retractable dew shield vs carry mat dew shield

[Daniel-K]

so now i have my ED 80 im wondering how many people bother to make a dew shield when they have retractable dew shield on the fracs. i have just bought my self a dew heater but is it still worth making a carry mat dew shield? also how warm should my dew heater be, before it affects my image?

Dan

[emadmoussa]

All my fracs have retractable dew shields, I never felt I needed any extra ''shielding''. Last night I spent 4 hours outside with 89% humidity , the  red-dot  and finder scopes got fogged up, but the scope with just fine.

[IanL]

I don't bother with an extra dew shield.  Provided you keep the metal shield warm enough to prevent dew forming in the first place you should be fine.  It is a bit hard to specify exactly how warm you should set the heater.  If you can actually feel the metal shield getting warm then that is likely to be too hot.  The metal should feel cool to the touch but dry on the outside (whereas the adjacent tube will usually feel much colder and damp once dew starts to form).

I put my heater band around the metal shield a few cm in front of the objective.  Your aim is not to actually warm up the glass objective by conduction.  Rather you want to warm the metal dew shield by conduction to net off against the radiative heat loss from the shield to the sky, plus a bit more heat so that you get sufficient radiation from the shield on to the objective to net off against its radiation to the sky.