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| Types of Telescopes / Terms |
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The
diagram at left show the differences between the two main types of
telescopes:
Refractor and Reflector. The Refractor (Galileo) telescope uses an
objective lens that bends light (refraction) for delivery to the
eyepiece. The Reflector (Newton) telescope uses a mirror to
collect the light, which it delivers to the eyepiece.
All optical instruments are measured and identified by two
numbers.
Aperture:
The width of the objective lens or mirror. The greater the width, the
greater the amount of light that can be collection. This affects
both the brightness and sharpness of the image.
Focal Length:
The distance from the optical center of the objective lens to the
optical center of the eyepiece. The longer the distance, the
greater the amount of the magnification.
Other Variables:
The amount of the magnification can be significantly altered by changing
the eyepiece and inserting or removing a Barlow lens.
(See below). |
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Telescope Characteristics / Optical Terms |
What qualities should you look for in a telescope? Most people
will think it is the amount of magnification, but the astronomer knows
better. What good is a large image if it fuzzy or so faint that it can
barely be seen?
Aperture (diameter of the objective lens or mirror)
This is the single most important factor in choosing a telescope. It
refers to the size of the opening used to collect light and the primary function of all telescopes is to collect light.
Refractor telescopes use a lens; reflector telescopes use a mirror. In
both cases, the
larger the aperture, the more light it can collect. This results
in a brighter image, greater detail, and image quality.
Resolution
This is the ability of a telescope to render detail, the higher the
resolution the finer the detail. The larger the aperture, the
greater the resolution.
Focal Length
This is the distance in an optical system from the lens (or mirror) to
the point where the instrument is in focus.
Magnification
A telescope is an optical system. The aperture collects the light
and brings it into focus. The image is then magnified by the
eyepiece. The combination is what yields the amount of
magnification. To determine the magnification, divide the focal
length of the telescope by the focal length of the eyepiece. For
example, if your telescope has a focal length of 500mm and if you use it
with a 4mm focal length, the magnification will be 500 divided by
4, or 125, which means 125 power magnification. |
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Chromatic Aberration
Newton's study of optics and light recognized that a lens is a circular prism.
Refraction - the
ability of a prism to bend light - resulted in different colors
of light bending by different degrees. This resulted in the
image of a bright object appearing with a ring of fuzzy colors
around it. This "chromatic
aberration," or fault with colors was the inherent defect
that led him to develop the refractor
telescope. Modern technology has solved this problem by
applying special coatings to lens. Today, such coatings
are applied to all quality lens. |
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