Celestron ASTROMASTER EQ Serie Manuel D'instructions page 18

SKY CONDITIONS
Sky conditions can significantly affect the performance of your telescope in three ways .
• Steadiness of the Air- On windy days, images of the Moon and planets will appear to wave or jump around in the
eyepiece; as if you are looking at them through moving water . Nights where winds are calm will offer the best higher
magnification views of the planets and the Moon . The best way to judge the stability of the atmosphere is to look at bright
stars with the naked eye . If they are "twinkling" or rapidly changing colors, the air is unstable and you are better off using
lower powers and looking for deep sky objects . If the stars are sharp and not twinkling, the air is stable and should offer
great high magnification planetary views .
• Transparency- How clear is the air you are looking through? If there is a high amount of humidity in the air, the faint light
from galaxies and nebulae can be scattered and diffused before reaching your telescope, causing a loss of brightness
in your image . Debris in the air from local forest fires or even distant volcanic eruptions can contribute to large loss of
brightness . Sometimes this humidity or debris can help stabilize the air, making for good planetary and lunar images, but
the loss of light would make it difficult to see fainter deep-sky objects .
• Sky Brightness- The amount of ambient light in the atmosphere can also effect deep-sky observing . How dark the sky is
can depend on your local surroundings . In the middle of a city, sky-glow caused by city lights being reflected back to earth
from the sky can overpower the faint light from distant galaxies . Getting away from the bright lights of a major city can
make the difference between seeing a faint deep-sky object and missing it altogether . Planets and the moon are plenty
bright enough on their own so the effect on observing them is minimal .
SELECTING AN OBSERVING SITE
If you are going to be observing deep-sky objects, such as galaxies and nebulae, you should consider traveling to a dark
sky site that is reasonably accessible . It should be away from city lights, a relatively unobstructed view of the horizon, and
upwind of any major source of air pollution . Always choose as high an elevation as possible as this can lower the effects of
atmospheric instability and can ensure that you are above any ground fog . While it can be desirable to take your telescope
to a dark sky site, it is not always necessary . If you plan to view the planets, the Moon or even some of the brighter deep-
sky objects, you can do this from any location, such as your own backyard . Try to setup the scope in a location that is out
of the direct path of streetlights or house lights to help protect your night vision . Try to avoid observing anything that lies
within 5 to 10 degrees over the roof of a building . Rooftops absorb heat during the day and radiate this heat out at night .
This can cause a layer of turbulent air directly over the building that can degrade your image . It is best if you set your
telescope up directly on a dirt or grassy surface . Setting up on any raised platform such as a wooden deck or a hard surface
like concrete or a sidewalk should be avoided as they transmit vibrations easily that can be transferred to the telescope .
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
Try not to 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 before dawn . Objects are best observed as they cross the meridian, the imaginary
line that runs from north to south through a point directly over your head . This is the point at which objects reach their highest
points in the sky and your telescope is looking through the least amount of atmosphere possible . Objects that are rising or
setting near the horizon will suffer more atmospheric turbulence since you are looking through a much longer column of air .
It is not always necessary to have cloud-free skies if you are looking at planets or the moon . Often broken cloud conditions
provide excellent seeing .
18 I ENGLISH