Astronomy Digital Picture – 16 seconds, ISO 400, 6.3mm f/2.8, on 30 Dec 2006 04:58AM
09 Jan 2007
Astronomers love light from space but hate light from Earth’s surface. Night sky darkness is interrupted by bright areas of artificial light from cities. The amount of light from Earth impacts our ability to see dim objects in space.
Unshielded lights send their light in all directions, including straight up and lit the sky. Dim object might not be visible even when it is tried to photograph due to the glow of sky by earth based light sources.
When it comes to the use of mercury vapour lamps, the problem is that they have enormous number of spectral lines in all parts of the spectrum from the infrared to the ultraviolet, disturbing the observation in these wavelengths.
We can reduce this problem, if we can make sure that lights sources are shielded so that most of the light is pointed downwards and not towards the sky. Use sodium vapour lamps instead of mercury lamps, Sodium vapour has only two lines in the entire optical spectrum!
When it comes to star light Vs city light, the later wins !
Tomorrow’s picture – Venus
Posted by naveenln 
Astronomy Digital Picture of the Day 