Hevelius's 60 foot telescope

A typical Galilean telescope with which Jupiter's moons could be  observed was configured as follows. It had a plano-convex  objective (the lens toward the object) with a focal length of  about 30-40 inches., and a plano-concave ocular with a focal  length of about 2 inches. The ocular was in a little tube that  could be adjusted for focusing. The objective lens was stopped  down to an aperture of 0.5 to 1 inch. , and the field of view  was about 15 arc-minutes (about 15 inches in 100 yards). The  instrument's magnification was 15-20. The glass was full of  little bubbles and had a greenish tinge (caused by the iron  content of the glass); the shape of the lenses was reasonable  good near their centers but poor near the periphery (hence the  restricted aperture); the polish was rather poor. The limiting  factor of this type of instrument was its small field of  view--about 15 arc-minutes--which meant that only a quarter of

the full Moon could be accommodated in the field. Over the next  several decades, lens-grinding and polishing techniques improved  gradually, as a specialized craft of telescope makers slowly  developed. But although Galilean telescopes of higher  magnifications were certainly made, they were almost useless  because of the concomitant shrinking of the field.



As mentioned above, the telescopic effect can be achieved with  different combinations of lenses and mirrors. As early as 1611,  in his Dioptrice, Johannes  Kepler had shown that a telescope could also be made by  combining a convex objective and a convex ocular. He pointed out  that such a combination would produce an inverted image but  showed that the addition of yet a third convex lens would make

the image erect again. This suggestion was not immediately taken  up by astronomers, however, and it was not until Christoph Scheiner published his Rosa  Ursina in 1630 that this form of telescope began to  spread. In his study of sunspots, Scheiner had experimented with  telescopes with convex oculars in order to make the image of the  Sun projected through the telescope erect.

But when he happened to view an object directly through such an  instrument, he found that, although the image was inverted, it  was much brighter and the field of view much larger than in a  Galilean telescope. Since for astronomical observations an  inverted image is no problem, the advantages of what became  known as the astronomical telescope led to its general  acceptance in the astronomical community by the middle of the  century.



The Galilean telescope could be used for terrestrial and  celestial purposes interchangeably. This was not true for the  astronomical telescope with its inverted image. Astronomers  eschewed the third convex lens (the erector lens) necessary for

re-inverting the image because the more lenses the more optical  defects multiplied. In the second half of the seventeenth  century, therefore, the Galilean telescope was replaced for  terrestrial purposes by the "terrestrial telescope," which had  four convex lenses: objective, ocular, erector lens, and a field  lens (which enlarged the field of view even further).

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.

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