Seeing Conditions - Celestron AstroMaster LT Instruction Manual

SEEING CONDITIONS

Viewing conditions affect what you can see through your telescope during an observing session. Condi-
tions 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 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 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 cer-
tain 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 den-
sities and, therefore, bend light differently. Light rays from the same object arrive slightly displaced creat-
ing 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 somewhere between these two extremes.
Figure 5-3
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