with than speculum metal, and significantly lighter, and replacing the coating can be done chemically rather than via regrinding the mirror (King 262). This method is generally so superior that all large telescopes are produced with some variant of this process, and such that the cutting edge telescopes produced today are large reflectors, ranging in size up to 10m. Bear in mind that that is ¾ the length of some of the larger telescopes that used speculum metal.

Lastly, there is a telescope about which few people know, but which is particularly interesting. The zenith telescope utilizing a mercury mirror is a particularity in the field of telescopes. Much work goes into making sure that the mirrors are aligned and stable, and are designed to have rigid mirrors to give the best viewing. Thus it is a bit counterintuitive that mercury, a fluid, should be used as a primary mirror. But when placed in a shallow rotating pan, centripetal force causes it to assume a parabolic shape, the depth of which corresponds to the speed of the rotation, and is independent of any small defects in the pan (Manly 12). This method is limited to pointing vertically, hence its use only in zenith telescopes, but is an effective solution.

In the past 400 years telescopes have evolved from very primitive tools to incredibly sophisticated devices. They have consistently remained on the leading edge of technical innovation, even spurring it in some cases. Over the years the nature of astronomy as a discipline has changed as well, from being a hobby of those with the intellect or means, to a formal discipline within the scientific community. (This was a general trend in all fields.)

Works Cited

Hoskin, Michael. Ed. “The Cambridge Concise History of Astronomy” Cambridge University Press, 2008.

King, Henry C. “The History of the Telescope” Sky Publishing Corporation, 1955

Manly, Peter L. “Unusual Telescopes” Cambridge University Press, 1991

Watson, Fred. “Star Gazer: The Life and Times of the Telescope” Da Capo Press, 2004.

Willach, Rolf. “The Long Road to the Invention of the Telescope” American Philosophical Society, 2008

Website of the Gemini Telescope, http://www.gemini.edu/node/110, accessed 6/5/2009


Figure 1. Public Domain, Mglg, uploaded to English Wikipedia, retrieved on 6/5/2009

Figure 2. Public Domain, Lucas V. Barbosa, uploaded to English Wikipedia, retrieved on 6/5/2009

Figure 3. Public Domain, Tmoore, uploaded to English Wikipedia, retrieved on 6/5/2009

Image 1. Copyright status unknown, assumed that it has lapsed into public domain. Author Unknown.

Image 2. Sketch made by Lord Rosse of the Whirlpool Galaxy in 1845, lapsed into public domain in the United States. Retrieved 6/5/2009.

Image 3. Author: NASA and European Space Agency, Jan. 2005. Public domain with attribution. Retrieved 6/5/2009.

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