When it comes to shooting the night sky then a wide angle lens is a must, and a large aperture (small number) to allow a lot of light in to your camera in a short period of time.  This will help minimise star trails.

As you are shooting at night it is often cooler and there is the possibilty of dew, a way of keeping the lens arm is paramount.

A cheap and easy way to prevent fogging is to use a hand warmer that is wrapped aroung the lens, these will easily stay warm for upto 8hrs.