Appendix: Tips for observing the sky
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Appendix: Tips for observing the sky
Sky conditions
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.
Selecting an observing site
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.
Choosing the best time to observe
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.
Cooling the telescope
Telescopes require time 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). A rule of thumb is to
allow 5 minutes per inch of aperture. For example, your telescope would require at least 30
minutes cooling off to outside conditions. Tip: use this time for polar alignment.
Adapting your 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.
