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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.
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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 farther the incoming light
rays are off-axis (i.e. passing through the edge of the objective lens) and virtually unnoticeable on-axis
(passing through the center of the objective lens). This type of aberration is only evident when observing
very bright sources of light, such as bright planets and very luminous stars (like Sirius). There are several
techniques that the observer can employ to suppress visible signs of chromatic 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 stop removed, you will allow all the incoming
light to pass closer to the center of the optical axis. Since, most planets are extremely bright objects
(visible to the unaided eye) any loss of light from reducing the aperture will be unnoticeable.
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 out particular planetary detail, such as the polar caps
on Mars or the bands and zones around Jupiter. The use of Celestron's Minus Violet Refractor Filter
(#94121) reduces the effect of chromatic aberration as well as improves contrast and resolution.
Figure 5-1
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 some where between these two
extremes.