Testing Huygens and others objective lenses

 As an amateur astronomer myself, I have always been very interested in the early development of telescopes in the 17th century. I consider it a privilege to tell you something about Christiaan Huygens and his telescopes. 

To me, Christiaan Huygens is a very fascinating person. Christiaan not only designed his telescopes, he also built them himself and he also used them. Christiaan’s first involvement with telescopes was in the autumn of 1652, when he was 23 years of age. He started to become interested in the art of lens grinding and he decided to get more information from the well-known instrument maker, Johann Wiesel from Augsburg, southern Germany. The optical instruments Wiesel made, such as spectacles, telescopes and microscopes, were sold throughout many parts of Europe. Even in London his price lists were distributed among potential buyers.

Based on information he got from his correspondence with Wiesel, Christiaan instructed a certain ‘Master Paul’ in Arnhem to build a telescope for him. This seems not to have been a success, because in the next year Christiaan decided to grind lenses himself, assisted by his older brother Constantijn.  According to the research carried out by Rob van Gent and Anne van Helden, the Huygen brothers bought their glass from various sources, among others in Amsterdam and London, but later also from the important glass factories in the city of Hertogenbosch ( Bois-le-Duc ) in the center of Holland even from England.

At first the Huygens brothers had to rely on the experience of professional Dutch lens grinders, such as Jan van der Wyck from Delft and Caspar Calthoff from Dordrecht. Especially Calthoff had a fine reputation, but unfortunately for the Huygens brothers, he soon moved to England. 

Lens grinding and polishing is very delicate and precise work. It is also very heavy manual work and very time-consuming. To lighten this hard labour for himself, Christiaan designed and built his own lens-grinding and -polishing machines. One of the greatest problems with lens grinding is to make a suitable lap by utilising metal moulds with a perfect spherical shape. Another problem for the Huygens brothers was to find glass of high quality and homogeneous in structure. However, not only the quality was important: they also needed to acquire glass of suitable dimensions to be able to make a large objective lens for their telescopes. According to research by Rob van Gent and Anne van Helden, the Huygens’ brothers bought their glass from various sources, among others in Amsterdam and in London, but later also from the important glass works in the city of ’s Hertogenbosch (Bois-le-Duc) in the centre of the Netherlands. 

What made it very difficult to produce usable lenses, as the Huygens brothers soon found, was that the whole art of lens grinding was surrounded by secrecy. They couldn’t benefit much from the experience of professional lens makers, because these lens makers weren’t very communicative and they vigorously protected their trade secrets from the competition. One of the most surprising astronomical discoveries Christiaan Huygens made with one of his first self-made telescopes dates from March 25th, 1655, when he discovered that Saturn has a moon, revolving around the planet in about 16 days. The moon was later named ‘Titan’. 

    The telescope he used for his discovery was equipped with an objective lens (also known as an ‘object glass’) with a focal length of 10 Rhineland feet (about 337 cm). The eyepiece he used was a single-lens of 3 Rhineland inches (79 mm) focal length, resulting in a magnification of about 43x. Christiaan himself writes that he used a telescope of 12 Rhineland feet, but at that time this meant the total length of the telescope tube including the eyepiece, not the focal length of the objective lens. The telescope itself doesn’t exist anymore, but its objective lens, which is by far the most important part of a telescope, has been preserved. Researchers are fully convinced that Christiaan used this lens for discovering Titan, because he scratched with a diamond, in his own handwriting, not only his name and the date of manufacture of the lens (February 3, 1655) along the rim of the lens, but he also scratched on it in Latin “Admovere oculis distantia sidera nostris” (“they brought the distant stars closer to our eyes”)

On June 13th 1655 Christiaan wrote a letter to Prof. J. Wallis in Oxford telling him that he had made ‘a discovery’ with his new 12-foot telescope. However, he didn’t tell him exactly what his discovery was, but, as was the custom in those years, he re-wrote the sentence explaining his discovery into the following anagram: Admovere oculis distantia sidera nostris.

It is therefore convincing that this lens (in literature it is often nicknamed the ‘Admovere’) was used in Christiaan’s 12-foot telescope when he discovered Titan. It is very lucky that we can nowadays see and admire the ‘Admovere’ lens, because it once was nearly lost! The lens is referred to in an auction catalogue dated 1722, and two years later it was mentioned by the Dutch physicist ’s Gravenzande in his foreword to his publication about Christiaan Huygens’ life and works ‘Opera Varia’. But after that, the lens was never again mentioned, nor seen. It seemed that nobody was really concerned. Then, in 1867 it was fortunately discovered by Professor Harting in a little old box, containing some old lenses, in the Physical Laboratory of Utrecht.

The famous Admovire lens through which Huygens discovered the Titan satellite.

Testing in OSLO of the Admovire objective lens at different diameters.

  The lens in question was tested on the sky in 1883 and 1884 by J. Outmans at a diameter of 5.2cm and the Titan satellite was at the limit of visibility, a bright star could be seen as a patch of light with irregularities, a star with 5 corners (rays ) coming out of the airy disc as well as fractional diffraction rings. The lens works well at a diameter of 4.4cm (44mm) and at 30mm much better with more contoured diffraction rings as well as a round airy disc with 3 outgoing rays at 44mm diameter and much better at 30mm.

  Huygens was able to improve the concentration of light at a diameter between 35mm and 44mm in a Focault Grid test it was able to establish that the lens has a resolving power of 3.8 arc seconds, confirmed when observing double stars.

   In September 1681, Christiaan Huygens left France definitively to return to The Hague in her native country of Holland. At the Royal Observatory in Paris, he had practiced observing the planets. He had already largely contributed to perfecting himself the astronomical glasses. The glasses of the time had for main optics a simple lens, planconvex or bi-convex, only possibility for the time. With such instruments, several causes had been identified as responsible for a pernicious spreading of light: on the one hand, the geometric aberrations linked to the spherical shape of the lenses, of which Huygens had calculated elements of theory, on the other hand the defects of realization relating to the inhomogeneities of the mass of glass and the imperfections of shaping of the surfaces, although the artisanal practice began to master them. In addition, there were the colored effects, now known as chromatic aberrations. Still poorly identified at the time, they manifested themselves in practice by their consequences. To accommodate all of these constraints overall, it had been  necessary to increase the focal length of the lens. Thus, to acquire  the required qualities, an objective of 8 cm in diameter already  required a pipe of approximately 8 meters long. 

The use of such  bulky instruments required ingenious procedures. Returning to Holland, Christiaan Huygens became interested again in the problem of glasses. His brother Constantijn proved to be motivated and talented for the production of glasses and a collaboration was established between the two brothers. With her machine for polishing optical glasses, Christiaan produced in 1683 lenses 12 to 13 feet in focal length (in fact, about 4 meters), two of which, 6.7 cm in diameter, are currently kept at the Boerhaave Museum in Leiden , one of them signed by Christiaan on May 30, 1683. Constantijn, who lived in Dieren near Arnhem, in turn undertook the pruning of lenses with the same characteristics.  The action of the two brothers was first to make large terrestrial glasses, with a sliding tube,  with eyepiece with three lenses correcting the image, according to the so-called 'Campanine' arrangement adopted by the Italian optician Guiseppe Campani. The correspondence exchanged between the two brothers  attests to tests on  the planets by Christaan, on landscapes by Constantijn; it attests to the difficulties encountered with the mechanical behavior of the long sliding tube. The Boerhaave Museum in Leiden keeps such a telescope. 

Objective lens of 6.7cm diameter polished by the Huygens brothers

  But above all, Christiaan studied the creation of more powerful glasses for astronomical applications.The habit had been established to express the capacity of an astronomical telescope by its length, given as the only parameter defining the power of the instrument. But Christiaan Huygens knew that the real ability of a telescope to bring objects closer is measured by the value of the magnification, which is not linked the length of the telescope but the diameter of the objective. He also knew that the ability to detect weak stars, such as planetary satellites, must be measured on the surface of the objective, ie the square of its diameter. This, provided that all the light collected by the lens is well concentrated in one point. Hence the need to produce very fine images, and consequently to use a large focal length for this.
  The two brothers had been able to supply glass slabs allowing the release of lentils 11 to 13 cm in diameter. The polishing machine could give the surface curvatures giving a focal length of 35 feet, or about 10 meters. After adding a slight diaphragm to hide the edges of the lens, which was still defective, the ratio between the focal length and the diameter of the objective (aperture ratio) was close to 100. This value was necessary to ensure the finesse of the image, it already led to very long tubes. Christiaan produced a lens 11.5 cm in diameter and 10.96 m in focal length and signed it on May 10, 1683. The lens had to be already available in February 1683 because we note in the Register of Astronomical Observations  an observation session on February 26, 1683 on Jupiter at 7h and Saturn at 7hl / 4. An appropriate mast had therefore been erected on the property of the Huygens. The scope must have been the conventional pipe type, but we no longer have any information. Until May 1683, there were still 6 observation sessions on the planet Mars and 3 on Jupiter.  During this period, Constantijn began to manufacture other similar lenses. In Dieren, he adapted his polishing machine and received from Christiaan the caliber (the 'basin') with a radius of curvature 10.6 m which he used to make his 35-foot lens. This lens was symmetrical bi-convex and therefore used the same shaping caliber for each of the two faces. 
In October and November 1683, Constantijn produced, in a few weeks, at least four similar lenses, preserved in the Boerhaave Museum, and later another in 1684.  The action of Constantijn on the polisher was to be slightly different from the way 'act of Christiaan, because the resulting focal lengths, designated 34 feet, are slightly shorter (on average 10.16 m). 

  

Huygens Objective lens D 13cm 34 feet

Huygens Objective lens D 18cm 84 feet

Huygens Objective lens D 13cm 34 feet 3

Huygens Objective lens D 14.5cm  43 feet

  Through the years they produced lenses with ever longer focal lengths. Eventually Constantijn reached a focal length of 210 Rhineland feet (almost 66 m).

As the grinding and polishing of lenses was a time-consuming business, the production of so many lenses must have taken much of their time. But it would have been unlike Christiaan if he had not taken measures in this respect. Since the 1660s Christiaan had been designing machines to ease the laborious job of grinding lenses. Another measure was not to do all the work themselves. It is not entirely clear if the brothers made their own eyepieces around 1660, but they did not do so later on. Occasionally their correspondence contains references to local craftsmen who prepared glass or ground eyepieces ; the brothers focused on the delicate work of grinding the object glasses.  In 1667 and 1668, Constantijn employed Cornelis Langedelf for polishing glass and grinding eyepieces, and in 1683 this same man delivered the tubes for one of Constantijn’ s telescopes. From 1682 the brothers preferred the services of Dirk van der Hoeven, who lived nearby in The Hague. At the same time the brothers also did business with a marbler van der Burgh, who supplied them with grinding laps and glass. 

The relationships the brothers had with these two craftsman were not identical. In the case of van der Hoeven he was often simply referred to as Dirk or the chimney-sweeper it was only his labour that was hired. The brothers supplied the material and tools, including the grinding laps. Van der Burgh, on the other hand, had a workshop of his own, and the Huygens were not his only clients.Moreover, one might expect this marbler to have been a more skilful grinder than his chimneysweeping fellow citizen. So, it probably was not the routine preparatory work that Constantijn asked of van der Burgh in April 1686, when he sent him two pieces of glass to be attened. Perhaps Constantijn wanted van der Burgh to produce the optical flats that he needed for his 170-foot lens and 60-foot lenses. However, the documents of the period do not provide any other clue on how these flats were made

Testing the Huygens Objectives

  Here are the optical tests of other objective lenses made by the Huygens brothers in OSLO, it is actually a simulation using a type of glass that imitates glass from the 17th century, having a refractive index of 1.52 and 1.58.

The simulations are made in polychromatic, monochromatic and focus having a focus point on a screen in focus and the light source comes from infinity.

We have tested all the objective lenses available at the Rijksmuseum Boerhaave, all of them have been analyzed even the ones that are not on display.

I can say that these lenses do not reach the level of quality attributed to Campani, but they are reasonable, the only mention is that all the tests were performed using the available diaphragms of these lenses, many of them do not have preserved diapghrams, so I brought them to a chromaticity coefficient attributed to Huygens between 1.5 and 1.6, calculated according to focal length/diameter and we reached a diameter at which the image is clear.

Most of the objectives of the 17th century worked like this, in a proportion of 70% or even 80% they kept a coefficient between 1.5 and 1.7, it even reached a coefficient of 2 or 2.5, sometimes even 4 - 5 like the objective lens kept in the museum Lowman made in 1656 is actually one of the first lenses polished by Huygens, its maximum diameter is 62mm and it works at 30mm diameter using a diaphragm placed in front of the lens that reduces the edge of the lens, the focal length is 3650 and at 30mm diameter it reaches a factor of 4 to provide a clear image.

With the passage of time, the brothers became more and more experienced in making objectives so that in the period 1682-1687 they made better and better lenses with bigger and bigger diameters, almost all of them worked at a chromaticity coefficient of 1.5 towards 1.6, so in my test i used them all at this coefficient.

During his stay in France (the Paris Observatory) he was able to have access to better lenses polished by Giuseppe Campani as well as to learn new tricks in lens making techniques.

Returning home as a new baggage of knowledge he started work together with his brother on making large quality lenses.

Here are the tests below.

Objective lenses polished by the Huygens brothers at the Rijksmuseum Boerhaave

Objective lenses polished by the Huygens brothers at the Rijksmuseum Boerhaave

Objective lenses polished by the Huygens brothers at the Rijksmuseum Boerhaave

   I also tested other objective lenses made by famous masters such as Hartsooker, Campani and Marcel Arnold to compare with the objectives of the Huygens brothers.

Giuseppe Campani being the most skilled in making lenses in the 17th century and the beginning of the 18th century, these instruments worked until the appearance of reflecting telescopes in the 18th century and even in that period such instruments were still being produced but mainly for terrestrial observations, aerial telescopes was used until the middle of the 18th century, so these instruments have a quite a long life.

Here are the tests of other authors: Campani lenses one from 1680 and others from Museo della Specola Bologna, Hartsooker objective from (1688) and finally Marcel Arnold.