More than 350 years ago, Christiaan Huygens proudly stated that his telescopes were the best on earth. He discovered Saturn's moon Titan with it, but Italian scientists were hot on his heels with their own telescopes. Research by Museum Boerhaave on the oldest lenses now sheds light on the grinding practices of the earliest telescope builders and on the question of who was the best.
By modern standards, the material with which the Dutch astronomer Christiaan Huygens scanned the sky in 1655 was child's play. Maybe even less than that. The performance of the telescope he built himself is nowadays surpassed by a children's telescope or binoculars. Yet it was enough to detect a speck of light near Saturn:Huygens discovered Saturn's largest moon:Titan.
Huygens proudly announced that he and his brother Constantijn made the best lenses in the world. Together they tow about thirty lenses, the majority of which is now in the possession of Museum Boerhaave in Leiden. “Huygens has mastered the sharpening process extremely quickly,” says Tiemen Cocquyt, curator of the museum, “but whether he and his brother really made the best lenses in the world is still open to question. With those statements he probably offended a few Italian lens sharpeners such as Eustachio Divini and Giuseppe Campani.”
Whether it was the Huygens brothers or their Italian 'competitors', in general little is known about the craft of lens sharpening at the time:an essential part of the tumultuous development that science was undergoing at the time. Sharpeners like Huygens left almost no description of their work. The only way to learn about the grinding culture now is to take a closer look at the lenses themselves. Ten to fifteen lenses have been preserved worldwide from the first half of the seventeenth century.
When Cocquyt received a grant from NWO in 2015 to research the polishing practices in the seventeenth and eighteenth centuries, he immediately knew that he should not only view the lens collection of Museum Boerhaave. He developed a mobile laser setup with which he visited several European museums and screened some of the oldest known lenses. In this way, the work of the seventeenth-century polishers is mapped down to nanometers. As a by-catch, we now know how good Huygens' lenses really were.
Gathering on eBay
It's science that actually started in Cocquyt's college days in the online marketplace eBay. He was then working on a prototype of such an optical measurement setup and scoured the site for parts. “I used to hope for a broken parts factory so I could buy them for next to nothing,” he says, opening a briefcase with a large shiny lens inside. “I estimate that this copy costs tens of thousands of euros new, I paid a fraction of that.”
Cocquyt had to rely heavily on his technical ingenuity. But various measurement setups further on, made of collected optics and self-written software, he managed to develop a relatively affordable and full-fledged optical measurement setup in ten years' time that is not inferior to professional machines for checking lenses. “They cost about a ton,” says Cocquyt. “We don't have that money.”
But why not use those existing devices, even if you don't have them yourself? Indeed, Cocquyt says that Galileo Galilei's lenses have been previously measured in an optical laboratory. “When you do comparative research, you prefer to measure all lenses with the same device,” he says. “I thought it was very important to look at these lenses systematically. But that's problematic. Museums do not like to transport these old and fragile items. I now overcome that by getting into the car with this setup and going to the lenses. The scope of the research is so much wider.”
Mapping lenses
The device is a so-called interferometer. It lets laser light pass through a lens (to be measured) twice and then interfere with itself:the two beams of light fall at a point and anaesthetize or amplify each other there. This trick maps the operation of the lens with high precision. “We measure, among other things, the shape of the lens, which is actually a kind of elevation map. It is usually precise to a few nanometers,” says Cocquyt. “This card says a lot about the cut quality of a lens.”
Over the past year, Cocquyt had several lenses in his lineup. That of Huygens in Leiden, but also that of Campani from a Swiss private collection. He drove to Berlin for a telescope from 1617 – one of the oldest surviving examples.
A perfect (convex) lens has the shape of a so-called hyperbola. Nowadays lenses are ground to a perfect profile to tens of nanometers, the seventeenth century grinders did significantly less accurate work. If you look at the height profiles, you will see large deviations from the ideal shape. As a result, the image they magnified became less sharp. Something that, incidentally, could partly be overcome by using a diaphragm, which shields the outer (usually less perfect) regions of a lens.
Huygens vs. Campani
Cocquyt had lenses by Dutch and Italian masters in his measuring set-up. Compared to, for example, Campani's lenses, those of Huygens were usually very large. For example, specimens of 4.6 centimeters (Campani) and 9.6 centimeters (Huygens) were compared. “In principle, a large lens is better, but then it must have sufficient quality,” says Cocquyt. “A lens of this size is more difficult to get into the right shape due to the larger surface area. If something goes wrong during sharpening, the entire lens is immediately unusable. There is a delicate balance between size and quality.”
Were those large lenses of the Huygens brothers really the best in the world? Cocquyt saw that the outer regions of their lenses deviated quite a bit from the ideal shape, especially the aforementioned extra large 9.6 centimeters. He also suspects that using a diaphragm saved him. “That actually allowed him to take out the 'worst' parts of the lens. The center of the lens was indeed good, which means that the performance of this lens can be compared with that of competitor Campani. So they were about as good," said Cocquyt.
From the middle of the seventeenth century the quality of lenses improved and the use of a diaphragm was no longer necessary from that time. Cocquyt says that Campani's lenses kept getting bigger and remained of good quality. The lenses had the right shape over the entire surface, unlike the decades before. We can only guess at how Campani did that. But Huygens' words that "his telescopes are preferred over those of other scientists" were certainly outdated by then. Cocquyt:“Among the arguing scientists, Campani eventually became the undisputed master of polishing.”
By meticulously measuring the lenses, Cocquyt knows exactly how they bend light and how they enlarge an image. This enables him to look 'fictionally' through Huygens' telescope and see, for example, how he must have seen Saturn in 1655. A modern (sharp) image of the planet enters the computer, which calculates how the image of the gas giant fell through the telescope. It shows the rings of the planet as two appendages. “If you really want to know how Huygens saw Saturn, you have to put the lens in a telescope and go up on the roof,” says Cocquyt. “Unfortunately, that is not an option with these unique pieces.”