Celestron 22007, Instruction Manual

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18 I ENGLISH
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 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 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
Fig. 18
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.