Travel Photography - Any ideas on how to prevent condensation during night photography?

I was out in the Swedish countryside last week and decided to try a star trail shot. I set up the camera to just keep firing 30 second exposures until the battery ran out.

The next morning I discovered that the camera lens quickly fogged over once the temperatures dropped, leaving 95% of the shots unusable. That project was quickly scrapped, and I left the camera with the lens off for half the day to make sure it dried out properly.

I'm sure some of you have run into this before. How do you get around it?

I've had similar problems when moving between indoor/outdoor temperature extremes. I've found that if you keep your camera stored in a plastic bag (zip-lock or similar) for a few minutes after going outside you can avoid the fogging over. This works because it allows the camera to warm up/cool off without exposing it to moisture in the air.

Of course, your problem seems to be that the temperature is dropping while you're outside, so the bag method probably won't work for you. All I can think of is to try and aim for an evening with low humidity. The less moisture there is in the air, the less will condense on your gear. Plus, lower humidity means a clearer evening for star gazing/photography...

Those little hand warmers...I think they're filled with some sort of iron and an oxydizer of some sort. An ample supply of those around the base of your gear should keep a little bubble of air around your camera that remains above the dew point.

Dew point is key. That's the temperature at which the air at it's current humidity will no longer be able to hold its water vapor, and the excess will be depoisted as dew. You only need a couple of degrees. Perhaps even a little tent or bivouac with a few hand warmers in it will give you the extra degree or two you need.

Dew heaters are the best for that. Back when I was doing astrophotography I used them quite a bit. Here's a site with many listed for example:

http://www.astronomics.com/main/category.asp/catalog_name/astronomics/category_name/X9LP94KKVRJJ8PUK3N2JFA0G05/Page/1

I can only attest to tropical climates, I usually carry silica gel in my camera back to stop moisture, and before I get ready to go out I put my camera bag outside with the zipper open a little so it can "climitize"...however I've never shot star trails in the cold