Shooting the night skies is a very rewarding field of photography, especially after you’ve sat in the dark cold night for 5 hours waiting for a time lapse or a startrails sequence to complete. But if you are shooting in a cold location (or a terribly cold night) you may lose the sequence to dew or condensation.
Once the camera hits the same temperature as the ambient temperature, some mist (or worse, dew) can start building up on the lens. If you are lucky it builds on the outside of the lens, creating a fuzzy blur to the photos. If you are unlucky, it can create drops of water inside the lens.
Mark Peter Thorpe (a.k.a pixelhobo) has a great description and a great solution: