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I have always thought that dew heaters must have an adverse effect, sort of like sticking your hand in front of the tube causes visible heat shimmer.
But I've never used a dew heater so I can't give any solid answer - the closest I come is to use a hairdryer to remove condensation on my secondary, and I know that totally destroys the imaging ability of my scope for about 5 minutes afterwards until the air around the secondary has cooled down.
Bird
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