Celestron 11007, Instruction Manual

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Seeing conditions directly affect image quality. These drawing 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.
FIGURE7-1
>> 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 .
>> 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 .
>> 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 .
>> 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 .
>> Observing Deep Sky Objects
Deep-sky objects are simply those objects outside the boundaries of
our solar system. They include star clusters, planetary nebulae, diffuse
nebulae, double stars and other galaxies outside our own Milky Way .
Most deep-sky objects have a large angular size. Therefore, low-to-
moderate power is all you need to see them . Visually, they are too faint
to reveal any of the color seen in long exposure photographs. Instead, they appear black and white . And, because of their low surface brightness,
they should be observed from a dark-sky location . Light pollution around
large 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 .