• Light pollution which reduces the contrast between the black of the sky and celestial objects. it can be artificial (urban lighting) or natural (moonlight).
• Dust and particles in the atmosphere and absorb some of the light rays.
• Atmospheric turbulence which is an image ripple caused by the movement of different temperature layers of air in the atmosphere. When atmospheric turbulence is important, the stars twinkle and high magnification, we see the stars as if under a stream of water. If they send telescopes into space, it is precisely to avoid this turbulence.
Adaptive optics devices allow to partially offset these parasites distortions. adaptive optics is a technique used on some ground-based telescopes, which corrects in real time deformation of a light wave. A system considers the perturbation due to the atmosphere, and then deforms a mirror so as to exactly compensate for this disturbance.
• Bad weather that prevent observation.
• Humidity of the air often found near the sea, lakes and rivers as sufficient to alter the observation. A halo around the moon indicates a very transparent atmosphere.
• The temperature difference between the telescope and the outside. He must leave his telescope at least one hour before observation for the tube and eyepiece adjust to room temperature.
So it is better to observe a celestial object :
► high to the horizon, because the least the atmospheric layer is be important, and the better it is visible.
► before sunrise or after sunset the moon, or during the new moon.
► far from population centers.
► aloft, above the polluted atmospheric layers into light and dust.
If, for example, the sun all day typing on a surface that stores heat, this place is going to stay warm at night. An astronomer placed here will then see the swirling air and waving image to his telescope, as the air above the asphalt on hot summer days.
It is advisable to stand on ground that has remained in the shade during the day or on a type of surface that does not accumulate heat (eg expanse of grass) to reduce the temperature differences between the soil and air. Likewise, it should not look at the telescope an object located above a house exhaling into heat, will swirl the air above it.
Adaptation to night vision : normally, our eyes can distinguish in the night sky 6 magnitude of the objects, but after spending half an hour in the night they manage to see objects magnitude 6.5. Not to alter this adaptation to night, use a flashlight with red light (or red led flashlight covered with a red filter).