Huygens and the improvement of the telescope

After the ‘invention’ of the telescope in 1608, the greatest Dutch contribution to the development of this instrument was made by Christiaan Huygens (1629-1695) together with his brother Constantijn Huygens Junior (1628-1697).

Huygens’ contribution to perfecting the telescope

It was Christiaan who would make the greatest theoretical contribution to the development of the telescope, mainly thanks to the invention of the Huygens ocular (named after himself) and the aerial (tubeless) telescope. He was also the first to reveal the potential of a micrometer.

Strongly inspired by their father Constantijn Huygens Senior, himself particularly interested in optical innovations, the Huygens brothers began grinding object lenses in 1654. A year earlier they had already had a telescope made by a certain ‘Master Paulus’ of Arnhem. Because they found this instrument disappointing, they decided to make one themselves. Christiaan Huygens in particular wished to use such a telescope for astronomical observations.

After consulting a number of well-known opticians, such as the scholar Gerard van Gutschoven of Leuven, the instrument maker Johannes Wiesel of Augsburg and the optician Jan de Wyck of Delft, they finally turned to the ‘pre-eminent tradesman in the country for this type of work’, the instrument maker Caspar Calthof of Dordrecht. The latter provided the Huygens brothers with their first grinding plates and other necessary equipment. In the spring of 1655, the first practicable telescope of 12 feet (approx. 3.7 meters) in length was ready. Soon after, Christiaan would use it to observe Saturn and discover a new moon, which would later be named Titan.

Christiaan was extremely satisfied with his new telescope. He wrote to a correspondent: ‘I recently built a telescope of 12 feet in length, and I believe you would be hard put to find a better, as I am sure that no one before has seen the wonder that I observed with it recently’.

As a result of his prolonged observation of the planet Saturn, Christiaan Huygens also developed a new theory for the ‘ears of Saturn’, first observed by Galileo, which he was able to prove was a ring around Saturn. From March 1656 he had a new telescope at his disposal to test his hypothesis. This telescope was 24 feet in length (approx. 7.3 meters) and once again, the object lens was ground by the brothers themselves.  This long telescope was raised in the garden of the Huygens’ residence using pulleys and a mast.

Huygens published his discoveries in 1659 in a book entitled Systema Saturnium. This publication also contains his first description of the workings of a micrometer in an astronomical telescope. In this telescope, the object lens creates a true image which is viewed through the ocular lens. By fitting a distance gauge onto the true image, the position of one heavenly body in relation to another could be determined with great accuracy.

The exterior of Christiaan’s telescope

Extremely satisfied with the progress he had made, Christiaan took the telescope with which he had made his discoveries with him wherever he travelled. The brothers originally mounted their lenses in simple tin tubes (an example can still be viewed in the Boerhaave Museum), and the ‘Admovere’ object lens (with which Christiaan discovered the moon Titan) was originally placed in such a tin tube too.

However, in October 1655, during a visit to Paris, Christiaan ordered a brand new telescope tube from an ‘artisan’, built according to the current French fashion. The main tube was now made of very thin wood covered in morocco leather (probably red) stamped with gold. The four telescopic tubes were clad with green parchment ‘as I have seen that is the way they do it here’. The new telescope was less than a quarter of the weight of the old tube. When fully retracted the length was now about 3 feet (approx. 91 cm).

However, in the end, the new tube proved to be rather unpractical. It tended to bend and collapse if only supported at one point. ‘A solution will need to be found for this problem’, wrote Christiaan to his brother in 1656. However, as far as the lenses were concerned, their telescope was unequalled, according to Christiaan: ‘Now that I have informed everyone of the discovery of Saturn’s moon [...] they cannot deny that my telescope is the best that ever was built’.

Hevelius and Divini protest against Huygens

Huygens’ claim that his telescope was better than all the others did not remain uncontested. For example, the astronomer Johannes Hevelius, who had been visited by younger brother Philippus Huygens in March 1656, was furious. Not only had Philippus labelled him an inferior lens grinder in his private correspondence with his brothers, Christiaan’s declaration, in the Systema Saturnium, that his new telescope was now the norm for all observers could not go unchallenged.

The Italian telescope-builder Eustachio Divini fought the hardest against Huygens’ claim that he made the best telescopes. In 1661, Divini published a small book, together with the Jesuit Honoré Fabri, in which he explained his own ideas about Saturn. However, although Divini was probably right ‘that his telescopes were of similar quality to Huygens’, he eventually lost the battle.

Christiaan Huygens’ aerial telescope (1684)

Around 1650, it was discovered that if lenses were used with a small radius of curvature, the problem of lens deviation (such as dispersion and distortion) could for a large part be  removed. The occurrence of these image distortions was a particular problem with astronomical telescopes. However, such a small radius of curvature of the lenses irrevocably led to long focal lengths, resulting in almost unmanageably long telescopes. Only very experienced users were able to handle such telescopes.

To get around the problem of wind sensitivity and the unhandiness of the long telescopes, Christiaan Huygens invented the so-called ‘aerial (tubeless) telescope’, of which he published a description in the book Astroscopia compendiaria in 1684. Huygens limited this instrument to a large object lens and an eyepiece. The object lens was hoisted up a mast in a holder and joined to an eyepiece or ocular lens by a rope. A small oil lamp was used to make it easier to aim.