Instruction manual

21
"Seeing"
The terms “seeing conditions” or “seeing” refer to the stability of the atmosphere, which directly effects the
amount of fine detail seen in extended objects. Essentially, extended objects are objects other than stars,
of some size, such as nebulae and galaxies.
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. Another
form of turbulence that effects seeing conditions is referred to as “air parcels”. These air parcels are the
smaller turbulence patterns in the air, within your immediate environment. The size of the air parcels,
compared to the aperture of your telescope, also determines 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 star images are diffuse. Seeing
conditions are rated on a five-point scale with one being the worst condition and five the best. Seeing
conditions can be classified in one of three categories. The definitions of these categories are based on
the causes of the different seeing conditions.
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 (stars) appear double. Type 1
seeing is caused by air currents within, or very close to, the telescope tube. These currents are caused by
a telescope that has not reached thermal equilibrium with the outdoor surroundings or heat waves from
people standing near the telescope. To avoid the problems associated with Type 1 seeing, allow your
telescope approximately 30 minutes outdoors to reach thermal equilibrium with the environment. If
observing with others, make sure no one stands in front of or directly below the telescope tube. The
StarHopper 10” and 12” telescopes come with a built-in fan behind the primary mirror that shortens the
time it takes to reach thermal equilibrium and eliminates tube currents.
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 contrast is low for extended objects.
Stars appear spread out and lack sharpness. Type 2 seeing conditions are produced in the lower
atmosphere, most likely by heat waves from the ground or buildings. To avoid the problems associated
with Type 2 seeing, select a good observing site. Look for broad hilltops or open grassy fields. Stable
thermal conditions found near lakes and atmospheric inversions tend to produce good seeing. Avoid sites
that overlook asphalt parking lots, plowed fields, valleys or shorelines. If you can't get a better location,
wait until the early morning hours when the surroundings are uniformly cool and seeing is generally better.
Type 3 seeing conditions are characterized by fast ripples that create shimmer in the visual field, affecting
otherwise sharp images. For extended objects, fine details are visible, but images shift around the field.
Stars are crisp points, but they rapidly shift small distances within the field of view. The cause of Type 3
seeing is turbulence in the upper atmosphere, which means the observer has no control over it. However,
the effects of Type 3 seeing are generally less pronounced than those of Type 1 and 2 conditions. Type 3
seeing conditions can’t really be avoided, so your best bet is to wait for moments of atmospheric
steadiness. If seeing conditions are extremely bad, you might consider waiting for a better night.
The conditions described here apply to both visual and photographic observations.
Figure 17