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Yep, I agree with Les.
There are three ways to prevent dew formation, heat, air flow and shelter.
Solid tube Newt's have their primary mirror sheltered down the end of a long tube. Those long 'light shrouds' that are often seen wrapped around open tube Newt's do the same thing, but they introduce other complications as they get wet. I use what I call a 'cuff' that comes up only about half way up the open tube from the mirror box. Since using this on my 17.5" I have never had dew problems plague it. My 12" the same, and save for impossible nights when even fog forms it has also not had dew problems.
Air flow is the way profession observatories stop dew formation on their mirrors. Vibration from the fans is the biggest hurdle here. Primary and secondary mirrors alike. Once the primary is at thermal equilibrium with the surrounds, this 'boundary layer' does not exist that can form from a warm primary. The fans then help prevent dew formation.
Heat can be used on scopes, but it needs to be carefully employed. Secondary mirrors benefit from gentle heating as it eliminates potential problems from fan vibration. But heating a secondary is more complicated than just sticking a heater onto it. The secondary mirror holder plays a vital part in this. The wrong holder and it will render any heating efforts futile. Heating of the secondary is very effective as it is small and the heating system can be designed to quite uniformly and gently heat it.
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