Celestron NexStar 8i manual Focal length

Focal length	The distance between a lens (or mirror) and the point at which the image of an object at
	infinity is brought to focus. The focal length divided by the aperture of the mirror or lens is
	termed the focal ratio.
J -
Jovian Planets	Any of the four gas giant planets that are at a greater distance form the sun than the
	terrestrial planets.
K -
Kuiper Belt	A region beyond the orbit of Neptune extending to about 1000 AU which is a source of
L -	many short period comets.
Light-Year	A light-year is the distance light traverses in a vacuum in one year at the speed of 299,792
	km/ sec. With 31,557,600 seconds in a year, the light-year equals a distance of 9.46 X 1
M -	trillion km (5.87 X 1 trillion mi).
Magnitude	Magnitude is a measure of the brightness of a celestial body. The brightest stars are
	assigned magnitude 1 and those increasingly fainter from 2 down to magnitude 5. The
	faintest star that can be seen without a telescope is about magnitude 6. Each magnitude
	step corresponds to a ratio of 2.5 in brightness. Thus a star of magnitude 1 is 2.5 times
	brighter than a star of magnitude 2, and 100 times brighter than a magnitude 5 star. The
	brightest star, Sirius, has an apparent magnitude of -1.6, the full moon is -12.7, and the
	Sun's brightness, expressed on a magnitude scale, is -26.78. The zero point of the
	apparent magnitude scale is arbitrary.
Meridian	A reference line in the sky that starts at the North celestial pole and ends at the South
	celestial pole and passes through the zenith. If you are facing South, the meridian starts
	from your Southern horizon and passes directly overhead to the North celestial pole.
Messier	A French astronomer in the late 1700’s who was primarily looking for comets. Comets are
	hazy diffuse objects and so Messier cataloged objects that were not comets to help his
N -	search. This catalog became the Messier Catalog, M1 through M110.
Nebula	Interstellar cloud of gas and dust. Also refers to any celestial object that has a cloudy
	appearance.
North Celestial Pole	The point in the Northern hemisphere around which all the stars appear to rotate. This is
	caused by the fact that the Earth is rotating on an axis that passes through the North and
	South celestial poles. The star Polaris lies less than a degree from this point and is
	therefore referred to as the "Pole Star".
Nova	Although Latin for "new" it denotes a star that suddenly becomes explosively bright at the
O -	end of its life cycle.
Open Cluster	One of the groupings of stars that are concentrated along the plane of the Milky Way. Most
	have an asymmetrical appearance and are loosely assembled. They contain from a dozen
P -	to many hundred stars.
Parallax	Parallax is the difference in the apparent position of an object against a background when
	viewed by an observer from two different locations. These positions and the actual position
	of the object form a triangle from which the apex angle (the parallax) and the distance of
	the object can be determined if the length of the baseline between the observing positions
	is known and the angular direction of the object from each position at the ends of the
	baseline has been measured. The traditional method in astronomy of determining the
	distance to a celestial object is to measure its parallax.
Parfocal	Refers to a group of eyepieces that all require the same distance from the focal plane of
	the telescope to be in focus. This means when you focus one parfocal eyepiece all the
	other parfocal eyepieces, in a particular line of eyepieces, will be in focus.
Parsec	The distance at which a star would show parallax of one second of arc. It is equal to 3.26
	light-years, 206,265 astronomical units, or 30,8000,000,000,000 km. (Apart from the Sun,
	no star lies within one parsec of us.)
Point Source	An object which cannot be resolved into an image because it to too far away or too small is
	considered a point source. A planet is far away but it can be resolved as a disk. Most
R -	stars cannot be resolved as disks, they are too far away.
Reflector	A telescope in which the light is collected by means of a mirror.
Resolution	The minimum detectable angle an optical system can detect. Because of diffraction, there
	is a limit to the minimum angle, resolution. The larger the aperture, the better the

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