Dew prevention

[Roo]

Hi,
I need some advice regarding mirror dew prevention. I live in Brisbane (subtropical climate). I am building a 10" f/5 truss tube dob.
1. I have an 80mm dia 12VDC fan to install in the mirror box. Should it go behind the mirror (between the collimation bolts) blowing toward the back of the mirror or on one side of the box blowing across the face of the mirror?
2. Should I be looking at spiders with dew heater options for the secondary mirror?
Appreciate any advice from practical experience.
Regards,
Andrew.

[rmcpb (Rob)]

Before getting too carried away with heaters try the simple solutions first.

The fan blowing onto the back of the mirror combined with a shroud should take care of most of the dew problems on both the primary and the secondary. If you still have problems then the addition of a dew/light shield should fix the problem completely except on the worst of nights that, thankfully, are very rare in your warmer climate.

Heaters introduce other problems we spend lots of time avoiding and should only really be used when all else fails.

Cheers

[Roo]

Thanks for the feedback. I was thinking that a fan mounted behind the primary mirror in a truss tube design wouldn't have much effect on the secondary mirror. The dew shield option appears to be the way to go for the secondary.
Regards,
Andrew.

[rmcpb (Rob)]

Don't discount the shroud, if nothing else it will stop you dropping eyepieces on your primary

Seriously it will help with channelling the air from the fan over your secondary.

[Omaroo (Chris Malikoff)]

I have to agree with Rob. Although my Newt is a solid tube, you can simulate that by fitting a tight-ish fitting shroud, which you should do in any case. My fan blows at the back of the mirror and if you stand in front of the scope you can feel a slight breeze on your face coming from the other end of the tube. I have never had either the primary or secondary gather any dew whatsoever while the fan is running. A fan will also help to break up the warm/cold air boundary layer hovering over the mirror even if the mirror hasn't yet reached ambient temp. The column of moving air from a fan doesn't affect your seeing as much as a static boundary layer hovering over your mirror if you weren't to use a fan. Everyone I've spoken to about it thinks that a steadily moving column of air doesn't share the unwanted refractive quality of a boundary layer of still air - at both ends of the scope.

Now... if only I could channel some of that air across my eyepiece......

Just my 2c worth...