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48 • Celestial Observing
Viewing conditions affect what you can see through your CM-1100 telescope
during an observing session. Conditions include transparency, sky illumina-
tion, and seeing. Understanding viewing conditions and the effect they have on
observing will help you get the most out of your CM-1100 telescope.
Transparency
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 clouds 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 transpar-
ency. 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. You can, on the other hand, observe planets and
stars from light polluted areas or when the Moon is out.
Seeing Conditions
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. 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. Seeing conditions are rated on a five-point scale where one is the worst
and five is the best (see figure 6-5). Seeing conditions can be classified in one
of three categories.
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 telescope,
or heated dew caps. To avoid the problems associated with Type 1 seeing,
allow your telescope at least 45 minutes to reach thermal equilibrium. Once
adjusted to the outdoor temperature, don’t touch the telescope tube with your
hands. If observing with others, make sure no one stands in front of or directly
below the telescope tube.
Viewing Conditions