I posted this question on the AP Yahoo Group and got some very informed replies.
The best was this:
http://www.dewbuster.com/faq-1a-ques...ml#PERFORMANCE
It describes in detail where to put the dew heater strip and how to prevent the dew heater from degrading the view which it can if improperly located and setup.
Its worth reading. You don't want weaker results imaging with your expensive APO wasting some of its performance on incorrectly placed and setup dew heaters.
Greg.