33.14 - Telescopes
Simple refracting (lens-based) telescopes have two lenses. A clever configuration of
these lenses enables the telescope to serve three purposes: to gather as much light as
possible from the object, to magnify the object and to provide as relaxed viewing of the
image as possible.
The first lens encountered by incoming light rays, the objective lens, is used to gather
as much light as possible. Light is at a premium in telescopes since faraway objects are
typically faint. You cannot see most of the stars in the sky because they are too dim, not
because they are too small (in angular size). Astronomers are willing to pay high prices
to obtain telescopes with large objective lenses. Amateur astronomers purchasing their
first telescope are often advised to pay more attention to its light gathering capability
than to its magnifying power.
The eyepiece of a telescope performs two important tasks: It magnifies the real image
created by the objective lens and creates a virtual image whose image distance is
essentially at infinity. Creating a distant image is useful because the human eye is most
relaxed when viewing objects at a distance.
At the right, you see the combination of lenses that constitutes a basic refracting
astronomical telescope. To analyze how this system functions, you must first determine
the location of the real image created by the objective lens. This image serves as the
object for the eyepiece.
Telescopes are pointed at faraway objects, so the light rays entering the telescope are
essentially parallel. Both the objective lens and the eyepiece are converging lenses.
The objective lens creates a real image of the distant object at its focal point. The
lenses are arranged so that this point is also at the focal point on the incident side of the
eyepiece. Putting the object here causes the virtual image created by the eyepiece to
be at infinity. The length of the telescope between the two lenses equals the sum of
their focal lengths. You see all this in the sketch to the right.
As you can see in Concept 2, the image produced by the objective lens is inverted,
smaller than the object, and real. The image produced by the eyepiece from this real
image subtends a larger angle than the object, and it is virtual. The larger angle is an
example of angular magnification, provided by the “magnifying glass” of the eyepiece. In
this case it is being used to view the real image created by the objective lens, rather
than a concrete object. The eyepiece does not invert the real image created by the
objective lens, so the final image is inverted compared to the original object.
The telescope configuration we have been discussing works well for astronomy. The image it produces is inverted, but this typically poses no
problem for astronomers, since they are accustomed to seeing telescope-rendered views of the objects they study. However, telescopes used
to study terrestrial objects (like birds) use other configurations, including perhaps a diverging lens or a set of prisms, in order to avoid
presenting an inverted image to the eye.
A different type of telescope designed for astronomy is called a Newtonian reflector. “Newtonian” refers to the developer of this type of
telescope, Sir Isaac Newton, and “reflector” refers to the use of mirrors. In a reflector, a relatively large concave mirror performs the “light
collecting” and first image creation duties that the objective lens performs in a refracting telescope. You see a diagram of a reflector to the right.
Light passes down the tube, is collected by a curved mirror, redirected by a flat mirror and then passes through the eyepiece.
Why mirrors? Consider the reflecting telescope at California’s Mount Palomar Observatory, which is famous for its 200 inch reflector. A glass
objective lens of the same diameter would have a mass of about 40,000 kg (and weigh about 50 tons), more than three times the mass/weight
of the mirror actually used. The weight of the mirror is still huge, but reducing the mass by more than two-thirds is a worthwhile
accomplishment. In addition, lenses are subject to chromatic aberration (different colors of light focusing at different distances), while mirrors
are not. A mirror is also easier to support (since it can be supported from behind), and only one side of the mirror needs to be manufactured
with optical precision, versus the two sides of a lens. These factors play out in the construction of large research telescopes: The largest
refractor in existence, at the Yerkes Observatory in Wisconsin, is only a little over one meter in diameter, while the largest reflectors, the Keck
telescopes, are nearly ten meters in diameter.Refracting telescopes
Objective lens gathers light
Eyepiece magnifies imageHow it works
Two converging lenses
Focal points at same location
Final image inverted, at infinity, virtualReflecting telescope
Uses mirror in place of objective lens
(^620) Copyright 2007 Kinetic Books Co. Chapter 33