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4
©2011 Celestron • All rights reserved.
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
transparency. Ideal conditions are when the night sky is inky black.
sky illuminatiOn
General sky brightening caused by the Moon, aurora, natural
airglow, and light pollution greatly affects transparency. While
this is 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 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 skies 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
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.
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