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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.

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