The six-inch Merz refractor is a lens telescope of 225 cm length and 16.6 cm diameter, with a unique wooden binocular tube from 1838 - older than the Old Observatory building itself. The telescope was on the roof of the Academy building in the first decades, where Frederik Kaiser used it as a private instrument.

Viewing the positions of planets and comets, observing binary stars and determining the size of planets are some examples of research for which this telescope has been used.

The ten-inch Repsold Refractor is a lens telescope from 1885, with a diameter of 26.6 cm and a length of 399.5 cm. The telescope is made by the German company Repsold und Söhne, but the lenses were made by lens maker Alvan Clark & Sons from the United States. A special combination for that time, since the German lens industry was very dominant in Europe.

The telescope was mainly used for viewing binary stars. Gerard Kuiper, now known by the Kuiper Belt, used this telescope for his doctoral research ‘Statistical Research of Binary Stars'.

The photographic double refractor from 1897, with its length of 524 cm, is the largest refractor telescope in the Netherlands. Inside the telescope are two parallel sets of lenses: one with an aperture of 33 cm and one of 15 cm. The English mounting in which the telescope is attached makes it easy to move the telescope.

Photographing astronomical objects was an emerging technique in astronomy at the end of the 19th century. For the first time, objects could be depicted that were not visible at all with the naked eye.

The photographic double refractor is the only historical telescope in Leiden built for photographic purposes. This used to be done using photographic plates: square pieces of glass with a light-sensitive coating. Nowadays, digital cameras can also be used to take pictures with this telescope.

The Zunderman Reflector is a mirror telescope from 1947 with a 46 cm mirror. The mirror makes it possible to keep the tube of the telescope very short, so that it can be hung in a fork mount.

When this telescope was built, the intention was to use it for photometric observations of variable stars. A number of projects have been carried out with this aim in mind. However, it became increasingly easy to build telescopes in remote areas where light pollution was low and clouds were scarce. So it become more common to use telescopes abroad, especially ESO telescopes, instead of the Zunderman Reflector in Leiden. In fact, this telescope has actually only been used professionally for a short period of time.

Nevertheless, in terms of optical properties, this telescope is the best historical telescope in the Old Observatory.

The heliostat is not a historical telescope, but the result of a crowdfunding campaign in 2015. It is also the newest telescope we have exhibited in the building.

The telescope is made to look at the sun. On the roof of the Old Observatory, a mirror is mounted on a motor that moves in an equatorial direction. When the weather is nice, this mirror reflects sunlight through a long tube to the visitors centre in the basement throughout the day. In the visitors centre the image of the sun is depicted on the wall. The image of the sun is roughly 1.5 meters across and the resolution is good enough to see individual sunspots.

During sunny days, the telescope can be seen operating in our visitor centre.