Appendix D - Glossary - SKY-WATCHER S11670 User Manual

bsolute Magnitude
The apparent brightness a star would have if placed
at a distance of 10 parsecs from the earth.
Achromatic Lens
A refractor lens, made of two or sometimes three
separate lenses, which has the effect of bringing
most of the viewed colors to a sharp focus, thus
reducing chromatic aberration.
Alt-azimuth
A simple mount that allows movement in altitude (up
and down) and in azimuth (side to side).
Anti-reflection Coating
A thin layer of film applied to an optical surface that
reduces the loss of transmission of light.
Aperture
The diameter of the primary mirror or lens.
arlow Lens
A "negative" lens which, when placed in front of the
eyepiece, increases
magnification and decreases the field.
ollimation
The process of aligning all the elements of an optical
system. Collimation is routinely needed in reflectors,
often in Catadioptric systems but seldom in
refractors.
eclination
Similar to Latitude on the Earth's surface, it is the
distance in degrees North or South of the Celestial
Equator (the projection of the Earth's Equator onto
the Celestial Sphere). The degrees can be
sub-divided into minutes and seconds.
Dew Cap
A tube extending forward from the front lens of a
telescope. It prevents dew from forming on the lens
as it cools down, and acts as a sunshade to reduce
reflections during the day.
Diagonal
A mirror or prism system which changes the angle
and orientation of the light rays coming from the
telescope to the eyepiece.
quatorial Mount
A telescope mount with an axis parallel to the axis of
the earth. This provides easy tracking of sky objects
and for photography when combined with a clock
drive.
APPENDIX D – GLOSSARY
the focal length and
Eyepiece
Also called an ocular. This is a small tube that
contains the lenses needed to bring a telescope's
focus to a final image in the eye. Telescopes usually
come with at least two eyepieces: one for low power
and a second for a higher power view.
Eye Relief
The distance between the eyepiece lens and the
position in which the eye must be placed to see
through the telescope. Telescope users who wear
eyeglasses while observing, appreciate the benefits
of longer eye relief.
Exit Pupil
This is the diameter of the beam of light from the
eyepiece which reaches the pupil of the eye. It is
usually expressed in mm, and determined by dividing
the diameter of the primary (in mm) by the
Magnification. Knowing this value and the diameter
of your dilated pupil allows you to choose the
eyepieces which will work best for you with a specific
telescope.
ield of View
The maximum view angle of an optical instrument.
The number, in degrees,
manufacturer is the Apparent Field of View. To find
the True Field of View (also known as the Actual
Field of View), divide the Apparent Field of View by
the Magnification.
Finderscope
A low power telescope attached parallel to the main
instrument which provides easy object locating and
telescope aiming.
Focal Length
The distance of the light path from the objective
(primary lens or mirror) to the convergence of the
beam. The convergent spot is called the Focus or
Focal Point.
Focal Ratio
This is found by dividing an optical system's Focal
Length by its Aperture. The resulting value is
sometimes called the system's "speed".
Focuser
A device which brings the light rays in a telescope to
a precise focus. Common designs include geared
(rack-and-pinion), gearless (Crayford-style) and
helical.
ens
A transparent optical element consisting of one or
more pieces of glass. A lens has curved surfaces
that bring distant light to a focus.
supplied by the
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