Celestial Observing • 41
Viewing conditions affect what you can see through your CR-150 HD telescope
during an observing session. Conditions include transparency, sky illumination,
and seeing. Understanding viewing conditions and the affect they have on observ-
ing will help you get the most out of your telescope.
TransparencyTransparency
TransparencyTransparency
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 atmo-
sphere from volcanic eruptions also affect transparency. Ideal conditions are when
the night sky is inky black.
Sky IlluminationSky Illumination
Sky IlluminationSky Illumination
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 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.
SeeingSeeing
SeeingSeeing
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 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 6-4). 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 telescope,
or heated dew caps. To avoid the problems associated with Type 1 seeing, allow
your telescope approximately 20 to 30 minutes to reach thermal equilibrium.
Viewing ConditionsViewing Conditions
Viewing ConditionsViewing Conditions
Viewing Conditions
