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46 • Celestial Observing
temperature, don’t touch the telescope tube with your hands. When pointing the
telescope, hold the telescope by the star diagonal. 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. Specifically, avoid sites that overlook asphalt parking
lots or ploughed fields. Stay away from valleys and shorelines. Instead, look for
broad hilltops or open grassy fields. Stable thermal conditions found near lakes
and atmospheric inversions also tend to produce good seeing. 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 steadi-
ness. If the seeing is extremely bad, pack up and wait for a better night.
The conditions described here apply to both visual and photographic observa-
tions.
Figure 5-4
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.