I dunno, dew can get pretty evil here in Canberra. I do a few things to help mitigate it.
1. I hard wax all my scope tubes (Turtle Wax Super Hard Shell). Yep, you bet, it makes the dew run off instead of pooling. I have gone out and seen my FSQ-85 with rivulets running off - I kid you not! I even hard wax my mounts, and make sure I leave wax residue in any gaps leading to the electronics to help water-tight those areas. Overkill? Not in my opinion. I reapply a new coat EVERY morning after a dew session after the scope has dried off. I don't pick the scope up by the tube anyway, so it's not going to slip out of my hands.
2. I put dew straps behind the objective as well as another on the dewshield itself. Sure, the one located behind the cell wards off dew from the lens (OK, sorry, I only am talking refractors as my VC-200L doesn't dew up!), but I found that dew was developing on the tip of the dewshield as well as the small baffle inside the dewshield on the FSQ. So, I now have another strap positioned over the baffle.
3. I put dew straps also on the guidescope objective. It's a necessity.
4. I do NOT EVER assemble the scope during night. My rig is ALWAYS assembled for how it is when imaging. This maintains a sealed system, so dew won't develop on CCD windows, flatteners, reducers, filters. I have had them dew up before, so I learned my lesson.
5. Heaters on sensors/CCD windows - haven't had a need because of the above.
The only times I have had to end a session due to dew was because I had NOT put on the dew heater straps because I was too blase, thinking "Nah, not going to dew tonight". When I have been out at Yass last winter, I had frost all over the scope and mount (a prior incarnation of Takahashi
- I've had a few
), but because I had the heaters on, the glass was clear and I kept going (was NOT fun packing it up! HAD to wear gloves)
And another thing I found entirely beneficial was keeping a few chemical hand warmers handy - they VERY quickly de-fog EP's you have accidentally breathed on and don't use up battery. They also keep your pockets and hands warm in the interim
. I have even strapped one onto the dewshield once when the old battery I had karked it in the field.