Hi Geoff, Do you have a dew shield on your scope? if not try this first as it will dramatically cut down on dew cutting short your viewing.
I have a 12" LB and without the shield the secondary dewed within an hour.
With the shield, on a very dewy night with the scope deliberately pointing straight up, NO DEW!!!!
The other thing you need is a fan on the base of your Primary, keep this going all night and it will keep both mirrors from fogging up. Cheers, John.
