Seeing Conditions; Transparency; Sky Illumination; Seeing - Celestron Ultima 11 Instruction Manual

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 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 refer to the stability of the atmosphere and directly effects 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
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