Evening

by w.hrybok

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Light pollution by definition, adds more artificial light to the atmosphere. But how does this affect astrophotography?

Do lights from the ground affect photographs of the sky (At the longer distance)?

What are the effects of light pollution in the resulting photo that a non-polluted photograph wouldn't have?

Reason on why with some example photographs would be very helpful.

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Sadly "affecting astrophotography" is an understatement :-( Something like "destroying every hope and crushing the soul of the poor guy who is spending a night chilling" would begin to approximate it, at least near a city. Then one says: ok I'll move a bit away, and maybe it will turn out that there is not a dark place in the whole country. Sigh. –  Francesco Sep 1 '12 at 6:37

2 Answers 2

For star trails (which is one type of astrophotography), light pollution has one major impact and some lesser impacts.

The prime impact is that light pollution will make less stars visible. Less stars visible means less trails and usually as a result, a somewhat 'sparse' sky in the image. I have seen nice images over populated areas but they have only a few trails and the primary subject tends to be the night-time view rather than the star trails themselves.

As an example of an image without much light pollution, I shot this a few weeks ago. Very little light in this area (an island offshore) and as is evident, captured a large number of stars.

Secondary impacts are that the sky will become colored rather than dark (which color depends on your white balance) and that the color of the stars themselves will be lost.

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There is a lot of thing out in the atmosphere, clouds are visibles but some vapor and dust are here even when the sky looks clear.

The fact is that all that stuff has the effect of reflecting and diffusing light, so does also the air itself.

For a very bright source the overall effect is a diffuse blue light, yep that's why the sky is blue by diffusing the sunlight (the diffusion goese like frequency to the 4th power so is much more efficient for the blue than for the red) this explain also why the sunset/rise is redish: having to travel a deeper air column the light loose all its blue componnent.

Ok back to the night. Light, from the moon or from a neighbourg city, will behave the same as sunlight: it will be diffused in the atmosphere and make it more luminous. This might not be visible at first glance but when you are trying to collect faint light from the stars this diffused light act as a background noise.

Having more noise for the same signal make it more difficult to observe (lowering the Signal to Noise Ratio (SNR)) at worst it may be as bright as some of the stars and totally hide them.

A luminous sky (even faintly) reduce also the maximum exposure time, at least if you want a black background, it gives a greyish looks to the pictures.

It also (as in fact it is a blue dominated noise) fade star's colors.

Astronomer choose high altitude place for their telescopes reducing the thickness of the air the light has to travel, the most radical solution beeing to go out of the atmosphere.

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Funny, my light pollution is always yellow-dominated, because most of the light from a city at night is usually from sodium-vapor lamps. –  Evan Krall Sep 2 '12 at 7:59

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