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ity of the atmosphere and directly affects the amount of fine detail seen in extended
s vary from time-to-time and place-to-place. The size of the air parcels compared to your aperture
eeing conditions refers to the stabil
S
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
isturbance
d
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.
Figure 5-1
Seeing condit
ions directly affect image quality. These drawings represent a
to excellent conditions
t lie some where
point source (i.e., star) under bad seeing conditions (left)
(
right). Most often, seeing conditions produce images tha
between these two extremes.
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farther the incoming light rays are off-axis (i.e. passing
rough the edge of the objective lens) and virtually unnoticeable on-axis (passing through the center of the objective
evident when observing very bright sources of light, such as bright planets and
ere are several techniques that the observer can employ to suppress visible signs of
p removed, you will allow all the incoming light to pass closer to the
ince, most planets are extremely bright objects (visible to the unaided eye) any loss of light
will be unnoticeable.
t particular planetary detail, such as the polar caps on Mars or the bands and
nes around Jupiter. The use of Celestron's Refractor Filter (# 94121) reduces the effect of chromatic aberration as
solution.
As mentioned earlier, all refractive optics will exhibit some amount of chromatic aberration due to the prism effect of
lenses. Chromatic aberration will become more pronounced the
th
lens). This type of aberration is only
ery luminous stars (like Sirius). Th
v
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hromatic aberration, these include; reducing the aperture and using filters.
The objective lens cap covering the objective lens of the telescope has a built-in aperture stop in the center. By leaving
the lens cap on the telescope with the aperture sto
center of the optical axis. S
from reducing the aperture
The lens cap should always be completely removed when observing deep-sky objects such as galaxies and nebulae,
where aperture (light gathering power) is essential and chromatic aberration is not an issue.
Another useful technique for reducing aberrations and improving planetary detail is the use of colored eyepiece filters.
Filters are commonly used to bring ou
zo
well as improves contrast and re
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