How to Stop Dew on secondary mirror?

[Garyh]

Hi again everyone,
Just a simpl question here. Does anyone have a method of keeping dew from forming on a secondary mirror. After about 2 hours mine fogs up and I don`t like sticking the old hair dryer down there as it takes forever for things to settle down. Would some form of heating with resistors work without making thermal currents? I don`t want a dew cover either as thatwould give me vignetting as this is a imaging newt...
Any ideas welcome..
Cheers Gary:thumbsup:

[iceman]

If you have a fan blowing from behind the primary up the tube, it will keep dew from forming on both the primary and seconday.

I had my scope out all night last night, got up this morning at 4:30am and EVERYTHING was wet, a very dewey morning, but my primary and secondary were both free from dew thanks to the fan blowing all night long.

I just had the recharger (240v) plugged into the 12v power pack running the fan all night.

[Starkler]

http://www.astrosystems.biz/dewgrd.htm

The dewguard is electrically heated, but under temp sensor control.

[janoskiss]

A dew shield does not need to be much bigger in diameter than the mirror to completely avoid vignetting.

[gbeal]

I made a rudimentary form of dew heater with nichrome wire, and it is part of the finder (objective and eyepiece) system. Runs straight from 12 volt, and the simple motorbike battery runs this and the Argo Navis all night.
Mikes idea sounds good though.

[Garyh]

Thanks Guys, for the info,
Looks like a low power heater does little harm to the difraction pattern,
might make a string of resistors to sit on the back of the secondary or go with a small fan!
I made up a foam dew shield with resistors sewn in for the genesis and it gets me through many hours with heavy dews..
Thanks again and CS...:thumbsup:
Gary

[Don Pensack]

A simple dewshield extension of the tube will likely prevent secondary dewing. It will also have the additional benefit of shielding the bottom of the focuser from extraneous light entering from over the end of the other side of the tube.
Don

[davidpretorius]

i also have a fan blowing under the primary (plus cooling) all night and the last two times i have got up in the morning, i have had a dewed up secondary. these are the first occurences i have had of this for a very long time. I will have in investigate further

[EzyStyles]

i made my own dew shield. It's some hard black plastic material which you can bend from Bunnings. It lasted my entire imaging session without any dew forming on the secondary.

Here's a pic:

http://www.iceinspace.com.au/forum/attachment.php?attachmentid=17747&d=1159809467