Aerial telescope

[5] As the objective diameter of these refracting telescopes was increased to gather more light and resolve finer detail they began to have focal lengths as long as 150 feet.

Their value as research tools was minimal since the telescope's support frame and tube flexed and vibrated in the slightest breeze and sometimes collapsed altogether.

[3][6] Around 1675 the brothers Christiaan and Constantijn Huygens decided to accommodate the very long focal length objectives they were creating by eliminating the tube altogether.

The telescope could be aimed at bright objects such as planets by looking for their image cast on a white pasteboard ring or oiled translucent paper screen and then centering them in the eyepiece.

Constantijn Huygens, Jr. presented a 7.5 inch (190 mm) diameter 123 ft (37.5 m) focal length objective[9] to the Royal Society of London in 1690.

[11] James Bradley, on December 27, 1722, measured the diameter of Venus with an aerial telescope whose objective had a focal length of 212 ft (65 m).

[12] Francesco Bianchini tried to map the surface of that same planet and deduce its rotational period in Rome in 1726 using a 2.6" (66 mm) 100 foot focal length aerial telescope.

The need for very long focal length refracting telescope objectives was finally eliminated with the invention of the achromatic lens in the middle of the 18th century.

An engraving of Huygens's 210-foot aerial telescope showing the eyepiece and objective mounts and connecting string.
1673 engraved illustration of Johannes Hevelius 's 8 inch telescope with an open work wood and wire "tube" that had a focal length of 150 feet to limit chromatic aberration.
An engraving of The Paris Observatory in the beginning of the 18th century with the wooden "Marly Tower" on the right.