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Alex, this dew issue has got me stumped. My reasons for this are as follows:
The secondary is not directly exposed to the night sky. Dew only really forms where the surface is in direct line of sight to the night sky. It makes no sense that it gets dewed up pointing down.
Added to this, our average winter humidity is around 80% and night time temps are way lower than Brisbane, so we go past the dew point really rapidly. Now where I have my observatory is on the south coast with on shore breezes we often reach dew point there about 30 minutes after dark. My obs usually is swimming in dew, but my secondary has not once dewed up on the RC.
Installing a dew heater might just negate the sharpness of the optics, it will certainly stop the dew. I reckon if anything using a dew shield is preferrable.
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