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...