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 affects 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 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 7
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.
Photography Hints
Your Celestron can be used for both terrestrial and astronomical photography. Celestron telescopes have fixed apertures and,
as a result, fixed f/ratios. To properly expose your subjects photographically you need to set your shutter speed accordingly.
Most 35mm cameras offer through-the-lens metering which will let you know if your picture will be under or over exposed.
This is more of a consideration when doing terrestrial photography where exposure times are measured in hundredths of a
second. In astrophotography, the exposures are much longer requiring that you use the “B” setting on your camera. The
actual exposure time is determined by how long you keep the shutter open. To reduce vibration when tripping the shutter, use
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