Meade Image Processing Manuel d'utilisateur télécharger pdf (Page 7)

4. Observations and weather

Atmospheric conditions matter much in astronomical observations. It is absolutely

necessary for students to understand how much the weather determines what we can do on a

given night. Even by the low standards of the Eastern US, the overall weather conditions for

astronomy in Mississippi can only be termed “outrageous”. And that is before thinking of

streetlights. Only about 30% of all nights are fit for any observation, and half of those nights

are useless for quality work due to atmospheric haze.

In our conditions, moisture in the air is the main problem. Of course, when clouds cover

half of the sky, or just there are moving clouds blowing in the wind, it is impossible to take

any images. One might use a telescope to visually observe the planets or the Moon when they

peek out among the clouds, but guiding images is impossible. One needs to try another night.

On many nights, however, there are only a few clouds, if any, still few stars are visible in

the sky and the background haze is bright in scattered streetlights. This is particularly so in

the summer months, from April to September, and it is due to moisture in the air. On such

nights, it is possible to take some pictures, but the background noise will be strong, and the

pictured objects will look much fainter, with few detail. This is due to a combination of

streetlights scattering on atmospheric water, and the absorption of light due to water as well.

The picture below indicates how bad light pollution is on campus. Notice that the true

image is the one on the left; we are “sweating blood” to dig out the object from the

background light. (This galaxy is the brightest of all, all others are even more immersed in

streetlights.) Unfortunately, cutting the background with software can only cut the even

background, but not the fluctuations of the background light – they remain large. Knowing

this, we realize that it is actually a great achievement to take any pictures at all!

Left: The Whirlpool Galaxy and the true sky background.

Right: The same image after artificially cutting the background.

Astronomers classify observational conditions as photometric when there are no clouds

and there is little moisture in the air. We know the night will be photometric when at sunset

the sky is dark blue all the way down to the horizon, and there are no clouds. In Mississippi

there are only about ten such nights a year.

A spectroscopic night is when some haze and some cirrus clouds exist, but the sky is

overall cloudless. Such nights are still good enough for imaging. At sunset, the sky is blue

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Meade Image Processing Manuel d'utilisateur Page 7