Startrails and mist

[GiacomoD]

Recently I went at night on a hill to try my first startrail. It was a new moon night and the sky was very clear. Apparently, the ideal conditions for a startrail.
I spent there a couple of hours, taking about 220 photos with a shutter speed of 30 seconds each. In the last ten pictures I tried to provide some extra lighting on the foreground trees using a handheld flash, but with poor results.
During that time, I could feel the air moisture growing, but I realized the consequences on the lens only at the end of the shooting session, when I removed the camera from the tripod and I found the lens completely misted over.
After post-processing, the major effect seems to be a gradual decrease in luminosity of stars, while the decreasing sharpness is probably unnoticeable after image resizing and final sharpening. Also, a very evident halo is visible on the foreground trees.

Any idea on how to avoid the misting over the lens during the shooting session?

[thecraig]

Heard it done with a filter kit and clear acetate cut to size.

When you see it starting to mist, swap it over for another clear film.

[Shadowman]

Very nice.