I agree about a fan doing the job. The fan on my 16" fortuitously directs air across the face of the primary. The primary never dews. At Snake Valley last November on an incredibly dewy night, a 16" lightbridge with shroud but a fan that only blew across the back of the primary dewed up totally, whereas mine with no shroud stayed clear. And it was a very dewy night. Peter modified his lightbridge to direct air across the face of the mirror (see
http://www.iceinspace.com.au/63-400-0-0-1-0.html ) and has had no more problems.