Seeing Conditions; Transparency; Sky Illumination; Seeing - Celestron SKY PRODIGY 70 Instruction Manual

urban areas washes out most nebulae making them difficult, if not
impossible, to observe. Light Pollution Reduction filters help
reduce the background sky brightness, thus increasing contrast.

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 effect they have on observing will help you get the most
out of your telescope.

Transparency

Transparency is the clarity of the atmosphere which is affected
by clouds, moisture and other airborne particles. Thick cumulus
clouds are completely opaque while cirrus can be thin, allowing
the light from the brightest stars through. Hazy skies absorb
more light than clear skies making fainter objects harder to see
and reducing contrast on brighter objects. Aerosols ejected into
the upper atmosphere from volcanic eruptions also affect trans-
parency. 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 for the 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 deep
sky viewing to moonless nights far from the light polluted skies
found around major urban areas. LPR filters enhance deep sky
viewing from light polluted areas by blocking unwanted light
while transmitting light from certain deep sky objects. You can,
on the other hand, observe planets and stars from light polluted
areas or when the Moon is out.

Seeing

Seeing conditions refers to the stability of the atmosphere and
directly affects the amount of fine detail seen in extended
objects. 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 time-to-time
and place-to-place. The size of the air parcels compared to your
aperture determines the "seeing" quality. Under good seeing
conditions, fine detail is visible on the brighter planets like
Jupiter and Mars, and stars are pinpoint images. Under poor
seeing conditions, images are blurred, and stars appear as blobs.
The conditions described here apply to both visual and
photographic observations.
Seeing conditions directly affect image quality. These drawings
represent a point source (i.e., star) under bad seeing conditions
(left) to excellent conditions (right). Most often, seeing conditions
produce images that lie some where between these two extremes.
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