There is no need to use heat to keep optics free from dew.
Heat actually complicates things. Not only does it kill batteries quickly, but it adds interference from infrared and introduces thermal issues to optics and mechanics.
Professional observatories do not use heat to keep optics dry from dew or frost. But for some reason we just do not look at how The Big Boys do things and why.
Heat does have a place, but its application is far less extensive than it is used. Red dot finders and eyepieces* can benefit from heating.
I have started two threads and written an article on using fans instead of heat with telescopes:
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SCT & Mak total dew control without heat - a solution!
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Refractor dew control WITHOUT heat - it is possible!
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Cooling of Newtonian Optics
The article "cooling of Newtonian optics" discusses the added benefit of dew control that comes from how to cool Newt. optics in a way that does not induce strain in the optics. The pic below of my 8" solid tube OTA shows how the mirror cell is all closed, with only a hole in it with the fan positioned so it draw air out from the scope, NOT blow onto the primary mirror.
Battery power is expensive and very bulky. Fans however use a fraction of power that heating does, and does not introduce the thermal problems that heat does. And by using a PWM unit, the fan's revs can also be regulated so even less power is consumed.
My home in Sydney is Dew Central at night. I do not experience my optics fog up with the way I have implemented these fans, even when EVERYTHING around me is soaking wet with heavy dew. And not one joule of heat being used. And when I go bush, I do not need to worry about having enough power to keep the optics dry because of excessive power consumption.
Alex.
* Using heating straps with eyepieces needs careful consideration. You should not be using the strap to introduce heat into the eyepiece - think about how an EP is built (a metal tube surrounding glass), that is set into a metal focuser, and very often that is fitted to a metal tube. The vast majority of the heat generated by the strap will be disipated into the metal, and the glass will warm up bugger all. Instead, look to keep your eyepieces in a heated case, this way when you put the warm EP into the scope, the heating strap will only be feeding a trickle of heat to keep the heat inside the glass, rather than trying to push crap uphill to force heat into a poor thermal situation.
Had no idea how quickly dew control drains the battery! Will be better prepared next time though
