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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
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
Sky Illumination
General sky brightening caused by the Moon, aurora, 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
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
Seeing conditions directly affect image quality These drawings represent a point source (i e , star) under bad seeing
conditions (left) to excellent conditions (right) Most often, seeing conditions produce images that lie somewhere between
these two extremes
Fig. 4-3
