Dew on large primaries is rarely a problem as the ambient temperature drops faster than the mirror throughout the night:
http://www.cruxis.com/scope/mirrorcooling.htm
Shedding this heat is much more important than worrying about dew, which why a fan is needed. In fact, data has shown that without some sort of active cooling the mirror will almost never get to within a degree above ambient. My open style 16" X 1.5" thick primary has never dewed in over 2 yrs of use.
Even if it was a problem the UC has stub tube that fits around the primary anyway that would conceivably act as a dew shield. Running a fan would also help stop dew from forming.
The secondary is different - in these open scopes generally a dew heater is used there.
