Thick questions about dew and dew heaters

[jnb]

I am sure this should be really simple but I have managed to confuse myself.

Why does dew appear on a cooling telescope? OK I know it's because the telescope cools below the dew point at which point water vapour condenses. But if the air temperature was at or below that temperature then water vapour would condense in the air, i.e. it's raining! If it's above that temperature then dew won't form. The telescope is presumably either at or above the ambient air temperature so what am I missing. Obviously something because it does form so I need a dew heater.

Practical questions. Why are PWM supplies used on dew heaters? With a large scope (which I don't have yet) presumably one has to be careful with positioning dew heaters to avoid uneven heating and how do you prevent the dew heater itself creating turbulence around the scope? OK the last effect would be very small but the dew heater is right next to the objective or mirror.

[Psychobilly]

PWM is a pretty loss less (more efficient)  way of controlling the power supplied to the dew heater - important when running away from a mains supply..The thermal time constants involved are so large you can use fairly low frequency PWM..

There  was some speculations that  some PWM Dew heaters and controllers were causing noise issues with cameras... 

I have always used dew heaters in prevention rather than cure mode.. i.e. They are on at a low level  from the start of a session rather than getting switched on when the dew starts to appear... The power getting ramped up if needed...

Peter...

[michaelmorris]

I am sure this should be really simple but I have managed to confuse myself.

... The telescope is presumably either at or above the ambient air temperature so what am I missing. ...

Your presumption is based on the assumption that the telescope loses heat only by conduction/convection.  In fact, heat loss through radition is going to be signficant.  The heat radiates out to space.  A dew sheild works by blocking the line-of-sight of infrared (heat) radiation to space and hence reducing radiative heat loss.  Dew shields work on anything.  I have a dew shield above the bookshelf in my observatory. The result is that whilst my tube of my scope may be dripping with dew, my books stay bone dry.

[clarkpm4242]

The primary mirror in a newtonian becomes cool enough to create a cold boundary layer of air next to the mirror surface.  Dew is formed from this layer.

One method of preventing dew is to keep air moving across the surface of the mirror preventing the cold layer forming.  Fans situated to the side creating a laminar flow across the mirror do this.

A very low power heater is best for the secondary, that is used to prevent dew in the first place.  Similarly a wraparound heater strip for refractors.

HTH

Paul