Huygens Aerial telescope, Leiden
During special occasions, like open days, one can find a green mast with a small brass tube on top of it behind the Old Observatory. This is one of the world's only working replicas of a Huygens aerial telescope. Modelled after a design by Christiaan Huygens from 1683, this telescope is made in a very unconventional way. Instead of having a long tube, it consists of two lenses and a 4-meter long cord. When used properly, one can align the two lenses perfectly by exerting the proper tension on the cord.
Huygens invented this kind of telescope because in his time it was very hard to make good lenses, especially with short focal lengths. Huygens was not deterred by this and instead used this to his advantage. He made lenses that had very long focuses and a brilliant quality. This was a victory, but the next hurdle was to make telescope tubes of this length that didn’t buckle under their own weight and were light enough to move around. Huygens again solved this by turning the problem into an advantage: he made telescopes without tubes. The telescopes became immensely popular upon their introduction, with most large observatories in the world buying one of at least 30 meters. Due to their size, these tele- scopes were very hard to use, and usually required a small army of assistants to aid the observer. And more often than not, a special structure was needed to hang up the primary lens. One can see an example of such structure on an old engraving of the Paris Observatory from the early 1700s.
Aerial telescopes quickly dropped in popularity as in the beginning of the 18th century, reflecting telescopes started to gain ground due to their compact design and strong magnification. The final blow to these telescopes came in the late 1750s when a new breakthrough in lens manufacturing allowed for very good lenses with a short focal length, removing any need for Huygens’ invention. Sadly, these telescopes were quickly removed due to their diffculty of operation. These days, only parts of the construction can be found in museums and historical observatories, most not working. Appart from this telescope only one other working Huygens telescope is known.
This telescope was funded by Hans de Rijk, whith the money that he won with the 2008 ‘NWO oeuvreprijs’ for his exceptional scientific communication. It was manufactured by the Leidse Instrumentmakers School and unveiled in May 2014 by Professor Vincent Icke during the first edition of the annual ‘Kaiser Lente Lezingen’ (Kaiser spring lectures).
About the telescope
D = 10 cm
f = 400 cm
From attics to domes, Four centuries of history of Leiden Observatory. (2018) ISBN: 978-91-639-7671-1
Last update 7/12/2018 by Alex