Astronomical telescopes

Early Inspiration

In 1980, when I was in middle school, a German comic book magazine for youngsters called Yps ignited my passion for astronomy. With each issue of the magazine, a toy gimmick was included: A small astronomical telescope (with plastic lenses), a star clock (using the big dipper and Polaris to estimate time), a small handbook about astronomy, and a phosphorescent star map. While the toys weren’t practical for serious observing, they sparked a lifelong fascination with the cosmos.

Homemade Telescopes: Utopia & Ganymede

In 1981, I got a small 2.4-inch refractor for Christmas. The views of the moon and the planets Jupiter and Saturn were awe-inspiring. While studying physics at the Federal Institute of Technology (ETH) in Zurich, I hand-ground the mirrors for two Newtonian telescopes - a 6-inch f/8 Reflector called Utopia (1990) and a 10-inch f/11 Reflector called Ganymede. Mirror grinding took place between 1992 and 1994, with final assembly of the telescope completed in 1998. I used Utopia for both observing and astrophotography and Ganymede for observing only.

Fig. 1 - The 6-inch f/8 Newtonian Reflector Utopia with the Dufourspitze (Switzerland's highest peak) in the background. This picture was taken close to the Gornergrat observatory at 3,100 m above sea level (10,170 ft).

Design Philosophy

With a focal ratio of f/11 and a tube over 3 meters long, Ganymede was a special planetary telescope. I was inspired by Harold Hill, author of 'A Portfolio of Lunar Drawings' and amateur astronomer Jan de Lignie's Newtonian Reflector. Looking at Hill's lunar drawings (obtained with a 10-inch f/10 Reflector) and looking through de Lignie's 8-inch f/8 Reflector convinced me that a long focal length offers superior performance.

The parabola of a mirror with a long focal length is easier to produce, one can employ a smaller secondary mirror (which enhances contrast and reduces diffraction effects), simple eyepiece designs (Ramsden, Kellner, etc.) offer better performance (off-axis aberrations like coma are less severe), the optics are easier to collimate and focusing is less critical (due to a greater depth of focus). Even under poor seeing conditions, Ganymede was a joy to use. Over time, the weight of the 10-inch f/11 telescope became increasingly difficult to manage.

Fig. 2 - The 10-inch f/11 Newtonian telescope Ganymede next to my girlfriend at the Swiss Star Party 2005 on Gurnigel in the Swiss Alps.

Fig. 3 - Ganymede next to the van for transportation. Ganymede featured a 10-inch f/11 Zerodur primary mirror and a secondary mirror obstruction of 12%.

Fig. 4 - Another view of Ganymede, showing the telescope fully operational with stray light protection mounted. The total length of the telescope slightly exceeded 3 meters (10 ft).

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