dew heaters and newtonian primaries

[Paddy]

Hi All,

Many thanks for all of the valuable discussion in these forums. I'm new to astronomy - l I got started in February when my wife bought me a Tasco 114mm reflector. I got addicted pretty quickly and bought a bintel 302mm dobsonian about a month later and I think she might regret having got me started. No, she's very supportive really.

I have found a lot of the discussion about modifying these telescopes and also about EPs very helpful.

I am thinking about building a dew heater for the primary of my 12", especially after reading somewhere that dew and dust can damage the surface of the mirror. I've got some resistance wire, a 12v 7 amp/hour battery and a potentiometer and am thinking about fixing the wire to the back of the primary. I've sort of modelled the design and power requirements on the Kendrick units that I've seen advertised.

I wonder if people have found dew heaters necessary or desirable on primaries in big long newtonians and if what I have in mind is a good way to go about it.

[Tannehill]

Welcome. Great scope, I've got the same one.

Dew rarely affects the primary mirror while observing, because it has fair thermal mass and is less prone to radiate itself down (to a temperature below the dew point) because the long tube acts like a dew shield -which is in effect an infrared shield of sorts.

The secondary mirror, being higher up and more able to radiate heat to the sky and smaller with less thermal mass, is the usual problem. That, and eyepieces.

Check out Bryan Greers website on thermal effects. I'd suggest that warming your primary is more likely to hurt your images by inducing thermal effects.

http://www.fpi-protostar.com/bgreer/index.htm

Many of us actually use fans to drive DOWN the primary mirror's temperature, and to remove the boundary layer.

But heating your secondary is a great idea. Also, fashioning a heater of some kind to sit in your eyepiece case is also good. EPs and secondaries (and corrector plates for those poor souls with Schmidt Cassegrains!) are dew prone to a terrible degree in some conditions.

Check our the SVFDS (Snake Valley Fog and Dew Society) pics here for some chuckles! http://www.iceinspace.com.au/forum/showthread.php?p=230878#post230878 :hi:

Regards,

Scott

[iceman]

I wouldn't use a dew heater on a primary - as Scott said we usually are trying to bring DOWN the temperature of the primary to keep up with the dropping ambient.

Dew on the primary can be a problem in some circumstances when your scope is pointed almost straight up a lot, but a running fan will all but eliminate primary mirror dew problems. Combine that with a dew shield for the front end, and you'll fix most of your dew problems for good.

The secondary can still dew up on really wet nights, but a blast from a 12v hairdryer into the focuser will solve that.

[Paddy]

Thanks Scott and Mike for your helpful replies - it will save me embarking on a counter-productive endeavour!

[Tannehill]

But I envy you the ability to make one yourself. If you have that inclination, you'll find there are hundreds, perhaps thousands, of small projects that can incrementally improve your scope function. Fans and dew heaters for the secondary and finder scopes, modifying the scope in other matters (Dobs are tremendously amenable to customization and modification, being mostly wood or particle board) to improve its motion properties, etc. Check out the Projects and Articles section for some examples.

Cheers

Scott

[Paddy]

I have been doing just that. I was getting very frustrated with the stickiness of the azimuth movement and was planning all sorts of modifications. Then I tried Mike's trick with the milk bottle washer that he wrote about in the projects sections - what a difference! Having all this input sure beats re-inventing the wheel!

[iceman]

Great stuff Patrick! I wish I had the skills to build a dew heater for my finderscope!

[astronut]

Mike,
With the skills needed to produce those jaw dropping images of yours, I guarantee that you have the skills to make a dew heater!!!!:D