Aerial telescope (Christiaan Huygensm Astroscopium Compendiaria,1684)

With the acceptance of the astronomical telescope, the limit on magnification caused by the small field of view of the Galilean telescope was temporarily lifted, and a "telescope race"  developed. Because of optical defects, the curvature of lenses  had to be minimized, and therefore (since the magnification of a  simple telescope is given roughly by the ratio of the focal  lengths of the objective and ocular) increased magnification had  to be achieved by increasing the focal length of the  objective. Beginning in the 1640s, the length of telescopes  began to increase. From the typical Galilean telescope of 5 or 6  feet in length, astronomical telescopes rose to lengths of 15 or  20 feet by the middle of the century. A typical astronomical  telescope is the one made by Christiaan Huygens, in 1656. It was 23 feet long; its objective had an aperture of several inches,  it magnified about 100 times, and its field of view was 17

arc-minutes.



Telescopes had now again reached the point where further  increases in magnification would restrict the field of view of  the instrument too much. This time another optical device, the  field lens came to the rescue. Adding a third convex lens--of  appropriate focal length, and in the right place--increased the  field significantly, thus allowing higher magnifications. The  telescope race therefore continued unabated and lengths  increased exponentially. By the early 1670s, Johannes Hevelius  had built a 140-foot telescope.



But such long telescopes were useless for observation: it was  almost impossible to keep the lenses aligned and any wind would  make the instrument flutter. After about 1675, therefore,  astronomers did away with the telescope tube. The objective was  mounted on a building or pole by means of a ball-joint and aimed  by means of a string; the image was found by trial and error;  and the compound eyepiece (field lens and ocular), on a little  stand, was then positioned to receive the image cast by the  objective. Such instruments were called aerial  telescopes.

Source: http://cnx.org/content/m11932/latest/