Celestial Observing • 43
rium. Once adjusted to the outdoor 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 associ-
ated 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 6-4
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.
