The reason your equipment is dewing up is because the surfaces are dropping significantly in temp not just one or two deg below.
Look at this calky
http://www.decatur.de/javascript/dew/index.html
for a normal scenario, 20C ambient 50%RH the dewpoint is about 9C.
The glass is looking into space that is -270C so it sheds heat real fast. That is why a clear night is so much colder than a cloudy one. This is also why your car bonnet can somehow be at -10C when the air temp only got to zero. One night about an hour after sunset my car roof was already below zero but the ambient was about 10C. Weird science!