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A simple solution may be an insulating blanket on the outside of the tube where the mirror is.
I bought and used a dew blanket for my refractor. It is really just one of those windscreen sun insulators with a bit of velcro to wrap it around on itself.
You simply wrap it around the APO tube where the objective is at the start of the night. The idea is it slows the temp drop of the objective so it stays above the dewpoint. It worked really well.
My dark site though tends not to be too bad for dew although the OTA has at times been dripping.
So one of those windscreen sun protectors, a bit of velcro strip from Spotlight and wrap it around the tube where the primary mirror is and perhaps that may work. It may not work as well on an open tubed scope as the heat can leave through the tube but it may.
I don't recall ever having dew on my RCOS 12.5 inch with a carbon fibre tube and it always had the 3 fans going pulling air down into the tube so perhaps that does work.
Greg.
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