IV. THE TOOLS: How we catch and manipulate
the waves
A. The naked eye.
As a tool for observing our world, the naked eye does an excellent job.
But as an astronomical tool, it leaves much to be desired, even though
it was the only tool available to mankind until the early 1600s.
Limitations of the human eye:
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Because we evolved as a diurnal (daytime) organism, the pupil, the opening
through which light passes, is small, a bit less than a third of an inch
(7 mm).
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Its magnification is limited.
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The range of frequencies that it can access compared to the entire EM spectrum
is also limited; for example, it detects only the visible portion of the
spectrum which is rather a narrow portion of the entire EM spectrum.
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The greatest handicap for the eye as an astronomical tool is the fact that
it cannot add up light; in other words, staring at a dim object for any
length of time will not make it brighter.
B. The telescopes
From the moment the telescope was invented it became universally used
because it overcomes some of the natural limitations of the naked eye.
Telescopes are used for the following reasons:
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They gather light. The larger the lens or mirror, the more light is gathered.
For example, a 200 inch (5000 mm) telescope will gather over 500,000 times
more light than the naked eye.
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They magnify the objects we look at.
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They resolve objects: they are able to make out details that cannot be
seen with the unaided eye.
There are two main types of optical telescopes: refracting telescopes
that use lenses and reflecting telescopes that use mirrors or some
other type of reflecting dish.
Types of telescopes
Optical
Refracting telescopes
Galileo did not invent telescopes but was he first to use and manufacture
them as astronomical tools. In their simplest form, they are simply two
lenses which bend light, hence they are called refracting telescopes. Starting
with a telescope of approximately nine power, he rapidly improved their
magnification to near 30 power.
Reflecting telescopes
While the refracting telescope continues to be used to some extent to
this day, it was rapidly replaced by the reflecting telescope which uses
mirrors rather than lenses, and was made practical by Newton**. There are
several powerful reasons for doing this:
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Each lens in a refracting telescope requires two surfaces to be ground
correctly; reflecting telescopes, only one, thereby reducing the chances
of distortion. Remember also that the mirror surface is on the front of
the glass and because the light does not go through the glass, as in a
lens, its path will not be distorted.
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Refracting telescopes’ lenses suffer from chromatic aberration, (the actual
focal point of the lens varies for different frequencies.) Mirrors do not.
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Every lens surface loses some of the incoming light, uncoated lenses as
much as 30% per lens. This is a tremendous handicap especially if we remember
that most astronomical objects are dim to begin with. In contrast, mirrors
are 99% efficient in reflecting light and therefore there is little light
loss.
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There are also structural reasons for preferring reflecting telescopes.
Lenses are heavier in the center and are thinnest where they are mounted
along their edges, causing gravitational distortion as they hang in the
telescope. In contrast, mirrors are thinner in the center, and thicker
along the edges, a structural improvement that is very important in larger
telescopes.
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Reflecting telescopes can be designed for a greater range of frequencies
than refracting, which are limited primarily to the visible part of the
spectrum. Reflecting telescopes range in frequency from the UV to the Infrared
Other Telescopes
Radio Telescopes:
Radio telescopes are directional antennas that survey the long end
of the EM spectrum. Many conceptually resemble reflecting optical telescopes
(dish-type telescopes). The satellite dishes we use are but one such example,
although they are not used for astronomical purposes.
Satellite Telescopes
In addition to optical telescopes such as the Hubble, we have establishes
space platforms that enable us to see the universe in frequencies that
are partially or fully blocked by the atmosphere, both in the high frequency
range from UV to gamma, and in the infrared.
Other tools
Cameras and CCDs
With the invention of cameras and photography, astronomers were
quick to see the value of a device that would allow the adding up of light
in long time exposures. Photography became especially useful with
the advent of fine-grained emulsions early in this century. The past two
decades have also seen the use of CCD devices (the same ones as
on video cams) that are now nearing such sensitivities that virtually not
a single photon of light that enters a telescope is lost.
**Newton is often incorrectly given credit for inventing the reflecting
telescope. In fact it was invented by an acquaintance of his, Gregory.
Gregory's complicated design was simplified and improved by Newton and
it is the latter's design that is still commonly used today. |