Dew shield - heated or not?

[lucid]

Hello,

So I'm looking at buying a few things very soon, amongst which is a dew shield for my CPC Edge 800.

But should I go heated or not?

Thanks in advance for your experience and advice.

Andrew

[alexjj67]

I have the same scope with a dew shield and still get dew.

I am just in the process of buying a band.

[PeterCPC]

I use the Astrozap heated dew shield and get no dew.

Peter

[lucid]

Simple - heated it is then.

[lewisjones]

You may also need the Dew Controller to regulate the amount of heat supplied

[Northern Soul man]

Hi Andrew - nice scope mate well done - regarding the dew controller it depends which dew heater you get - some operate on full power like the dew zapper - but I have the Astrozap heated dew shield like Peter and find that you can run on full power because it only just supplies enough heat (to the CPC 1100 anyway).  I have the Hitec Astro 4 band dew controller - only using the 1 channel, but the controller has a built in power point so that you can run power to the scope which keeps the wiring to a minimum - I just tie the controller around one of the fork arms - so only need a very short power lead from the controller to the scope - when operating and moving the mount around, it doesn't allow for any cord wrap around the base of the mount just above the tripod.

Paul.

[perfrej]

Good evening!

Having worked closely with top meteorologists in the Arctic for 15 years I have had the privilege of discussing dew formation with them at some length during sauna sessions on the ice breakers.

Dew does not "fall". It forms on particles or surfaces, and the prime surface to protect is your lens or mirrors. The dreaded dew forms when the dew-point and the temperature of the object meet, so the primary job here is to keep the surface temperatures above the dew-point. The protected object is usually thermally connected to the tube of the OTA, so keeping the tube itself warm is a good start. The first step in keeping the tube warm is to make sure it is not black, as black surafces radiate their heat to the open sky - and we can assume that the sky is indeed a good "sucker" of heat as we do not use the stuff when it is cloudy. So, white tube, or - and this is an interesting solution - insulated tube.

Back to active heating. My take is that a dew-band is best positioned as close to the protected surface a possible. For a refractor or an SCT, this would be on the outside of the tube at the top of the scope, and for an open reflector the back of the two mirrors. Heating a dew shield is less efficient and slower, but can cause a cushion of warmer air to form in the enclosed air-space, and that way indirectly heat the front glass. We have to recognise that the indirect heating is less efficient and will cause more air turblulence in front of the objective than will a more direct heating of the objective.

Isn't it lovely how complicated we can make things?

Bottom line... Dew band as close to the top glass as possible. A heated dew-shield is most efficient when the built-in heat-band is positioned as close to the edge of the top lens as possible.

I use a single heat-band positined on the tube as close to the lens as possible. On my refractors I use only the built-in dew shield, and on the 190MN I use a dew shield with a buit-in heat-band that is at the bottom of the shield, which in turn puts it right at the corrector plate.

Don't use too much power. You only have to bring the temperature of the top lens just above the dew-point. And if you have a black tube, insulate it with camping mat material or similar.

Ah! Another rant!

All the best,

Per

[AstroJOE]

As others have said.... I also use a heated astrozap dew shield with a Hitec Astro 4 band dew controller on my CPC (1100) and I have never had a problem with dew formation or any noticeable air turbulence. I run mine on half power even on the coldest nights (minus 3ish) and it still seems to work fine.

James

[Rusty-Gunn]

Hi. I've seen notes about camp mats. Do we wrap the entire tube like a "jacket"? Any pictures would help understand. Thank you.

[perfrej]

I did

[Rusty-Gunn]

Thank you. I do appreciate it.

[perfrej]

No worries. I don't have the scope anymore, but it never dewed up on me after insulation was put in place

/per

[Rusty-Gunn]

With the camp mat insulation, can I assume the tube takes longer to cool down? Sounds reasonable. My tube has a built-in fan which ought to help though.

Another question (don't mean to hijack) did you use those wrap-around dew zapper bands underneath the mat? Would I need to if I used a mat?

[perfrej]

I ran without heaters with this configuration. I am right on the Baltic coast, so quite moist, but I don't think it would have been enough for an August night. On the other hand, August nights are not dark enough for imaging here...

/per

[ollypenrice]

Cool down for cool down's sake is not the point. The point is to avoid tube currents which, if I'm not wrong, are provoked by strong differences between internal and external temperature. If you slow that interaction down as Per is suggesting then you'll also reduce tube currents. Such is my thinking...

The hyper-insulation of SCT tubes is quite a big thing in some imaging communities and is said to be very effective.

Olly

[alan potts]

An interesting question and funny enough one I am asking myself this year after cut short sessions a few times as the weather had had a say on my observations. After looking at shields with heater bands on and off and adding up the cost for my scope collection it became clear I could buy 2-3 electric radiators for a house for the same price just depends on the size.

I am just wondering what i will find to spend on when this is sorted.

Alan

[michaelmorris]

Since using dew heaters, I have never lost a single observing session due to dew.

[Rusty-Gunn]

Thanks guys.

[lucid]

Yes, thanks for the advice all. I've gone for heated