22
OBSERVING THE SKY
Sky conditions are usually defined by two atmospheric characteristics, seeing, or the steadiness of the air, and
transparency, light scattering due to the amount of water vapor and particulate material in the air. When you
observe the Moon and the planets, and they appear as though water is running over them, you probably have
bad "seeing" because you are observing through turbulent air. In conditions of good "seeing", the stars appear
steady, without twinkling, when you look at them with unassisted eyes (without a telescope). Ideal "transparency"
is when the sky is inky black and the air is unpolluted.
S
ky conditions
S
el
ec
ti
ng an obs
er
ving site
C
hoosing the
best time
to
obs
er
ve
C
ooling the telescope
Travel to the best site that is reasonably accessible. It should be away from city lights, and upwind from any
source of air pollution. Always choose as high an elevation as possible; this will get you above some of the lights
and pollution and will ensure that you aren't in any ground fog. Sometimes low fog banks help to block light
pollution if you get above them. Try to have a dark, unobstructed view of the horizon, especially the southern
horizon if you are in the Northern Hemisphere and vice versa. However, remember that the darkest sky is usually
at the "Zenith", directly above your head. It is the shortest path through the atmosphere. Do not try to observe any
object when the light path passes near any protrusion on the ground. Even extremely light winds can cause major
air turbulence as they flow over the top of a building or wall.
Observing through a window is not recommended because the window glass will distort images considerably.
And an open window can be even worse, because warmer indoor air will escape out the window, causing
turbulence which also affects images. Astronomy is an outdoor activity.
The best conditions will have still air, and obviously, a clear view of the sky. It is not necessary that the sky be
cloud-free. Often broken cloud conditions provide excellent seeing. Do not view immediately after sunset. After
the sun goes down, the Earth is still cooling, causing air turbulence. As the night goes on, not only will seeing
improve, but air pollution and ground lights will often diminish. Some of the best observing time is often in the
early morning hours. Objects are best observed as they cross the meridian, which is an imaginary line that runs
through the Zenith, due North-South. This is the point at which objects reach their highest points in the sky.
Observing at this time reduces bad atmospheric effects. When observing near the horizon, you look through lots
of atmosphere, complete with turbulence, dust particles and increased light pollution.
Telescopes require at least 10 to 30 minutes to cool down to outside air temperature. This may take longer if
there is a big difference between the temperature of the telescope and the outside air. This minimizes heat wave
distortion inside telescope tube (tube currents). Allow a longer cooling time for larger optics. If you are using an
equatorial mount, use this time for polar alignment.
A
dapt
ing
yo
ur eyes
Do not expose your eyes to anything except red light for 30 minutes prior to observing. This allows your pupils to
expand to their maximum diameter and build up the levels of optical pigments, which are rapidly lost if exposed to
bright light. It is important to observe with both eyes open. This avoids fatigue at the eyepiece. If you find this too
distracting, cover the non-used eye with your hand or an eye patch. Use averted vision on faint objects: The
center of your eye is the least sensitive to low light levels. When viewing a faint object, don't look directly at it.
Instead, look slightly to the side, and the object will appear brighter.
To Next Page
Loading...
Need help?
General
