Celestar Manual
Celestial Observing
50
Figure 7-2
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, aurorae, natural airglow, and light pollution greatly effect
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
