The telescope which discovered the Venus’s atmosphere

Posted in telescope on July 7, 2012 by yuri80232

©2012 Yuri Petrunin

What is common between John Dollond (1706-1761) and Mikhail Lomonosov (1711-1765), why are their portraits are placed here side-by-side? The former is English optician, the latter is Russian polymath, they were born at the beginning of the 18th century, and both died just few years after making their major achievements: John Dollond had invented the achromatic telescope, and Mikhail Lomonosov had discovered the atmosphere of Venus. Let‘s see whether we can find more connection between those two great men.

The invention of the telescope early in the 17-th century has dramatically changed our understanding of the solar system and the universe, but the telescope itself did not change much for a while. Even 150 year later, the telescopes had the same type of the objective – a single lens. The changes over that time were mostly related to the quality of the optics, the size of the aperture which modestly increased by 10-15 times, the length of the instruments changed a lot from 1 foot hand-held telescopes to 210 foot long ones (C. Huygens) and longer. Increase of the length was a necessary step to overcome the main problem of the refractive optics – chromatic aberration. The eyepiece had been changing as well: from a single lens of the Galilean or Keplerian type to a 3-lens Schyrlean (1745) or 4-5 lenses eyepieces for terrestrial observations. In 1662 C. Huygens invented a new two-lens eyepiece for astronomical observations.

The year of 1758 has brought to the market a new two-lens objective telescope from the optical shop of John Dollond & Son in London. A combination of flint and crown glasses had greatly reduced the chromatic aberrations and that was a breakthrough in the refractive optics since the Lepperhey’s invention of the Galilean telescope! The Royal Society had appreciated Dollond’s work by awarding him the Copley Medal of in 1758. The invention of the two-lens achromat principle was not only theoretical, but very practical, as the astronomers got a new type of relatively compact powerful refractors.

This new instrument changed the classification of telescopes. They now were divided to terrestrial and astronomical ones, reflectors and refractors and the latter ones were classified by the number of lenses in the objective: single-lens (singlet) and double-lens (doublet) achromatic. A three-lens (triplet) objective was added to the classification by Peter Dollond in 1763 as a further step in improving the refractors by reducing spherical aberrations. That was not as big of a step, as the first one, it was more of a technical solution: by dividing the optical power of the crown lens in two parts and making two crown lenses instead of one, the spherical aberration which is proportional to the square of the lenses curvature has been significantly reduced… Going further in this direction is an interesting subject by itself, but the main purpose of this article is different – namely, identification of the telescope that was used to discover the atmosphere of Venus.

As it was written in our previous article (Early gears for seeing Venus atmosphere) we have quite limited information about the instrument used by Lomonosov for observations of the 1761 transit of Venus.  Let’s go into the available details. The best source of any historical research should be original articles and documents, and I will be using scanned pictures from the Lomonosov’s original article published in July 1761 “The appearance of Venus on the Sun, observed at the St. Petersburg Imperial Academy of Sciences on May 26, 1761”. For those, who cannot read Russian, I would recommend the most detailed modern English translation by V. Shiltsev [1]

The article contains a plate with several drawings of the event, and the two upper ones Fig.1 and Fig.2 are the most important for identifying the Lomonosov’s telescope.

Fig.1 shows the ABC sketch of the Sun. The beginning of the Venus transit (ingress) is at the point B on the right, and the end of transit is in the point A on the left. As we see the trajectory of the Venus goes through the upper half of the Sun’s disc from right to left. Comparing this sketch to the actual path shown on the image below gives as the first key info: Lomonosov used the astronomical telescope that gives an inverted image.

From the text of his article we know that he used a 4.5-feet long two-lens refractor, while Krasilnikov – the other observer of the St.Petersburg team – observed the event with a 6-feet two-lens refractor.  Both of these two-lens design telescopes can only be of one type – the recently invented (1758) Dollond’s achromatic refractor. It took a bit of time for a new expression “achromatic” to become a commonly used term. In the very beginning, the difference between the new refractors and the old ones was the existing of the second lens (“glass”) in the objective. Therefore, we have now concluded that Lomonosov’s was a 4.5 foot doublet astronomical telescope.

The only type of the eyepieces available for the astronomical telescopes of that time was the Huygenian eyepiece with a typical field of view (FOV) of 20 – 25 degrees.

Fig. 2 from the Lomonosov’s article shows the image of the Sun with Venus on it as it would be seen between the second and third contacts. Lomonosov described the off-axis performance of his telescope in details:

 “…it was clearly noticed that as soon as Venus moved off the axis of the tube and approached the proximity of the edges of field of view, a fringe of colors would appear due to the light rays refraction, and its [Venus] edges seemed smeared the farther [it] was from the [tube] axis X (fig. 2).”

We have no possibility to test the original Lomonosov’s telescope as it did not survive the World War II. What we can do is to create a model based on actual data of the early Dollond Flint-forward telescope made in 1758-1760 [2], and see whether Lomonosov’s description of the off–axis performance of his telescope is similar to such modern model.

The spot diagram of the telescope objective is shown on the left and for the eyepiece – on the right. The images are given for linear off-axis shifts from 0 to over 6mm, that corresponds to field angles from 0 to 0.3degrees for the objective, and from 0 to 10 degrees for the eyepiece (half field of view).

The box size is 200 micron for objective diagram; and 100 micron for eyepiece. Circles correspond to the Airy disc. As we can see, both objective and eyepiece have almost diffraction limited performance near the center of FOV, while the off-axis performance of eyepiece is affected by aberrations.

The next diagrams show:

The combined performance of the telescope and human eye (on the left) and  Zemax generated image of an artificial star as it would be seen near the edge of view (on the right). And again, the telescope-eye system spot size is approximately equal to the size of the Airy disc near the center of the FOV where we have quite decent image quality. The monochromatic images near the edge are affected mainly by coma and to a lesser extent by spherical aberration and astigmatism. The lateral chromatism extends the monochromatic images of different wavelength across the axis and produces an image of colored strip of few arc seconds long – which is similar to Lomonosov’s description of the off-axis performance of his telescope. The only difference is that the modeled image is for an artificial star, while Lomonosov observed black disc of Venus over bright background of the Sun. In his case the image of the Venus (when planet is near the edge of FOV) did extended in the radial direction from the point X (Fig. 2), and the edges of Venus were also colored radially with the outer edge being more colored and deformed. The shape of the Venus disk when being near the edge will be radially extended – and that what we see on the Lomonosov’s sketch Fig. 2.

Therefore, we can assume that the computer model of the early telescope is close to the telescope used by Lomonosov.

One would see similar distortions not only for the Lomonosov’s telescope, but for any other refractor citrca the 18th century Venus transits with Huygenian eyepiece. The off-axis performance of the telescope could be count as questionable from today’s perspective, but the 18th century astronomers had no choice. After all, perfect on-axis performance of Huygenian eyepiece and 2-3 times wider FOV makes Huygenian eyepiece a winner comparing to a single-lens eyepieces.

The solution of above mentioned problems is simple and given by Lomonosov in the same article:

“Therefore, during the entire observation the tube was permanently directed in such a way that Venus was always in its center, where its [Venus’] edges appeared crispy clear without any colors.”

Lomonosov’s Fig.2 tells us that Venus was  near the edge of the FOV of the eyepiece, therefore one can conclude that the FOV was approximately equal or slightly larger than the angular size of the Sun, i.e., about  half a degree.

Knowing that a typical FOV of the eyepiece is at most 20-25 degrees, we can calculate that the Lomonosov’s telescope had magnification of about 40x or slightly more.

We now have enough evidence to to summarize Lomonosov’s presumed telescope:

Type of the telescope: astronomical doublet-achromatic refractor

Focal length: 1200-1400mm

Aperture: ~50mm

Power: ~40X

Maker: J.Dollond & Son

Country of origin: England

The direct evidence of the origin of telescope can be found in the digest of articles on the 100th anniversary of Pulkovo Observatory (prepared in 1939, printed in 1945, right after the end of the WWII).  The last article of that book by A. Nemiro “Museum Astronomical Observatory at Pulkovo” quotes:

”…the most numerous group of tools are the instruments ordered to observe the transit of Venus across the solar disk in 1761 and 1769. This group includes achromatic telescopes by Dollond, the reflectors by Short of the Gregorian design with lens-heliometers, and quadrants by Sisson. One of these Dollond refractors was used by famous Lomonosov to make the biggest discovery – he discovered the existence of an atmosphere of Venus during its passage through the disk of the Sun in 1761”.

So, we can now add to the story of the two great men that John Dollond made the telescope used by Mikhail Lomonosov to discover the Venus’s atmosphere!

arXiv:1206.3489

2 Rolf Willach “ New light of the Invention of the Achromatic Objective”. Notes and Records of the Royal Society, London. 1996, pp. 203-204.

The author would like to express his gratitude to Dr. Valery Terebizh for the Zemax evaluation of the early telescope-eyepiece combination. Discussions on the subject with Alex Rychenko and Dr. Igor Nesterenko are greatly appreciated, as well as Dr. Vladimir Shiltsev’s help in editing the manuscript.                                                  YP 07/07/2012

If any questions, please contact author via email: yp3141@gmail.com

Early gears for seeing Venus atmosphere

Posted in telescope on June 4, 2012 by yuri80232

©2012 Yuri Petrunin

The beginning of the 17th century brought to the life a new device – a telescope – and that helped to discover many new celestial bodies.  The observable world tremendously expanded and human fantasy started to populate it with people, animals and plants. Moon and the closest planets were the first candidates to search for the extraterrestrial life. The atmosphere, was thought to be required for any life forms of the other worlds, was in mind of the observers and was “in the air” – it was “wanted” and actually discovered; the trees on the Moon (W. Herschel) and the canals on Mars (G. Schiaparelli) were observed later.

The mid-18th century was quite a different world from ours – simple life, simple food, no TV and internet gave people more time to look at the sky, which was dark, free of satellites and light pollution. Kings sponsored construction of Observatories, patron science, equipped expeditions and supported scientists. England was leading the industrial revolution, supplying the first class optical instruments to other countries. English masters were invited to work abroad while exchange apprentices (“industrial spies”) from other countries came to work at the best instrumental shops of the “foggy Albion”.

A new type of telescope, two-lens (doublet) achromatic refractor was patented by Englishman John Dollond (1758) and immediately introduced to the market. Comparing to its predecessor – a single-lens objective (singlet) telescope, which was the dominant astronomical gear from 1608 (H.Lipperhey)  to the end of the 17th century, the new telescopes were much shorter and better quality,  as the chromatic aberration was reduced by combination of crown and flint lenses. Several of those new instruments were used for observations of the first Venus transit of 18th century and many dozens during the second one.

These rare astronomical events of 1761 and 1769 gave an opportunity for measuring the size of the solar system and find the fundamental astronomical unit (AU) – the mean Earth-Sun distance. Hundreds of astronomers from many countries participated in the observations and the first results on the AU were obtained at the end of the year 1761 (J. Short).

This article is about the telescopes used to see the Venus atmosphere phenomena during the transit of 1761. Let’s go back to 1761 to check at least the observations with known size and type of instruments and among those who saw the phenomena.

J.B.Chappe d’Auteroche, French, assistant astronomer at the Royal Observatory (Observatoire de Paris). Observed in Tobolsk, Siberia, Russia.  Telescope: 19 Paris feet refractor (possibly singlet), powers ~75X.

S.Dunn, Englishman, Master of the Academy. Observed at Ormonde House in Chelsea. Telescope: 6-feet Newtonian reflector with 6-inch primary mirror made by Mr. Dollond (Peter), power used 110X and 220X. Telescope was used before the transit on observation of the Jupiter’s satellites occultation, solar maculae, etc. During the Venus transit 1761 he observed: the last half of the transit, interior contact (the 3rd contact) and exterior contact (the 4th contact) at the egress. Border of the light surrounded Venus while on the disc of Sun was interpreted as caused by the Venusian atmosphere.

B. Ferner, Swedish, Professor of astronomy at Uppsala. Observed the transit near Paris, France. Telescope: “good” 28-inch focus reflector, 5” aperture, powers 80X.

M. Lomonosov, the first Russian-born Academician of the St. Petersburg Imperial Academy of Sciences, observed the transit from St. Petersburg. Telescope: 4 ½ feet two-lens refractor. Observed entire  transit and saw luminous phenomena: blurriness of the Sun’s edge at the beginning of ingress (contact 1) and end of the egress (contact 4), “hair-thin bright radiance” between Venus and Sun shortly before the second (internal) contact, arc of light around the part of Venus off the Sun’s disc at the egress (after the 3rd contact) ; correctly explained the mechanism of phenomena and came to conclusion that “the planet Venus surrounded by a considerable atmosphere equal to, if not grater one which envelops our earthly sphere”. Lomonosov also was the first to publish the report about the Venus atmosphere in Russian (June 1761) and in German (Augusrt 1761).

P.W. Wargentin, Secretary of Sweden Academy of Science, Stockholm, Sweden. Telescope: “an excellent” 21 feet refractor, powers ~80X. Reported: “at the exit, when part of Venus was quite emerged, it was visible faintly illuminated. But whether this was owing to an inflection of the Sun’s rays or to a reflection in atmosphere of Venus, he leaves to others to determine”.

The telescope described in most detail was Dunn’s Newtonian with the mirror made by Peter Dollond. The exit pupil was 1.4 – 0.7mm.

The reflector of 5” aperture used by Ferner was possibly of the Gregorian type counting primary relatively short focal length of 28” and high power used 80X, the exit pupil of 1.6mm.

Chappe and Wargentin used both similar instruments, possibly singlet refractors with the exit pupil of ~0.8mm.

Lomonosov was observing with possibly one of the first Dollond achromatic refractors. Such 4.5 feet refractors had objective diameter of 1.5-2.0” and magnifying power of 30-40X. The Exit pupil ~1-1.5mm. Similar instrument was used during transit of Venus 1761 by French astronomer N. de LaCaille.

As we see there are significant variations in the type, apertures (from 2 to 6 inch), and magnifying powers (from 30X to 220X) of instruments used for the observations, but the exit pupil used by the observers was in the range of 0.8-1.5 mm – that corresponds to an average maximum resolution of the human eye. If the instruments were of high quality, then one could quite believe that the observers should be able to see the fine details such as thin shiny line around the Venus when entering or exiting the Sun’s disk. It might be a combination of good instruments, proper solar filter, good seeing condition on Earth, favorable states of the solar photosphere and Venusian atmosphere and skills of the observers that made the discovery of the atmosphere at the Venus happened at that time. The optics for telescopes made by famous named opticians that were staying behind their work should not be questioned.

Can the above observations be repeated during the 2012 transit? We may find in collections or museums same or similar instruments used for the transit of Venus 1761, we may find place with good seeing, but  seeing the phenomena still might be questionable since we cannot repeat one condition – the solar activity at that time. That might be a key factor that made the Venus atmosphere discovery happens in 1761. It is known from chronicles that 1761 was a year of high solar activity and of a maximum number of Sun spots. Counting that we all live inside of the atmosphere of the Sun, additional research should be made to support the idea of easing seeing phenomena of the Venus atmosphere in correlation with activity of our closest star.

Addendum:

We have very limited information about instruments used by Lomonosov and other Russian observers during the first of the transits of Venus in the 18th century, but there are documentary records that St. Petersburg Imperial Academy of Sciences had ordered for the second event 6 years later in 1767: 17 achromatic two-lens refractors made by Dollond, 6 of them of 12 feet, 8 of 3 feet, 2 of 8 feet with Graham micrometers, and one of 10 feet length with two eyepieces. Another achromatic telescope had a three-lens design objective (triplet) – that was a new Dollond’s development introduced in 1763. Also ordered were two Dollond’s divided micrometers for Gregorian telescopes; two 2-feet Gregorian telescopes by J.Short and one 3-feet Gregorian telescope by the same maker (that most expensive telescope was prepared not for scientific purpose, but for VIP person, who sponsored all the preparation for the transit of Venus 1769 observations in Russia – Empress Catherine the Great).  So, as Russia was equipped with such large number of Dollond instruments for the second transit of Venus in the 18th century, one may reasonably assume that Russian observers were equipped by the instruments of the famous London maker for the first transit as well.

Reference: http://www.transitofvenus.nl/aureole.html

If any questions, please contact author via email: yp3141@gmail.com

The First Telescope

Posted in telescope on February 27, 2012 by yuri80232
©2012 Yuri Petrunin Edited by Thomas Dobbins

A typical caption for this iconic illustration reads: “Hans Lipperhey, a spectacle-maker in Middelburg, held a concave lens in front of his eye and a convex one farther away, combining them into the first telescope.” Would the presence of both convex and concave lenses in spectacle shops be sufficient to make a telescope? Why would the craftsman hold lenses in such a way? Perhaps he was simply checking the quality of the surface of the convex lens with a magnifier? That certainly makes more sense. Checking the surface polish during lens fabrication was a routine procedure. Could more details be seen on the surface of the convex lens by placing a concave lens near the eye? No, the convex lens appears diminished in size when viewed through a concave lens. However, a convex lens combined with a convex magnifier gives the Keplerian rather than the Galilean type of telescope invented by Lipperhey. So which type of telescope was made first?

Introduction

The invention of the telescope occurred several centuries after the introduction of spectacle lenses. Why did it take so long to make a telescope from spectacle lenses? The answer can be found in Rolf Willach’s book, a detailed account of the technology and art of lens making from early glasses to the first telescope [1]. However, one seemingly small question has remained unanswered: How did the inventor of the telescope discover his ingenious method of lens improvement, or how did Lipperhey think of the idea of using a diaphragm? This article will delve deeply into the technical aspects of lens making and provide new insights about the invention of the telescope.

Spectacle lens versus telescope objective

In spectacles, a pair of lenses placed a modest distance in front of the eye helps to focus images on the retina. For the average eye, the pupil is dilated to a diameter of only 2 to 3 millimeters in diameter under daylight conditions, so only the small central part of a spectacle lens actively assists direct vision.  The remainder of the lens serves chiefly for orientation.

When a lens serves as a telescope objective, it is placed a considerable distance from the eye and its entire surface forms the image of a distant object at its focal plane. An additional lens placed near the eye is required to see an enlarged image.

The lenses in spectacles must be free of small-scale defects such as scratches, bubbles, and prominent striae, at least near the center. Being comparable in size to the daylight opening of the pupil of the eye, any of these defects will have a pronounced effect when viewing through this lens.  Such defects can be detected when selecting a suitable piece of glass prior to actually fabricating a lens. Large-scale defects like surface irregularities and variations of the refractive index within the body of the lens are not as critical because only a small part of the lens is in use.

When a spectacle lens serves as a telescope objective, these small defects do not have as pronounced an effect on the quality of the image because they have a relatively small combined area. Conversely, an objective lens is more sensitive to large-scale defects like surface irregularities and internal inhomogeneity. These large-scale defects can seriously degrade the image, but they are not easily detected during lens material selection and may only be seen after the fabrication of the lens is complete. Considering that spectacle lenses were relatively thin, inhomogeneity is less problematic than surface irregularities.

Now we must consider the lens itself. It is a round piece of transparent substance with at least one of its surfaces curved in order to change the convergence of light rays.  Glass was the most common substrate for lenses. From early times, lenses had one or both surfaces convex and were used as magnifying and burning glasses, or as a tool to correct the most common vision condition of “old eyes,” presbyopia or far-sightedness, the inability to focus on nearby objects.

Let us divide the spectacle lens into zones according to their function. The central zone measuring two to three millimeters in diameter is used for direct vision, while the surrounding zone of at least 10 millimeters diameter is used for peripheral vision. Only the central part of the lens must be of good optical quality. The surrounding outer zone must be transparent, but it does not play a role in correcting vision. However, as a consumer product spectacle lenses must be polished to the edge for cosmetic reasons.

In the case of a telescope objective, the entire surface must be perfectly spherical in order to form a sharp image. Let us also check if the spherical and chromatic aberrations would be affecting the view through a telescope made with spectacle lenses. For most common group of customers aged 40 to 50, [2, 232] the focal length of spectacle lenses was about 300 millimeters. This result comes from averaging the focal lengths of 12 spectacle lenses measured by Willach [1]. With a typical diameter of 30 millimeters for the spectacle lens, spherical aberration is negligible. In fact, a 30 millimeter diameter spherical lens with a focal length of 300 millimeters is diffraction limited for monochromatic light. Chromatic aberration for such a lens would not be as easy to overcome, but we must consider that the first telescopes magnified only 2 to 4 times. At such low magnifications, the spurious color fringes around the image would not be readily visible because they would be near the eye’s resolving power in size. Consequently, irregularities in the outer zone of the lens would remain the major limitation.

Lens making techniques and the internal quality of glass in early times were sufficiently advanced to produce spectacle lenses that used only one to two percent of lens surface to help the eye to focus, but a lens of such diminutive size would not be sufficiently large to serve as the objective of a telescope, a device for “making far objects appear close” that requires higher resolving power than the naked eye. To obtain superior resolution, the good part of the lens has to be larger than the opening of the pupil of the human eye. Otherwise the subject would be seen larger, but no details would be added. Furthermore, if the larger outer zone of the lens had an irregular shape and did not bring light rays to a common focus, the resulting image would be blurred unless the peripheral “bad” part of the lens were masked off.

The difference between the size of the useful area of the lens for spectacles and for telescopes was the major factor in keeping the invention of the telescope at bay for several centuries. However, the year 1608 brought a new device to life. Hans Lipperhey, a spectacle-maker from Middelburg, “found a certain art with which one can see all things very far away as if they were nearby“ and demonstrated a working model of the first telescope at the end of September [3,11].

Lens Making Techniques of the 17th century

Let us focus on the routine work of an ordinary spectacle shop. Work began with the selection of glass. The most common material for lenses was ordinary plate glass made for windows [4] or pieces of Venetian glass made for mirrors. Both surfaces of these materials were polished. Selection was performed by a preliminary visual inspection to find pieces with minimal inclusions and bubbles and good polish, at least in the central part of a future lens. The next step was to determine which side of the glass had better flatness by examining oblique reflections from each surface. The side with less distortion of the reflected image was kept untouched and the opposite side was ground to a nominally spherical curve.

Once pieces of glass were selected they were rounded or shaped to a common size. Typical spectacle frame openings measured about 30 millimeters in diameter.

Rough grinding to impart a convex surface was performed using concave metal tools. This process continued until the ground surface of the lens assumed the opposite form of the tool and became uniform from center to edge.

Fine grinding was performed using the same tool using successively finer abrasives until rough grinding marks and scratches disappeared. At the end of this stage of work the lens surface could produce a reflected image of a bright object and the worker could determine the uniformity of the polished surface by looking at the surface at an oblique angle [5]. The completed finely ground surface usually had a decent spherical shape of opposite sign to the tool that created it. It should be noted that a metal tool wears at a much slower rate than a glass work piece, a vital factor when producing a multitude of lenses with the same curvature.

Once residual particles of abrasive were washed away at the end of fine grinding session, adding a few drops of water rendered the surface of the lens transparent (at least in the central part). The surface was then carefully examined using a magnifier. If scratches were detected, the lens was returned to one of the previous fine grinding stages. It was important to eliminate scratches at this stage so that less time would be consumed during the next step, polishing.

Polishing operations were performed using a different tool. This took the form of a round piece of wood with a flat or slightly curved surface larger in diameter than the lens diameter. Covered with felt or deer skin, it could be affixed to the workbench or rotated on a spindle. During the grinding stages the surface of the lens was gradually made to conform to the hard concave surface of the metal tool, which served as the “master” reference surface, but during the polishing stage the lens surface itself served as the “master” for the pliable surface of the polishing tool.

There are several ways to polish a lens, and not all of them give the same result. Variables include the speed of tool plate (if the tool is rotating) or the speed of lens moving across the tool (if tool is fixed); the relative positions of the lens and the surface of the tool; the number, length, direction, and configuration (straight, circular, oval, etc.) of strokes. All of these factors play a role in determining the quality of a lens.

During the polishing operation, the surface of the lens becomes shiny at the center first and gradually progresses toward the edge because the smaller particles of polishing agent tend to concentrate near the center and larger particles toward the periphery. As a result, the central portion of the lens polishes out first. A spherical shape is imparted to the central zone of the lens, but the outer zone must be polished out by holding the lens holder at an angle relative to the tool, which is not conducive producing a uniformly curved surface. When the entire surface of the lens is polished out, only the central zone will have comparatively good optical quality. The surface of the outer zone will have an irregular shape, but for use as a spectacle lens this result is satisfactory.

If a more steeply curved tool is used to polish out the outer zone, or if the original grinding tool is covered with felt or deerskin for polishing, or if the polishing tool is applied in such a way that the edge of the lens polishes out first, the effect will be to impart an irregular surface at the edge zone early in the polishing operation. This irregular outer zone will spread from the edge towards the center as polishing proceeds, destroying the relatively decent spherical surface that existed when the grinding operations were complete. If the entire surface of the lens has an irregular shape, it will be unsuitable even as a spectacle lens.

Lenses for spectacles were not produced singly but in batches of the same focal length, passing from together from the rough grinding to the fine grinding to the polishing stage. When a batch of similar lenses was completed, the sole remaining operation was to mount them in frames.

Lens Conditions and Combinations

By the dawn of the 17th century the typical spectacle-making shop produced lenses to correct both myopia (“near-sightedness”) and presbyopia (“far-sightedness”). In other words, both convex and concave lenses were made. Two possible combinations of such lenses could be configured to produce a telescope of one of the early forms. Combining a positive (convex) objective lens with a negative (concave) eye lens gives the Galilean form of telescope, while combining a pair of positive lenses gives the Keplerian form.

The historian Albert Van Helden has concluded that the Galilean form had a better chance to be invented first [6, 17]. In my opinion, however, the probability of a telescope of either form was vanishingly small due to the uniformly low quality of spectacle lenses, which only had a decent optical figure over a very small central zone.

The availability of both positive and negative lenses is a necessary, but not a sufficient condition for obtaining a sharp, un-inverted, magnified image. An objective lens of typical spectacle lens quality must either be opaque near the edge (due to incomplete polishing) or its irregular outer zone must be masked off with a diaphragm. When we envision the optical scheme of a telescope, finding the right combination of lenses seems like an almost trivially simple exercise. But why did Lipperhey, a mere spectacle-maker, succeed and not the likes of Roger Bacon, Giambattista della Porta or Girolamo Fracastoro, who actually conducted optical experiments?

The Inventor

Experienced lens makers were aware of two possible ways to polish a lens. Evangelista Torricelli, well known during the mid-17th century for his comparatively large lenses of high quality, left us with rare descriptions of early optical fabrication techniques. One of several important remarks contained in a 1643 letter to his friend Raffaello Magiotti concerned polishing: “…never should one polish [a lens] on the same tool plate that has worked it, because it gets polished first at the edge, and then very late in the middle, and they [lenses] do not turn out well” [5, 30]. Torricelli’s remarks referred to telescope objective lenses, but they applied to spectacle lenses as well.

Willach’s tests [1, 95] of 57 spectacle lenses made between 1500 and 1630 showed that only five examples had a good central zone of 10 millimeters diameter that potentially could surpass the resolution of the naked eye, making them suitable for use as telescope objectives provided that the remainders of their surfaces were covered. The overwhelming majority of lenses was of very poor optical quality and probably made using the polishing techniques described earlier.

Should we regard Lipperhey as an experienced maker of spectacle lenses? He was 38 years old and had been making spectacles for six years prior to the invention of the telescope. No examples of his handiwork survive, so we can only speculate about the quality of his work. But we do know that the performance of his first telescope impressed the burghers of Middelburg, who looked through it. Moreover, within a short time he was able to produce three “improved” instruments suitable for seeing with both eyes, namely binocular telescopes. These facts certainly suggest that he was a maker of lenses of exceptionally high quality. Making a binocular telescope is a demanding task not only for the precision required to align two telescopes in parallel into a single instrument, but also for matching the focal lengths of the lenses used in both telescope tubes to a degree that is far more exacting than would be required for a pair of lenses in single spectacle frame. In a six-month period Lipperhey made at least seven telescopes that are referred to in contemporary documents, the first specimen as a single instrument, followed by three pairs of binoculars. While he was not awarded a patent for his invention, he was handsomely paid for his work. The sum was sufficient for him to buy a large house where he continued making spectacles and telescopes.

Some readers may consider the details about the grinding, polishing, and testing of lenses as rather boring.  However, the details of the routine work of a spectacle-maker’s shop – a thorough understanding of the steps involved in making a lens — contains the key to answering a very important question: How did the inventor of the first telescope arrive at the idea of covering the part of the lens that had an irregular shape and gave distorted images?

The Act of Invention

It is probable that Lipperhey’s invention of the telescope stems from a chance occurrence when he was performing routine tests on batches of lenses, either the end of the fine grinding stage or at the beginning of the polishing stage. At these times the central part of the lenses becomes transparent, but their edges remain opaque. This is the only condition in which a spectacle lens is capable of forming an image that is not seriously degraded by irregularity of the outer zone. Checking the surface of a lens for imperfections with a magnifier by looking at and through it against the uniform background illumination of the sky was probably the fortuitous situation when Lipperhey was able to discern the magnified image of distant objects like the rooftops of the houses across the street.

The only problem with this combination of a pair of positive lenses is the inverted image it produces. One would certainly hesitate to demonstrate an instrument that showed everything upside down, let alone apply for a patent! But Lipperhey had in his shop both convex and concave lenses. Trying to cure the problem of inverted images, he would have replaced the magnifier with a concave lens and combined it with the same unfinished convex lens. By moving the convex lens farther from the concave lens he obtained an uninverted magnified image of distant objects. According to Albert van Helden, the telescopic effect is very easy to discern with the Galilean lens configuration [6, 18], and I certainly agree with the author in this instance.

It seems probable that Lipperhey knew of both the Keplerian and the Galilean form of telescope. He stumbled upon the Keplerian form while performing a routine inspection of spectacle lenses with a magnifier, then he deliberately devised the Galilean form that employs a negative concave eye lens when he attempted to solve the problem of the inverted image. We can regard the Keplerian form as a discovery, while the Galilean form deserves the status of an invention. Although he was not a scientist, Lipperhey deserves the lion’s share of the credit for the novel instrument that he invented by chance. He left the astronomical discoveries for others. He may not have even looked at celestial objects through his telescope – what would be the reason? There were so many interesting earthly subjects for the magic of his new instrument.

Since spectacles lenses had to be finished with polishing to their very edge, Lipperhey would have determined that masking the outer part of the lens gives virtually the same result as leaving an opaque unpolished edge zone. This was the birth of the diaphragm or aperture mask, an elegantly simple way to prevent the optically imperfect outer part of the lens from destroying the sharp image created by the good central part. This idea was soon repeated by others.

Because the telescope was a practical invention, not a theoretical one, Lipperhey was first simply because he was using better polishing techniques than his contemporaries.  Compared to optical scholars who had a “lack of attention to lenses” [7], Lipperhey was obliged to pay very close attention to the art that was the source of his livelihood, and he had to test a host of lenses. The State General not only accepted his creation but soon asked him to construct a more complicated device – the binocular telescope.

Conclusion

The invention of the telescope at the beginning of 17th century became possible only when the central part alone of a spectacle lens was used to form the image. Full-aperture lenses performed adequately for spectacles, but they were not suitable for telescope objectives. This requirement makes it clear that the stories about children playing with lenses being responsible for the invention of the telescope or Girolamo Sirtory’s tale [3, 19] of the stranger who visited the shop in Middelburg, tested lenses in front of their maker, who then got the idea of constructing a telescope, are all sheer fantasy.

The leap from the low-power terrestrial telescope to the first astronomical instruments made by Galileo required using a diaphragm as well. That simple refinement remained a key optical aspect of the telescope for over a century.

Author’s Moment of Revelation

I make my living making modern telescopes.  Antique telescopes are my hobby, an escape into the past from the everyday routine of the opto-mechanical workshop. My thoughts first turned to the invention of the telescope in September 2008 during a visit to Middelburg, where I attended a conference on the occasion of the 400th anniversary of the invention of the telescope along with other members of the Antique Telescope Society. Even after listening to several interesting lectures at that conference, notably those by Rolf Willach, Giuseppe Molesini, Marvin Bolt, Sven Dupre and studying the subject further, I did not arrive at any original insights.

The breakthrough came two years later after a friend in France asked me to solve a problem with the 110-millimeter aperture air-spaced doublet objective of his century and a half-old refractor made by Secretan. The achromatic objective did not give sharp images. Cursory tests revealed poor color correction and a very objectionable amount of spherical aberration. My first thought was that the objective had been incorrectly assembled, but that was not the case. In fact, the lens elements of the objective had been made very accurately, but it had been improperly designed.

Sometimes less effort is required to simply make a new lens from scratch than to repair a defective one. My friend simply wanted to observe with the telescope, so I suggested that I simply make a new objective lens to replace the defective original.  Glass blanks were purchased and rudimentary tooling was made. When the replacement objective was successfully completed, I decided to make several additional objectives from the remaining glass.

I was working on them for a couple of months after normal working hours and on weekends, performing all the stages of lens making. Once, when I was examining the surface of a lens element during the final stage of fine grinding using a magnifying glass in an attempt to determine if any scratches remained near the edge, it dawned on me that an unpolished edge acts like a diaphragm – an opaque outer zone surrounding a transparent central zone. This is the lens condition in which the irregularities of figure in the outer zone do not detract from the quality of the image. I said to myself this was probably the same circumstance that Lipperhey encountered when he was testing his lenses and observed the telescopic effect.

February 29, 2012.

If any questions, please contact author via email: yp3141@gmail.com

1 Rolf Willach, “The long route to the invention of the telescope”

2 Vincent Ilardi, “Renaissance vision from spectacles to telescopes”

3 Huib J. Zuidervaart, “The ‘true inventor’ of the telescope.  A survey of 400 years of debates” in the book “The origins of the telescope” (2010)

4 Rolf Willach, “The development of lens grinding and polishing techniques in the first half of the 17-th century” in “Bulletin of the Scientific Instrument Society, #68 (2001)

5 Giuseppe Molesini, “Telescope lens-making in the 17th century” in OPN Optics & Photonics News, April 2010

6 Albert Van Helden, “The invention of the telescope” (2008)

7 Vasco Ronchi, “The Nature of Light: A Historical Survey”.

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