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OBJECTIVE -
The objective is the front lens of a
telescope. The listed measurement for objective
lenses is the lens diameter. A larger objective
allows more light to enter a telescope and provides
a brighter image. The objective diameter is
also sometimes referred to as the aperture of a
telescope.
FOCAL LENGTH -
The focal length of a telescope
is the distance from the point where light enters
a telescope (the objective) to the point where the
image is in focus. In telescopes with the same size
objective, a longer focal length will provide higher
magnification and a smaller field of view.
MAGNIFICATION -
The magnification of a
telescope is determined by the relationship between
the focal length of the telescope and the focal length
of the eyepiece used. A greater difference in these
focal lengths results in a greater the magnification
of the telescope. Every telescope has a maximum
useful magnification of about 60 times the diameter
of the objective in inches. Magnification beyond the
maximum useful magnification will provide dim, low-
contrast images.
FOCAL RATIO -
The focal ratio of a telescope
is a description of the relationship between the
focal length and objective lens size of a telescope.
Visually, a smaller focal ratio (also called f-stop)
provides a wider field of view. Photographically,
the lower the f-stop, the shorter the exposure time
needed to capture an object on film.
LIMITING MAGNITUDE -
The limiting magnitude
of a telescope describes the faintest object you
can see with a telescope. The magnitude of a star
describes its brightness. The larger the magnitude
of an object, the fainter it appears to be. The
brightest stars have a magnitude of 0 or less.
RESOLVING POWER -
The resolving power, or
Dawes’ Limit, of a telescope is the ability to view
closely spaced objects through a telescope. The
resolving power of a telescope is measured in
seconds of arc. The smaller the resolving power, the
better you will be able to separate binary stars when
viewing through your telescope.
ABERRATION -
Aberrations are degradations
in image, which can occur due to optical system
design or improper alignment of optical system
components. The most common types of
aberration are chromatic aberration, spherical
aberration, coma, astigmatism, and field curvature.
COLLIMATION -
Collimation is the alignment
of optical components within an optical system.
Improper collimation will distort an image and
may result in aberrations present in the image.
Most reflector telescopes have collimation
adjustments which can be made in order to
reduce aberrations and image distortion. Refractor
telescopes do not require collimation nearly as often
as reflector telescopes.
TELESCOPE TERMINOLOGY
