Celestial Observing • 39
Viewing conditions affect what you can see through your G-9
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telescope during
an observing session. Conditions include transparency, sky illumination, and
seeing. Understanding viewing conditions and the affect they have on observing
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 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 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 incom-
ing light rays. The amount of bending depends on air density. Varying tempera-
ture 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 condi-
tions, 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 5-4). 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. Once adjusted to the outdoor
temperature, don’t touch the telescope tube with your hands. When pointing the
Viewing ConditionsViewing Conditions
Viewing ConditionsViewing Conditions
Viewing Conditions
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