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Seeing
Seeing conditions refer 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 atmospheric turbulent cells 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. Seeing conditions are rated on five-point scale where one is the worst and five is the best (see Figure 9-1). Seeing
conditions can be classified in one of three categories which are based on the cause.
Type 1 seeing conditions are characterized by rapid changes in the image seen through the telescope. Extended objects,
like the Moon, appear to shimmer while point sources (i.e., stars) appear double. Type 1 seeing is caused by currents
within or very close to the telescope tube. These currents could be caused by a telescope that has not reached thermal
equilibrium with the outdoor surroundings, heat waves from people standing near the telescope, or heated dew caps. To
avoid the problems associated with Type 1 seeing, allow your telescope approximately 45 minutes to reach thermal
equilibrium. Once adjusted to the outdoor temperature, don't touch the telescope tube with your hands. When pointing
the telescope, hold the Ultima by the handle on the rear cell. If observing with others, make sure no one stands in front of
or directly below the telescope tube.
The images produced by Type 2 seeing conditions don't move as quickly as those produced by Type 1 conditions, but the
images are quite blurry. Fine detail is lost and the contrast is low for extended objects. Stars are spread out and not sharp.
The source of Type 2 seeing is the lower atmosphere, most likely heat waves from the ground or buildings. To avoid the
problems associated with Type 2 seeing, select a good observing site. Look for broad hill tops or open grassy fields.
Stable thermal conditions found near lakes and atmospheric inversions also tend to produce good seeing. Avoid sites that
overlook asphalt parking lots or plowed fields. Stay away from valleys and shorelines. If you can't get a better location,
wait until the early morning hours when the surroundings are uniformly cool and the seeing is generally better.
Type 3 seeing conditions are characterized by fast ripples, but sharp images. In extended objects fine detail is visible, but
the images shift around the field. Stars are crisp points, but they shift small distances rapidly around the field. The cause
of Type 3 seeing is turbulence in the upper atmosphere which means the observer has less control over it. However, the
effects of Type 3 seeing are generally less pronounced than the other two types. You can never really avoid Type 3 seeing.
Your best bet is to wait until moments of steadiness. If the seeing is extremely bad, pack up and wait for a better night.
The conditions described here apply to both visual and photographic applications.
Figure 8-1 Seeing conditions directly affect image quality. These drawings represent a point source (i.e., star)
under bad seeing conditions (left) to excellent condition (right). Most often, seeing conditions produce images that
lie somewhere between these two extremes.
