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DECLINATION (DEC.)
The astronomical equivalent of latitude. Declination describes the
angle of a celestial object above or below the celestial equator. The
sky over the Northern hemisphere has a positive declination. The
sky over the Southern hemisphere has a negative declination. For
example, Polaris (the North Star) which lies nearly directly over the
North Pole, has a declination value of 90°.
RIGHT ASCENSION (R.A.)
The astronomical equivalent of longitude. Right Ascension measures
the degree of distance of a star to the east of where the ecliptic
crosses the celestial equator. R.A. is measured in hours, minutes,
and seconds as opposed to degrees. It is different than the term
“meridian”, which is used in referring to lines of longitude. Right
Ascension is referred to in “hour circles”. There are 24 hour circles of
right ascension which run from the north to the south celestial poles.
CELESTIAL EQUATOR
The celestial equator is the line of declination which lies directly
above the Earth’s equator. The celestial equator lies halfway between
the north and south celestial poles and serves as the 0° point in
measuring declination.
ECLIPTIC
The ecliptic is the apparent path of the sun through the sky over
the course of the year. Since we view the sun from different angles
throughout the year, it appears to move in relation to other stars.
The vernal (spring) and autumnal (fall) equinoxes lie at the points
where the ecliptic intersects the celestial equator. The vernal equinox
is where right ascension is at 0h (hours). The autumnal equinox can
be found at 12h R.A.
ASTRONOMY
TERMINOLOGY
ZENITH
The zenith is the point in the celestial sphere directly above your
head. The zenith varies depending upon your location. In general,
the declination point of your zenith is equal to the latitude at which
you are standing on Earth.
EPHEMERIS
The ephemeris of a planet or the sun or the moon is a table giving
the coordinates of the object at regular intervals of time. The coordinates
will be listed using declination and right ascension. Other information
such as distance and magnitude may be listed in ephemerides
(plural of ephemeris).
ALTITUDE
The altitude of a celestial object is the angular distance of that object
above the horizon.The maximum possible altitude is the altitude of
an object at the zenith, 90°. The altitude of an object on the horizon
is 0°. Altitude is measured from your point of observation and does
not directly correlate to points on the celestial sphere.
AZIMUTH
Azimuth is the angular distance around the horizon measured eastward
in degrees from the North Horizon Point. Thus, the North Horizon
Point lies at an azimuth of 0°, while the East Horizon Point lies at
90°, and the South Horizon Point at 180°. Azimuth is measured
from the point of observation and does not directly correspond to
points on the celestial sphere.
ANGULAR DISTANCE
Angular distance is the size of the angle through which a telescope
tube or binocular aiming at one object must be turned in order to
aim at another object. If you must rotate the equipment from the
zenith to the horizon, the angular distance between the two points
would be 90°.
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
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