Please Explain DewShields

[Zaine UK]

Hi,

I'm going to get a DewSheild with my 8" Celestron, but i don't understand them. I read they're heated so i assume would need power? How do you power them? Off the Powertank?

Thanks for your help...

[Cornelius Varley]

Dew shields and dew strips are two different animals. A dew shield is a tube that fits around the top of the telescope and is about 1.5 - 2 x the diameter of the tube in length. The shield forms a column of stable air infront of the front element and helps stop dew forming. A dew strip is a length of material with a resistance wire running through it. A current is passed through the wire causing a small increase in temperature in the strip which is conducted into the telescope tube near to the front element, this helps to keep the temperature of front element of the telescope above the dew point. Dew strips should ideally be powered from a separate power source (not the same powertank used to drive the mount) because the current drain is high. If you are using a psu with a high enough current rating you can power the strip from this.

Peter

[Zaine UK]

Thanks. I'm planning on using the AstroZap ones on a Celestron CPC 800 GPS (XLT) with the Celestron Powertank.

[Merlin66]

Zaine,

One powertank or two????

[Zaine UK]

Just one. Either a Celestron one, or a Maplin one.

[Merlin66]

Hmmm

You may need one for the telescope and another for the dew strip heater.

[Zaine UK]

Hmm, think i might give the DewSheild a miss for the time being. How long would you guess i'd get before the telescope was unsuable because of Dew on a average nights viewing?

[AlistairHowie]

Consider an Astrozap flexible dewshield with an integrated heater strip. That way you can start off without the dew controller (a box that sits between your power source and the dew strip and is used to regulate the current/temperature in the strip), just using the dew shield passively. And if you do decide you need heat to keep the dew at bay longer, then you can purchase the controller and off you go. Its a nice neat solution as the heater strip is integrated into the base of the dew shield. Available from FLO.

Not sure you'll ever get an answer to your sensible question about an average night's viewing and dew - everyone will probably say, "it depends" ... which is does. Depends particularly on your location, not just on the weather. Start with a passive solution first - just the dew shield - and see how you get on. It also helps to keep stray light out of the corrector plate. I used my new CPC1100 without a dew shield on a windy first night out, and the corrector plate got a whole load of dust on it!

[michael.h.f.wilkinson]

I have a dew shield and separate dew strip (with controller) I got much later. On almost all nights the dew shield (passive) is enough. Just occasionally I need to switch on the dew strip. At full power it can deplete a powertank quite quickly, but it can be used at lower power for much longer (with the regulator).

[AlistairHowie]

Just as a data point, at full power, my Astrozap heated dew shield for my 11" OTA draws 1.5Ah.

[Zaine UK]

Thanks for the great advice.