This reflecting telescope belongs to the Observatory Collection. It was created by Rudolf Wolf (1816-1893). He was a professor of astronomy, head of Semper Observatory, director of the library of the Polytechnic (today ETH) and collector of astronomical instruments. He created an inventory of his collection in 1873. This shows that the reflecting telescope was added to the collection after it had been gifted to Rudolf Wolf by Emil Kern.

The route to the collection

In a letter to Rudolf Wolf dating from 1889, Emil Kern, a son of the founder of Kern & Co. in Aarau, writes, “knowing that you are an avid collector, I am taking the liberty of sending you the prints of some of the instrument [figures] that I had made last year.”¹ This letter and the other documents are now kept in the ETH Zurich University Archives.

Several instrument drawings are enclosed with the letter, including one of the reflecting telescope. Unfortunately, the surviving letters contain no mention of the reflecting telescope gifted to Rudolf Wolf by Emil Kern.

The invention of the reflecting telescope

When the telescope was invented in the 17th century, it marked a turning point in astronomy: even though the first telescopes provided only low magnifications, they led to new insights. Reflecting telescopes were already invented shortly after the original telescope was invented in 1609.²

The fundamental difference between telescopes and reflecting telescopes lies in the optics.³ The former have convex lenses, the latter concave. The advantage of the latter is that they have no colour errors, as convex lenses do. The first efforts to use concave mirrors were made as early as 1616; various scientists tried to make reflecting telescopes. One of these was Scottish mathematician James Gregory. He built his first reflecting telescope in 1660, publishing it just three years later in his work Optica promota.⁴

A new feature of Gregory’s reflecting telescope was the introduction of a counter-mirror or secondary mirror. This is described in the 3D model under the third annotation.

Gregory reflecting telescopes show an upright image, similar to Earth telescopes. The reason for this is the counter-mirror. The purpose of this being reversal: Light is reflected by the main mirror, producing a real image. The light after the reflection falls on the counter-mirror and again produces a real image, which can then be viewed through the opening in the concave mirror using the eyepiece.⁵

Gregory’s main achievement was theoretical work, so he had to rely on instrument makers and opticians to build reflecting telescopes. Not satisfied with the work of even London’s best opticians, he apparently abandoned his project. He died in 1675 at the age of 36. Robert Hooke had succeeded in building a reflecting telescope just one year earlier. In all likelihood, Gregory’s work was also of great importance to Isaac Newton, who also worked on reflecting telescopes from 1668 onwards. Indeed, in the defence of his reflecting telescope, Newton mentioned that he was familiar with Gregory’s Optica promota.⁶ Reflecting telescopes based on James Gregory’s were built mainly in the 18th century.

Jan van der Bildt, manufacturer

This Gregory reflecting telescope was built by Jan van der Bildt. He is considered a well-known manufacturer of reflecting telescopes in Franeker (province of Friesland). According to one anecdote, before van der Bildt tested one of his telescopes in 1755, he met a young doctor who was on his way to the mayor. The mayor lived just outside the city. The young doctor and van der Bildt met again a few days later. When asked how the test with the telescope had gone, van der Bildt replied that he could see far beyond the city and that the doctor had better draw the curtains the next time he visited the mayor’s wife.⁷