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Dew on primary-help please

Started by Tom Bourg, 09/04/2005 01:05AM
Posted 09/04/2005 01:05AM Opening Post
Hello everyone,

I really enjoy using my new 16" truss dob, but I am having trouble with dew forming on the primary. Big surprise here in Louisiana when the humidity is usually around 110 percent. :S

Anyway, I haven't purchased a fan for the primary, yet and that is next on my card but I doubt that a fan alone will do the job. A heater for the primary runs about $300 all total and gets me into the battery business which I don't want to do. One reason I bought this scope is because it is simple and doesn't require electricity. The fan, using a small battery fits into my scheme, but a large battery doesn't and I don't want to plug stuff into my cars lighter, if possible.

What I have hit on is the possibility of using styrofoam poster board as sort of a dew shield for the main mirror. If I remember correctly it is available in black. If is isn't last I heard spray paint is available in flat black. It is super light weight and I can curve a couple of sheets around the mirror. I haven't tried it yet because Katrina has cut into my driving around this town.

Does anyone have any other ideas? I know some of you have come up with some really great solutions to some of your problems.

I believe I've learned more from reading you guy's posts here on Astromart than I have in all the books I've read on astronomy.

Thanks in advance and clear skies.

Tom B.
Baton Rouge, LA

One
Posted 09/04/2005 03:30AM #1
Tom,

I'm not surprised at the dew. I live in Houston and have been shocked that I haven't had a problem with my 12.5" truss dob. I planned on it and built in 2 fans; one on the back and one on the face. The fans should take care of the dew issue. Just insulate the mounting well so you don't introduce a vibration issues.

On the battery issue, look at the Obsession site. You can add a 12v sealed battery into the tail gate with some velcro and simple wiring. I built a simple control plate like they have using sheet metal and flat paint. Just add a place to plug in a charger and plug in when the scope is not in use. You can get away with small batteries and even run 6v batteries in series to get to 12v's.

If you are shying away from batteries, it leads me to one thought. How sure are you that your primary is dewing up? Typically your seconday will dew up long before your primary will. If you do not have power then you do not have a dew heater on your secondary. I'd add this one first and check out the results before any other mod's. Check out the spiders from Protostar. They seem to work pretty well.

Good luck with it.

Ed
Posted 09/04/2005 06:53PM #2
We don't have a big dew problem here, but it does happen.
If a fan is used on the front of the mirror it will help a lot. The one on the rear won't help so much as it causes the temperature of the mirror to cool to below dew point but does only so much to help keep the upper surface free of dew.
Here is a picture of my upper fan that I make for Dobs. You can use any design that works for you.
Now some are concerned with the use of Velcro for holding the fan, so I am in the process of a new design that will not use Velcro, but uses wood screws instead.

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Posted 09/05/2005 05:08PM #3
Tom,

Do you use a shroud? They absorb a lot of the dew. I've had 3 truss tube dobs and the primary has rarely been a problem. It's always the secondary and finders that dew up for me.

Demetrios