Glossary
Glossary
Here are some of the terms used in our astronomy pages. We are working on a comprehensive glossary and will post it when it is complete and verified for accuracy. In the mean time, there are a number of outstanding glossaries available on line.
SEDS (
Students for the Exploration and Development of Space ) has a wonderful
glossary at:
http://seds.lpl.arizona.edu/billa/tnp/help.html#au
At the end of the SEDS glossary are a number of additional links to other great sources of information.
(AU) astronomical unit:
The average distance from the Earth to the
Sun. 1 AU = 149,597,870 km or 92,960,116 miles
Aperture:
Aperture represents the size (diameter) of the
primary mirror or objective lens of a telescope.
Aperture Fever:
Many amateur astronomers suffer from a
condition known as Aperture Fever. This is an ongoing desire for more
light gathering power. The larger the aperture, the more faint
objects one can see in their telescope.
Barlow Lens:
A lens designed to increase the magnification
of an eyepiece. The eyepiece is slipped into the Barlow lens which is
in turn placed in the focuser. Barlows come in many flavors. There
are Barlows that will double (2X), triple (3X), and even quintuple
(5X) the power of your eyepieces.
Binary Star:
Two stars which revolve around each other's
center of gravity. Mizar, the 2nd star in the handle of the Big
Dipper is a binary star. Have a look at it. The sky is brimming with
binary star systems.
Catadioptric:
Catadioptric telescopes are essentially a
combination of a refractor (lenses) and a reflector (mirrors). These
telescopes fold the light path 3 times allowing for a much shorter
tube. Further, because of their clever use of corrective lenses and
lack of a spider to hold the secondary mirror, they are free of many
of the optical defects that can plague refractors and reflectors.
Chromatic Aberration:
Chromatic Aberration is a defect in some
optical systems that adds false color to images. These colors usually
take the form of a light purple halo around the observed object.
Collimation:
Collimation is the term used for the alignment
of the optics in a telescope. Improperly collimated optics will not
deliver optimum performance and will add a number of undesirable
distortions to the image.
Coma:
A condition associated with some optical
systems that has the effect of making objects at the outside edge of
the field view have the appearance of being wedge shaped or look like
little comets.
Contrast:
For astronomical purposes, contrast is simply
how dark the sky appears behind the objects that are being viewed.
The more contrast, the darker the sky and the more detail one can
make out in the object being viewed.
Diffraction:
The effect caused by light bending around
obstructions in an optical system. (More coming on this including
some graphic illustrations.)
Ellipse:
Basically,(well almost) an oval. Webster's
calls it a closed plane curve generated by a point moving in such a
way that the sums of its distances from two fixed points is a
constant: a plane section of a right circular cone that is a closed
curve. Let's go with "kind of
like an oval" till
I can explain this with a few graphics.
Focuser:
A device used to move an eyepiece towards or
away from the focal plane of an optical system like a telescope.
Light-Year:
The distance light travels in a year.
9,460,920,000,000 km or 5,880,000,000,000 miles or 63,239 AU
Light-Speed:
186,000 miles/second or 299,274 km/second
Newton (Issac):
http://www.treasure-troves.com/bios/Newton.html
Magnitude (or apparent magnitude):
Magnitude or apparent magnitude is the term
used to indicate the degree of brightness of a celestial body. This
system of measuring brightness was conceived
by the Greek astronomer Hipparchus. Since his time, this
system has been expanded to accommodate
fainter objects. This is a numerical scale where the brightest star (
Sirius ) other than our sun (Sol) has a magnitude of -1 and the
faintest star visible to the naked eye (of most human beings)
have a magnitude of 6. An
increase in one magnitude is equal to 2 1/2 times the brightness
of the previous step in the magnitude scale. Our sun has a magnitude
of -26. This is almost 6 trillion times brighter than a 6th magnitude star.
The chart below demonstrates how this scale functions from a magnitude 6 star to a magnitude 1 star in terms of increase in brightness.
|
Magnitude of star |
Times brighter than magnitude 6 |
|
1 |
97.7 |
|
2 |
39.1 |
|
3 |
15.6 |
|
4 |
6.3 |
|
5 |
2.5 |
|
6 |
Objective:
An objective is the main or largest lens or
mirror in a telescope. If it is a mirror, it is often called the
primary mirror.
Paraboloid:
A concave surface with a three-dimensional
figure. ( We will try to get a nice graphic up explaining the
differences between spherical, parabolic and paraboloidal surfaces.)
Reflex Sight:
A reflex sight is a zero power finder that
greatly simplifies aiming your telescope. It works by projecting an
illuminated reticule (pattern) on to a flat piece of glass. This is
very reminiscent of a heads up display. This reticule is often shaped
like a bullseye. You simply look through the glass and move the
bullseye over the object in the sky that you want to look at with
your telescope. The Telrad Reflex Sight is the most popular of these
types of finders and in our opinion, should be installed on every
telescope as it takes ALL of the frustration out of finding objects
in the sky. At $40.00 to $50.00, it is very economical and probably
the most cost effective and helpful accessory one can add to a telescope.
Seeing:
Seeing is the term used by astronomers to
describe the quality (transparency) of the atmosphere on any given
night. Our atmosphere is extremely dynamic. Most nights, the stars
twinkle. Twinkling is caused by the light of the stars bending
through our turbulent atmosphere. Poor seeing results in poor image
quality at the eyepiece. Imagine looking at a quarter at the bottom
of a swimming pool. Now imagine that the swimming pool has big waves.
Get the idea? If you have a night of good seeing, grab that higher
power eyepiece and use it. Otherwise, be happy with those lovely,
wide angle lower power views.
Zenith:
The point in space directly above you.