Notice that the arrangement of lenses for the telescope in Figure 19.17 results in an inverted image. Most astronomical telescopes are arranged like this. It really does not matter to an Astronomer if she is viewing a star cluster upside down or right side up. However, for viewing something on Earth, a terrestrial telescope needs to have a final upright image. Binoculars often do this by reflections with prisms that correctly orient the image to be upright as well as shortening the distance between the objective and eyepiece lenses.
Figure 19.18 shows another way of obtaining an upright image.
Opera glasses or field glasses use
this arrangement of lenses. In this arrangement, the eyepiece
is a diverging lens and intercepts the light before the real image
of the objective lens is produced. This arrangement is also beneficial
because it shortens the distance between the lenses. Just as with
an astronomical telescope, the objective lens forms a real image.
While this real image is small, it is nearby so observation is
far easier than for the original object. A diverging lens is now
used as an eyepiece and is positioned so its focal point coincides
with the real image. This produces an upright virtual image at
The magnification of a telescope is given by
M = fobj / feye
where fobj is the focal length of the objective lens and feye is the focal length of the eyepiece. That means a large magnification results from a long focal length objective lens and a short focal length eyepiece.
All lenses suffer aberrations of various kinds. chromatic
aberration is the problem that glass bends or refracts
light of different colors different amounts. This means that light
of one color is focused as a slightly different position than
light of another color. One way to eliminate chromatic aberration
is to substitute a mirror for the objective lens.
Such a telescope was first invented by Sir Isaac Newton and is
illustrated in Figures 19.20 and 19.21. A large spherical or parabolic
mirror focuses the light the same as the objective lens we have
just studied. A small mirror is placed in front of this objective
mirror and diverts the light to one side where the image may be
viewed with an eyepiece. A variation of this, called a Cassegrainian
telescope, is also shown in the figure. There, a small mirror
reflects the light through a hole in the objective mirror. Almost
all large astronomical telescopes use mirrors, such as the one
in Figure 19.22. Such reflecting telescopes have several advantages
over refracting telescopes-lack of chromatic aberration, only
one surface to grind and polish, and the weight of a large mirror
can be supported along its back (the weight of a large lens can
be supported only at its edges and this will cause a lens to "sag").