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How far can I see with a telescope
"Seeing" is the term astronomers use to describe the sky's atmospheric steadiness. Transparency is used to describe atmospheric clarity.

The atmosphere is in continual motion with changing temperatures, air currents or wind and weather fronts, causing stars to twinkle. (The images of twinkling stars are highly distorted and the amount of light reaching us through the air is also affected, so it changes from instant to instant and the star twinkles.) If the stars are twinkling considerably, this is known as "poor" seeing conditions. When the star images are steady, this is known as "good" seeing conditions. Poor seeing is most noticeable when observing planets, close double stars and the moon, whereas deep sky objects such as nebulae and galaxies are less affected by poor seeing conditions.

For deep-sky objects, the most important factor is the transparency of the atmosphere (a measure of how clear the sky is on a given night - determined by clouds, dust and haze). Seeing conditions and transparency will vary widely from site to site, from season to season and from night to night.
How far can I see with a telescope
The farthest you can see in the sky with your telescope will depend on its ability to gather light and where you observe.

The most distant objects visible with amateur-sized telescopes are faint galaxies and the brightest quasars. They will be brighter and easier to see in a scope that gathers more light and has a greater magnitude limit (two topics discussed elsewhere in the Knowledgebase). Here bigger is better and a larger aperture scope will see more remote objects.

Location is also important. Even with a big scope, you’ll see fainter, deeper and farther out into the universe from an isolated dark-sky site than from the heart of a megalopolis.

For example: quasars are all very faint as seen from the earth. The brightest one is 3C 273 in Virgo. It is magnitude 12.9. A good 4-inch scope is capable of seeing it in dark-sky conditions. You’ll need a larger scope to see it at all from a city.

This quasar is an active galactic nucleus that is located about 2.6 billion light-years from the earth.

What does magnitude mean?

Astronomers use a system of magnitudes to accurately measure the brightnesses of astronomical objects. An object is said to have a certain numerical magnitude. The larger the magnitude number, the fainter the object. Each object with an increased number (next larger magnitude number) is approximately 2.5 times fainter. The faintest star you can see with your unaided eye (no telescope) is about sixth magnitude (from dark skies) or magnitude 6.0, whereas the brightest stars are negative numbers. Sirius is the brightest appearing star in the sky and is magnitude -1.5.
Apparent magnitudes of selected objects:

Sun                             -26.7
Full moon                   -12.6
Venus (maximum)     -4.7
Sirius                          -1.5
Saturn (maximum)     -0.2
Naked-eye limit         6.0
Neptune (maximum) 7.7
Quasar 3C-273         12.9
Pluto (maximum)       13.7
8-inch telescope        14.2 (limit)
Hubble Telescope     30.0 (limit)

Information on this page courtesy Celestron’s web site celestron.com
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