8
S
ky conditions
S
electing an observing site
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 vapour 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.
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. If you try to observe
on any structure, or even a sidewalk, movements you make may cause the telescope to vibrate. Pavement
and concrete can also radiate stored heat which will affect observing.
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. However this may
take longer if there is a big diference 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.
Do not expose your eye to anything except red light for 30 minutes prior to observing. This allows your pupils
to expand to their maximum diameter and biochemical light adaptation to occur. It is important to observe
with both eyes open. This avoids fatigue at the eyepiece , allows you to check against reference material,
and is a good habit to develop if you sketch at the eyepiece.. If you find this too distracting, cover the non-
used eye with your hand or an eyepatch. 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.
C
hoosing the best time to observe
C
ooling the telescope
U
sing your eyes
OBSERVING THE SKY
