by Hendrik J. Gerritsen (Professor Emeritus of Physics, Brown University) and Michael L. Umbricht (Ladd Observatory Curator) April 14, 2009
We have tried to express in the somewhat peculiar sounding title what the content of this short essay will be. We will present an overview of how telescopes came into being and then lead into the question why the Ladd telescope is the way it is namely why the choice, made in 1891, was to buy a 12 inch diameter refractor and not - for example - a large diameter reflector of the kind Brown recently bought, a 16 inch Meade LX200.
A refractor uses several lenses that break (from latin fract-) the light, so that it comes to or from a focus after it falls on a lens. A reflector, invented and first made by sir Isaac Newton, uses as its objective a large mirror. So it seems worthwhile to ask when lenses were first invented and how they were combined into a telescope and after that look into the use of curved mirrors in connection with telescopes. It will then become understandable why Prof. Winslow Upton decided at the end of the 19th century for the telescope we still cherish and use, more than a century later.
Lenses, made from round polished quartz, but also from glass, were used in antiquity as ornaments (ref. 1a), as burning glasses and occasionally for magnifying purposes (ref. 2) dating back to 2300 B.C. They were found among the treasures of old Troj and throughout ancient Europe and North Africa. The oldest description of the lens as magnifier goes back to Roger Bacon (1210-1294) and the earliest documented case of lenses in spectacles goes back to a painting from 1352 (ref. 1b). There is evidence that they were invented around 1285 perhaps by the Italian Salvino d'Armati on whose tombstone it says (ref. 3a):
Here lies Salvino degli Armati of the Armati of Florence.
Inventor of Spectacles. God pardon his sins. A.D. 1317.
Spectacles were such a boon to mankind that from then on their development spread from Italy all over Europe. Before this invention aging monks needed large notes and text to sing their Gregorian songs. Soon we find besides from positive, convex, reading glasses for aging eyes, also concave lenses in use, to correct for nearsightedness some people are born with when their eye lens is focusing too strongly.
The concave mirror, this essential component in a reflecting telescope, was also invented long before it was used in a multicomponent optical instrument. In fact their use seems to have been mainly for purposes of setting burnable materials on fire with rays of the sun and is credited with driving away a fleet of Persian ships planning to invade Greece.
Given that the essential parts to make a telescope have been available for centuries, why did it take till the early seventeenth century before the telescope was invented? ( The word itself comes from the Greek from tele = far and skopein = to see) and was given to Galileo's first "telescope" at 1610.
We translate and quote (ref. lc) thoughts by the very ingenious and inventive physicist / astronomer / telescope builder Christiaan Huygens (who lived in Holland from 1629 till 1695). "Only a superhuman genius could have invented the telescope on the basis of theory" He believed that the invention was due to chance and was somewhat surprised that it did not happen sooner, given all the spectacle lenses in use.
We will never know with certainty who first invented the telescope nor where. Most historians consider the Netherlands as the country of origin. There in Middelburg, Oct. 2, 1608, Hans Lippershey, a spectacle maker who was born in Wesel, Germany, applied to the States General for a patent on his "kijkglas" (spyglass). There was much interest in it partly for its military value. At that time Holland was engaged in a long struggle for its independence from Spain. The Dutch prince Maurits of Nassau bought a few of the double version (binoculars) but Lippershey did not obtain a patent. This was because meanwhile another Dutch spectacle maker, a native of Alkmaar called Jacob Adriaanszoon (later latinized to Metius) laid claim to the same invention on Oct. 17, 1608. Finally a third inventor Zacharius Jansen - a competitor in the same town of Middelburg - claimed priority. The latter has generally been credited with inventing the first compound microscope (ref. 1d, 3b) though.
About Lippershey's invention two stories are told. According to one of those (ref. 4) he discovered the telescope when he examined two lenses to look for flaws by holding them to the light. One was plano concave, the other was plano convex. Suddenly a distant church tower seemed very close. He then used the device to lure customers to his spectacle shop. In another version (ref. 3c) incorporating the serendipity that Christiaan Huygens saw as the only possible mother of this invention, two children - playing in Lippershey's shop with lenses - discovered a funny effect and showed it to the shop owner who immediately realized its importance. There is even some, meager, evidence that the invention was described earlier in obscure and unclear language by the Italian Johann Baptista delia Porta (ref. Ie and 3c).
The "Dutch trunks" or "perspectives" as they were called, appeared very rapidly all throughout cultural centers so that one year later, 1609, Galileo Galilei a forty-five years old professor of mathematics in Padua, heard an accurate description about them from a former student of his who had seen one. Galileo set out immediately to construct such an instrument, to improve upon it and to use it to study the sky with, in particular the moon and the planets.
Galileo never claimed to have invented the telescope, he credited "the Dutchman" with that (ref. 5). He remarks, however, that while that invention was by chance, his (Galileo's) telescopes were the "work of the reasoning and intelligent mind" (ref. 3d). It should, however, be pointed out that the state of knowledge of optics at that time was rather primitive, restricting the work of the intelligent mind mostly to systematic trials. For example, the basic law of refraction - on which lens action is based - was only discovered in 1621 by Willebrord Snell. (ref. 6) also in the Netherlands.
Galileo worked hard at grinding and polishing his own lenses and in 1610 discovered - with his 30x magnifying telescope - craters on the moon, four satellites of Jupiter, the phases of Venus, sunspots, many stars invisible to the naked eye in particular in the Milky Way, and the strange appearance of Saturn. All this he published in the booklet "Sidereus Nuncius" (ref. 5) translated as "Message from the Stars" or "Starry Messenger" published in 1610 in Venice, a copy of which is in Brown's John Hay library.
The book raised an enormous amount of interest and a year later Galileo was made a member of the Academia del Lincei, the first society dedicated to advancement of science and philosophy.
That evening of Galilei's initiation was also devoted to star gazing and during it one of the members suggested to give the name "telescope" to what had been called before "spyglass" or "perspiculum".
The Starry Messenger was avidly read and admired by many astronomers among them Johannes Kepler in Prague. It stimulated him to obtain one of Galileo's telescopes in 1610 and publish in 1611 a book "Dioptrice", devoted to the study of lens combinations and lens errors. In it Kepler pointed out the advantage of a larger field of view if one would use two positive, convex, lenses instead. The image is then turned upside down but that seemed a small price to pay. In fact Kepler's telescope became soon afterwards the prototype of all refractors including Brown's. The "Dutch telescope" is now only still used in rather low power opera glasses. (See schematics attached about both types)
Refractors grew impressively in size and magnification during the next decades (ref. 1,3,4). However, with the growth of optical theory and the improvement of the quality of glass and lens grinding, some limitations became apparent, in particular "chromatic aberration". This effect caused objects to have some rainbow like colors around them and is due to the fact that glass breaks violet light most strongly and red light the least. Thus red focuses further away from a convex lens than green and green further away than violet. Because one did not have our present day concepts about color, progress was slow and erratic, until Isaac Newton studied white light and the colors it consists of. This research led him to construct a reflective telescope in 1671, in which the large diameter convex lens of the refractor was replaced by a large concave mirror (ref. 7a). Newton argued that reflection is the same for all colors and that chromatic aberration would not occur using a concave mirror (ref. 7b). Newton realized that in principle chromatic aberration of lenses could be compensated for, by using two lenses consisting of different types glas cemented together. However, the few types of glass and other transparent materials he had available, did not have the required optical properties and Newton concluded erroneously that lenses could not be corrected for chromatic aberration (ref. 7.c) by using combinations.
However, the reflecting telescope did certainly not push the refractor out of use, for the following technical reasons: the mirror had to be made from metal. This made it very heavy and tarnishable as well. Newton used bell metal, six parts copper, two parts tin (ref. 3e). To maintain a reasonable reflectivity required frequent careful repolishing. This problem would plague reflectors for two centuries, till finally silver-on-glass mirrors and, still later, evaporated aluminum-on-glass mirrors were invented at the end of the nineteenth and beginning of the twentieth century, Because Newton's reflector required two mirrors, each with its rather low reflectivity and its tarnability, typically only 16% of the incident light was used (ref. 3f, 7d) effectively. In 1721 a Newtonian reflector of 6 inch diameter, 62 inch focal length was ground from speculum metal by John Hadley and compared with Christiaan Huygen's 7 ½ inch diameter, 123 foot (!) focal length aerial refractor of 1692. The definition of both was comparable, Huygen's telescope gave brighter images but was much harder to maneuver. Incidentally, Hadley was the first grinder who managed to reduce spherical aberration by parabolizing the surface. In the mean time, while mirror telescopes were improved and increased in size, newer glass compositions created the possibility to make the chromatic aberration correction that Newton had claimed was not possible. The Ladd Observatory telescope uses such lenses.
The situation of priority for this invention is somewhat curious (ref. 1f, 3g). The first inventor seems to have been Chester Moor Hall in 1729 but he was very secretive about it even when, much later in 1758, a patent for the achromatic doublet was given to the silk weaver / amateur optician John Dollond of London. By using a combination of two glasses of opposite power, a concave of flint and a convex of crown glass he constructed a net positive lens but with compensation for the color dispersion, that is the lens had the same focal length for red and blue-violet light.
The achromatic doublet certainly renewed interest in the refractor which had begun to be pushed out by giant reflectors such as William Herschel's. He believed in the achromatic advantage of Newton's reflector - notwithstanding the major problems of casting, grinding, polishing and often repolishing such a mirror. Herschel became famous for his discovery of Uranus in 1781 with his home-made 6 inch diameter, 7 foot focal length metal mirror reflector. Several years later he would complete the then world's largest metal mirror reflector, "one of the wonders of England and a great tourist attraction" (ref. 4b). It had a diameter of 48 inch, a weight of more than a ton and a focal length of 40 foot. It took five years and three rejected mirror castings to complete it in 1788 (ref. 4c).
It was not long before the idea of the achromat coupled with advances in glass technology, when compared with the trouble with mirror tarnish, tipped the scales again in favor of refractors. For about one century a revival of advanced refractors took place. In 1824 Fraunhofer completed a refracting telescope with a perfect 9 ½ inch diameter crown-flint air spaced achromat, the Dorpat refractor and one of the finest telescopes in the world. Its equatorial mounting and clock drive with weights are similar to those of Brown's refractor. Nevertheless, reflectors still were being built, most notable being Lord Rosse's "Leviathan of Parsonstown" , 6 feet across with a 54 feet focal length and a mirror that weighed four ton (ref. 4d) and was completed in 1844. Lord Rosse used it to make beautiful, detailed, sketches of many spiral nebulae.
Many of the large refractors built in the 19th century were erected in the USA which had a late start in observational astronomy, but was rapidly catching up with Europe. American instrument making also developed quickly to satisfy the new need and builders like Amasa Holcomb (who made reflectors in Massachusetts), Alvan Clark and sons (a former portrait painter who started work in 1850 and built the first American refractor telescope factory in Cambridge, Mass.) and John Brashear (more about him later) made telescopes (though with imported glass blanks) comparable to Europe's best. "In the mid nineteenth century the refractor was more than ever before the basic instrument' in both private and national observatories" (ref. 3h):' They were preferably refractors in an equatorial mount, sometimes clock driven. A few of the very large refractors and reflectors with dates of completion and makers are shown in the tables below:
Telescope / Institution
Dr. G.E. Hale and Alvan Clark
Mount Hamilton, California
Alvan Clark & Sons
Telescope / Institution
Keck 1 and 2
Mauna Kea, Hawaii
California Association for Research in Astronomy
Mauna Kea, Hawaii
National Astronomical Observatory of Japan
Palomar Mountain, California
Dr. G.E. Hale and Corning Glass
California Institute of Technology
Mount Wilson, California
Dr. G.E. Hale
Victoria, British Columbia
The trend is clear. Around 1900 the larger instruments became again reflectors. The reasons are several. Reflectors were by then made by silvering properly shaped glass. This reduced cost and weight. Although the silver layer still tarnished, the process of resilvering is much simpler than repolishing of a metal mirror. The silvering process was first described by Justus von Liebig (ref. 3h) and used in 1856 on a silvered reflector. It was introduced in the USA by Prof. Henry Draper and modified in 1861. Further and final improvements to silvering glass in a reliable way were made in 1870 by J. Brashear (ref. 3i). This process is reliable, not very expensive and does not produce heat which can crack a mirror. This signaled the end of the metal speculum era. The last such telescope was built in 1862 to be used in Melbourne, Australia. It had serious metal tarnish problems (ref. 3j).
For a while refractors and reflectors coexisted and most telescope builders (in the USA notably Alvan Clark and John Brashear) produced both types.
What may then have gone into the decision a good century ago to buy the 30 cm refractor?
First there is the consideration of resolving power. It is given by Dawes criterion; the smallest resolvable angle ? is given by ? = 11.6/d arc sec, where d is the diameter (in cm) of the lens or mirror. If one chooses a 30 cm diameter objective, ? is 1/3 arc sec. Such high resolution is not obtainable due to turbulence in our atmosphere (twinkling), which at best allows a resolution of ½ arc sec. So a 12 cm diameter objective of excellent quality could resolve all we can see from earth. The larger diameter just helps to bring out dim objects. For study of the planets and of double stars a 30 cm diameter is excellent.
Indeed, our telescope can under the proper atmospheric conditions see the Cassini division in Saturn's rings, Mars' polar cap, Jupiter's bands and red spot and the white dwarf in the Sirius double star system equally well as the largest telescope. A telescope with a 30 cm diameter seems to be a good compromise between cost and light gathering power, while having the maximum resolution possible as limited by atmospheric conditions. Why a refractor instead of a somewhat less expensive silvered reflector? Here, I believe a major consideration may have been that silver tarnishes and needs periodic resilvering. A refractor can be used for many years before a simple cleaning process (to remove dust) is applied. Another additional benefit is that a refractor is closed by a lens on both ends, thus reducing tube air turbulence, which is known to be a factor in reflectors.
John Brashear was probably chosen as the builder, because he had made excellent, medium size, refractors, aided by designs from Prof. Charles Hastings of Yale University (ref. 3k). Brashear wrote an autobiography in which he mentions the lens made for Ladd Observatory and gives an interesting insight in his development as telescope builder (ref. 8).
As telescope development continued, Brashear's silvering process gave way to vacuum aluminization. Aluminum is preferable to silver because it does not tarnish, and reflects the UV better. A thin transparent coating of Al203 protects the metal quite effectively. Vacuum aluminization was first used in 1932 on a 12 inch reflector (ref. 3l) and was subsequently used in 1943 on the 84 inch reflector at the Dominion Observatory at Victoria B.C. and in 1948 on the 200 inch mirror in Pasadena. One of the driving forces for vacuum deposition of aluminium metal was the need of the growing radio communication industry which required an excellent vacuum for electrons to move in and work as amplifiers of weak radio signals.
Further progress in telescopes of the future will probably not be so much in a further increase of mirror diameter but most likely in using various strategies to detect and correct for air turbulence including using telescopes such as the Hubble telescope in outer space, and to interconnect several smaller mirrors. But while these developments and other will go on, such is the logic of science, we can here on earth enjoy the beautiful views offered by our century old treasured refractor.
- "Fernrohre und ihre Meister" by R. Riekher, VEB Verlag Technik Berlin (1957) p 13, 17, 9, 21, 18, 86 (a - f)
- "The Principles of Optics" by A. C. Hardy and F. H. Perrin, Mc Graw-Hill Book Company, New York (1932), p. 1
- "The History of the Telescope" by Henry C. King, Charles Griffin & Company Ltd. London (1955). p 27, 31, 30, 36, 74, 77, 144, 262, 270, 264, 351, 382 (a - l)
- "The Telescope Makers" by Barbara Land, Thomas Y. Crowell Company, New York (1968), p. 2, 99, 100, 122 (a - d)
- "Sidereus Nuncius" by Galileo Galilei translated by E. S. Carlos, 1880, p. 10.
- "Principles of Optics" by Max Born and Emil Wolf, Pergamon Press, Oxford, 1975, p. XVI
- "Opticks" Sir Isaac Newton (original 1704) Dover Publication Inc., New York 1979, p. 109, 82, 102, 105 (a - d)
- "John A. Brashear" (Autobiography) The American Society of Mechanical Engineers, New York 1924, p. 247.