A typical Galilean telescope with which Jupiter's moons could be
observed was configured as follows. It had a plano-convexobjective (the lens toward the object) with a focal length of
about 30-40 inches., and a plano-concave ocular with a focallength of about 2 inches. The ocular was in a little tube that
could be adjusted for focusing. The objective lens was stoppeddown 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). Theinstrument's magnification was 15-20. The glass was full of
little bubbles and had a greenish tinge (caused by the ironcontent of the glass); the shape of the lenses was reasonable
good near their centers but poor near the periphery (hence therestricted aperture); the polish was rather poor. The limiting
factor of this type of instrument was its small field ofview--about 15 arc-minutes--which meant that only a quarter of
the full Moon could be accommodated in the field. Over the nextseveral decades, lens-grinding and polishing techniques improved
gradually, as a specialized craft of telescope makers slowlydeveloped. But although Galilean telescopes of higher
magnifications were certainly made, they were almost uselessbecause 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 outthat such a combination would produce an inverted image but
showed that the addition of yet a third convex lens would makethe 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 withtelescopes with convex oculars in order to make the image of the
Sun projected through the telescope erect.
The Galilean telescope produces an erect image of an object
viewed directly but an inverted image of a projected object;by substituting a convex for the concave ocular, this
situation is reversed.
But when he happened to view an object directly through such an
instrument, he found that, although the image was inverted, itwas much brighter and the field of view much larger than in a
Galilean telescope. Since for astronomical observations aninverted image is no problem, the advantages of what became
known as the astronomical telescope led to its generalacceptance 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 theastronomical telescope with its inverted image. Astronomers
eschewed the third convex lens (the erector lens) necessary forre-inverting the image because the more lenses the more optical
defects multiplied. In the second half of the seventeenthcentury, therefore, the Galilean telescope was replaced for
terrestrial purposes by the "terrestrial telescope," which hadfour convex lenses: objective, ocular, erector lens, and a field
lens (which enlarged the field of view even further).
Hevelius's 60 foot telescopeHevelius's 140 foot
telescope(Machina
Coelestis, 1673)
