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RadioShack 12volt DC Cooling Fan
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My current 'scope, Orion SkyQuest XT12 IntelliScope, is the 3rd telescope I've owned. Living 40-something miles outside of Chicago has always presented challenges with stargazing so I've always held to the notion that the bigger the tube and the more filters you have will generally yield a good night under the stars! My first scope was a Sears catalog special... but I saw Saturn hanging in the eyepiece when I was 14 and that did it for me! 25 years later I'm lugging around a 12" Dob. ;-) Before this metal-tube 12" Dob I had an 8" Orion paper-tube Dob.
Since weather here in the midwest changes as drastically as anywhere else, I decided to add a cooling fan to my 'scope. The idea here, for those wondering, is to get that giant chunk of glass to "thermal equilibrium" with the ambient air temperature as quickly as possible. Experts believe that the huge chunk of highly polished glass at the bottom of a Dobsonian will flex and cause errors in focusing. This is caused when the mirror is warmer than the ambient air. Another problem is when a mirror is cooler than the ambient temperature. This allows dew or fog to appear. Adding a fan would help... so the experts say.
I purchased a 3 inch, 12volt DC Cooling Fan from RadioShack to "cool my mirror".
(This review is about the fan, not the mounting theory, i.e. mounting and block-off plates, which can be a opinionated discussion on thermodynamics. This fan mounted cleanly to my dob's mirror mount! IMHO, I don't see the need for a full block-off plate. Forcing ambient air onto the rear surface of the mirror with a fan more quickly brings the mirror to thermal equilibrium than with no fan at all.)
So do these things work or is it all Snake Oil? The answer is a classic "Yes and No" as well as "Your mileage may very". Running the fan makes a pretty noticeable differerence... when the mirror is cool. Example: if I take the 'scope from inside my air conditioned home to a hot/muggy night I need to run the fan to keep any dew/fog from appearing. On the other hand, if the 'scope is warm and the night turns cool (as with summer and autumn viewing) or If I bring it out for some winter viewing, I can't see the need for a fan. Although the mirror may be "off" due to a temperature difference it just wasn't noticeable to me. Ever. Your mileage may very but for "cool mirror, warm night" viewing, a fan does help!