most scopes come nitrogen purged now so they don't fog up unless there is a break somewhere in the seal of the scope.
i had an old redfield that would kill me. everytime i saw a buck and the heavy breathing would start; i had to constantly wipe it off so i could see out of it
bought a leupold and i have no more worries cause it stays in the gunsafe all year long cause them extreme fiberoptic pins on my darton don't fog up at all
cat crap should do the trick