I often use a T-thread CLS-filter from Astronomik when I photograph. The CLS takes away much of the disturbing light from the atmosphere but lets the light from the universe through.
It is ideal when photographing nebulae and makes a light polluted sky look almost pitch black.

Above is two pictures where the difference with CLS is seen.
The object is M31 – Andromeda galaxy and the camera and the exposure is exactly the same in the two frames. The pictures are taken from Älvsjö in Stockholm, Sweden.