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What the person has is a dew shield, or pair of them. The protective dew shield slows down the formation of dew on the front optics. This is a passive system, as the dew shield (a circular tube, a bit like a very long lens shade on a camera lens) literally “shields” the objective lens (or SCT corrector plate) from the rapidly cooling ambient air.
An active system would be the fitting of a dew heater. These are low powered electrical strips made from resistance wire which you wrap around the objective lens cell. The low power slightly warms the cell and air around the objective, to maybe just ½° C above ambient, and this is sufficient to prevent dew from condensing on the optics.
Heat from your eyeball can also contribute to condensation forming on eyepieces on those very humid nights.
I’m not sure of the price of these motorised chairs – it was a manufactured unit rather than home made. If I can find a price, I’ll let you know.
Cheers
Dennis
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