I have been using a 12 Volt computer fan on the back of my 10" mirror for well over a year or so. At first I had the fan attached to the mirror cell with elastic bands and it worked well from what I gathered. I then did a bunch of research and saw dozens of fans attached to neoprene or wood or a metal enclosure so that the fan would be blowing air on the mirror, but the fan was "closed off" and not "open" like I had mine.
So I went with the closed system and I do find it works, although I am not sure if it works any better? I find that after a night of observing in the extreme cold in the winter that after bringing my telescope inside, it doesn't matter how long I leave it capped,(I put the dust caps on both ends BEFORE I bring it inside), as I do leave the end caps on for about 30-40 minutes, and the mirror will always fog up with condensation after checking the mirror in 30-40 minutes.
Well, when I see this happening, I need to take the backing off with the fan on it so that the warmer air can get to the mirror and have the condensation go away. This is WHY I think the open concept is better because I won't have to keep taking the back part off with the fan on it and put it back on again, once the inside of the tube, including the mirror has dried over time.
Would it be ok to just leave the fan and backing piece on the back of the mirror all of the time? I know eventually the warm air in the house will dry everything off, but it will take a lot longer. Do any of you switch your fans back on once the scope is inside the house to keep the mirror from fogging up?
I just want to know which concept is better with the info I have mentioned here ~ Open mirror with the fan on elastic bands, or closed up with the back piece on the mirror. I also went out last night and bought 8 "D" cell batteries to replace what I was using which is "AA" batteries and all I need now is a D cell battery holder to attach to the fan.