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Just a quick update. I bought a cheap foam mat and made a dew shield for my 8" Newtonian, following the rule of thumb that is often mentioned of making the shield extend 1 and 1/2 times the diameter of the aperture. So far, this seems to have worked. I have only had the chance to use it twice under dewy conditions, during 5-6 hours of observation on both occasions, and at the end of each session there wasn't a hint of dew on the secondary (or primary) mirror, even though the telescope itself (along with everything else) was covered in it. So reasons to be optimistic. If in the end this doesn't prove adequate I might make a dew strap out of one of those usb heaters and instead of placing it on the back of the secondary, wrap it around the end of the OTA underneath the dew shield.
It is a bit cumbersome, and ads to the moment arm, so care should be taken to use light weight material, but material strong enough to hold its shape, and also, of course, to balance the scope with it on.
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