A | B |
C | D |
E | F |
G | H |
I | J | K | L |
M | N |
O | P | Q |
R | S |
| U | V | W | X | Y |
Nanometer - One billionth of a
meter. This is the standard unit for measuring
wavelengths of light. The visible spectrum (what the human eye sees)
covers the wavelength range from 400-700 nanometers (nm). Sometimes
visible wavelengths are given in Angstroms, 1/10th
of a nanometer.
Narrowband Filter - Narrowband
filters allow a very specific small portion of the light spectrum to be
transmitted. Isolating a particular wavelength can allow certain details to be seen that are otherwise
invisible or difficult to observe. For stargazing, an OIII filter is used
for enhancing the view of nebulae by blocking light
pollution. For solar observing, an H-alpha filters
is used to view solar flares and prominences. Narrowband filters are also
often used for CCD imaging of nebulae.
Nebula - A type of
deep-sky object, nebulae are clouds of glowing
gas. There are several types of nebulae, including clouds of gas where
stars are forming and clouds of gas left by stars that have died. Diffuse
nebula are huge clouds of mostly hydrogen gas, which collapse under gravity to
form stars. The stars then give off radiation that causes the gas to glow,
allowing it to be seen through a telescope.
Planetary nebulae form when stars die and shed their gaseous atmospheres.
Supernova remnants are clouds left when giant
stars die and explode. Nebulae are among the most colorful objects in
photographs, but little to no color can be seen visually through a telescope
(due to the way the human eye works). Examples of nebulae include the
Great Orion Nebula and the Lagoon Nebula (both diffuse nebulae), and the Ring Nebula
(a planetary nebula).
Neutral Density Filter - A neutral
density (ND) is used to
attenuate the amount of light coming through a telescope. Normally such a
filter is used for observing bright objects such as the moon and planets.
White-light solar filters are also a type of neutral density filter, although
much darker than those used for lunar observing. Lunar ND filters usually
transmit 10-50% of the light, while solar ND filters transmit only 1/1000th of
1% of the light.
North Celestial Pole - This is the point on the
celestial sphere directly above Earth's north pole. The Earth's rotation
axis is aimed at this point in the sky, so during the course of the night, while
the rest of the sky appears to turn, this spot in the sky remains in the same
position. The moderately bright star Polaris is located very near (but not
exactly at) the north celestial pole.