Observing Deep-Sky Objects
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“Seeing” Conditions
Viewing conditions affect what you can see through your telescope during an observing session. Conditions include transparency, sky illumination and “seeing”. Understanding viewing conditions and the affect they have on observing will help you get the most out of your telescope.
Transparency
Transparency refers to the clarity of the atmosphere and is affected by clouds, moisture, dust and other airborne particles. Thick cumulus clouds are completely opaque, while cirrus clouds can be thin, allowing light from the brightest stars through. Hazy skies absorb more light than clear skies, making fainter objects hard to see and reducing contrast on brighter objects. Dust particles and gases ejected into the upper atmosphere from volcanic eruptions also affect transparency. Ideal conditions are when the night sky is inky black.
Sky Illumination
General sky brightening caused by the moon, aurorae, natural airglow and light pollution greatly affect transparency. While not a problem when viewing brighter stars and planets, bright skies reduce the contrast of extended nebulae, making them difficult, if not impossible, to see. To maximize your observing, limit
"Seeing"
The terms “seeing conditions” or “seeing” refer to the stability of the atmosphere, which directly effects the amount of fine detail seen in extended objects. Essentially, extended objects are objects other than stars, of some size, such as nebulae and galaxies.
The air in our atmosphere acts as a lens, which bends and distorts incoming light rays. The amount of bending depends on air density. Varying temperature layers have different densities and, therefore, bend light differently. Light rays from the same object arrive slightly displaced, creating an imperfect or smeared image. These atmospheric disturbances vary from
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