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My experience is that Dew shields were not very effective, they were OK at 45 - 60° elevation, but when getting close to looking straight up, then they were useless.
I also stopped using hair dryers, they blow dust onto the optics and you also keep chasing focus for the next 2-3 minutes, and it dews up again after 15 minutes.
Dew heaters are the way forward I think.
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