Planetary Observers Club Chair:
Aaron Clevenson
19411 Cluster Oaks Drive
Humble, TX 77346
E-mail: aaron@clevenson.org
GLOSSARY OF TERMS
Albedo The proportion of light falling on the
surface of a body that is reflected.
Ansae Latin for 'handles'. The appearance of
the protrusion of the rings of Saturn on either side
of the planet's disc.
Aperture The diameter of the primary light
collecting element of a telescope, be it a mirror or
lens.
Apparition The period of time when it is possible
to observe an object in the sky.
Arc second A unit of angular measurement. One
60th of an arc minute or 1/3600 of a degree.
Eclipse The occurrence of one celestial body's
shadow temporarily falling on another body.
Egress To emerge from, as a star or satellite
emerging from behind another planetary body.
Ingress To disappear behind. The opposite of
'egress'.
Lunar Limb The extreme edge of the visible
Moon.
Meridian A line on the celestial sphere passing
from the North Pole through the Zenith to the South
Pole. Can also be the center line of a planetary body
drawn through it's poles.
Objective The main light gathering optic in
a telescope or binocular.
Occultation The passage of one celestial body
moving directly in front of another.
Opposition The point at which a planet appears
in our sky directly opposite the Sun.
Phase Defect The extent to which the illuminated
area of a spherical body such as a planet or moon differs
from a complete circular disk.
Prograde Motion of a celestial body moving
in its usual direction on the celestial sphere.
Red Spot A large hurricane like oval feature
in the clouds of Jupiter larger than the Earth. It has
been visible since first reported in 1664. Its color
density varies over time, sometimes being difficult
to see.
Retrograde Motion of a celestial body moving
in the opposite direction of its usual motion.
Terminator The boundary between the lighted
and unlighted portions of a celestial body's surface.
Transit The crossing of a celestial object
across the observer's meridian caused by the daily apparent
motion of the celestial sphere. Also the passage of
a planet across the face of the Sun or of a planet's
satellite across the primary's disk.
BRIGHTNESS SCALE
| MAGNITUDE* |
COMMENTS |
| Very bright stars |
|
| -4 |
Venus at its brightest |
| -3 |
Jupiter at its brightest |
| -2 |
Sirius in Canis Major, the brightest star in the
sky |
| -1 |
Betelgeuse in Orion |
| 0 |
Vega in Lyra |
| +1 |
Spica in Virgo, Deneb in Cygnus, Pollux in Gemini |
| +2 |
Polaris, the north star |
| +3 |
Megrez, the faintest star in the Big Dipper |
| +4 |
|
| +5 |
Probable naked eye limit in the suburbs |
| +6 |
Probable naked eye limit in the country |
| +7 |
|
| +8 |
Neptune |
| +9 |
Approximate limit of typical binoculars |
| +10 |
Approximate limit of a 60 mm telescope |
| +11 |
Approximate limit of a 3-inch telescope |
| +12 |
Approximate limit of a 4-inch telescope |
| +13 |
Approximate limit of a 6-inch telescope |
| +14 |
Approximate limit of an 8-inch telescope |
| Very dim stars |
(Above Comments assume as dark a sky
as possible.) |
* The difference in brightness between any successive
two numbers is a ratio of two and one half times. Magnitudes
are approximate.
FILTERS FOR VISUAL OBSERVATION
A good resource at the telescope is a set of colored
filters. Filters can be acquired from various sources.
Consult your astronomy magazines. Kodak's Wratten series
can be purchased in over a hundred colors and densities
and can be mounted in slide mounts and simply held between
the eyepiece and the eye. For longer observations, as
when sketching, screw-in filters are available for both
1-1/4 and 2-inch eyepieces. You don't need to choose
between dozens of colors though, only a few will do.
Filters can reduce glare, improve
image definition and enhance tonal contrast. Here are
some suggestions:
A BLUE filter, such as a Wratten
#44A, 47B, or 80A, can be used for the detection of
high altitude clouds on Mars, white ovals and spots
in the belts of Jupiter, and the zones of the clouds
of Saturn. It can also be used to cut down glare on
a bright Moon.
A GREEN filter, such as a Wratten
#58, allows you to see more clearly the edges of the
Martian polar caps and enhances the belts and Great
Red Spot in the clouds of Jupiter.
A YELLOW filter, such as a
Wratten #8, 12, or 15, can improve markings in the clouds
of Venus and enhance Martian dust storms.
An ORANGE filter, such as a
Wratten #21, is one of the more useful ones you can
have. It is used for bringing out detail on Mars, and
enhancing some of the zonal detail on Jupiter. An orange
filter darkens the blue sky so daytime observations
of Jupiter, Venus and the Moon are much improved.
A RED filter, such as a Wratten
#23A, 25, or 25A, can also be used to enhance contrast
on Mars, Jupiter and Saturn. A red filter, however,
is fairly dark, so it works best on larger aperture
telescopes which give brighter images. Flipping back
and forth between red and blue filters can sometimes
bring out subtle colorations on the Moon.
A POLARIZING filter can cut
down glare when observing a nearly full Moon, making
it easier to see ray structure. It will also cut down
day-time glare.
ASTRONOMICAL SEEING
| LEVEL ONE |
Severely disturbed skies: Even low power* views
are uselessly shaky. Go read a good book |
| LEVEL TWO |
Poor seeing: Low power images are pretty steady,
but medium powers are not. |
| LEVEL THREE |
Good seeing: You can use about half the useful
magnification of your scope. High powers* produce
fidgety planets. |
| LEVEL FOUR |
Excellent seeing: Medium-powers are crisp and
stable. High-powers are good, but a little soft. |
| LEVEL FIVE |
Superb seeing: Any power produces a good crisp
image. |
*The PRACTICAL LOWEST power magnification for
any telescope is approximately 7 times for each inch
of aperture. Example: 28X for a 4-inch (100mm) diameter
telescope.
*The PRACTICAL HIGHEST power magnification
for any telescope is approximately 50 times for each
inch of aperture. Example: 200X for a 4-inch (100mm)
diameter telescope.
|
