Seeing Conditions; Sky Illumination - Celestron Astromaster 90 AZ Mode D'emploi

• Hazy skies, fog, and mist can also make it difficult to focus when viewing terrestrially . The amount of detail seen under these
conditions is greatly reduced .
• If you wear corrective lenses (specifically glasses), you may want to remove them when observing with an eyepiece
attached to the telescope . When using a camera, however, you should always wear corrective lenses to ensure the
sharpest possible focus . If you have astigmatism, corrective lenses must be worn at all times .

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 .
14 I ENGLISH