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Viewing Conditions
38 © Celestial Observing
Viewing conditions affect what you can see through your C4.5 telescope
during an observing session. Conditions include transparency, sky illumina-
tion, and seeing. Understanding viewing conditions and the affect they have
on observing will help you get the most out of your C4.5 telescope.
Transparency is the clarity of the atmosphere and 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.
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Sky Illumination
General sky brightening caused by the Moon, aurorae, natural airglow, and
light pollution greatly affect transparency. While not a problem for the
Moon, planets, and brighter stars, bright skies reduce the contrast of ex-
tended 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 refer to the stability of the atmosphere and directly effects
the clarity of star images and the amount of fine detail seen in extended
objects like the planets. 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 dis-
placed creating an imperfect or smeared image. These atmospheric distur-
bances 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. Seeing conditions are rated on a five-
point scale where one is the worst and five is the best (see figure 6-4). Seeing
conditions can be classified in one of three categories which are based on the
cause.
Type 1 seeing conditions are characterized by rapid changes in the image
seen through the telescope. Extended objects, like the Moon, appear to
shimmer while point sources (i.e., stars) appear double. Type 1 seeing is
caused by currents within or very close to the telescope tube. These currents
could be caused by a telescope that has not reached thermal equilibrium with
the outdoor surroundings, heat waves from people standing near the tele-
scope, or heated dew caps. To avoid the problems associated with Type 1
seeing, allow your telescope approximately 20 to 30 minutes to reach thermal
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