Dew - fog and drops, turbulence and undulations...

[BGazing]

Surely someone will have a good advice on this.

I use a heated dew shield. When it starts dewing from the inside of the corrector plate a bit (foggy appearance) I crank it up and it goes away.

However, irrespective of how effective this is I always seem to have tiny dew drops on the corrector when i shine the light down the tube and see reflections. Is that inevitable on a humid night, or only so if observing objects over certain elevations (which, again is preferable). As dew falls down, I reckon it is impossible to prevent the drops form actually falling on the corrector, but the heated one makes them evaporate, correct? 

Also, it seems to me that if I crank it up too much I see slight undulations (turbulence), but not on nebulae...rather on watching starry objects, like M11 and M13 at medium power. I watched under some serious jet stream...so I am wondering whether it was the jet stream or the dew heater which might have caused this...

[Stu]

Have you tried a hairdryer on a warm (but not hot!) setting? I often find that I get dewed up even with dew gear working but a quick blast with the hairdryer clears things very well and has only a short term effect in terms of convection currents.

[ChrisLX200]

On a Summer evening the air is thick with microscopic particles - not just water droplets but tree sap and pollen too. Just waft a laser beam around on a 'clear' night and you can see them reflecting the light. These inevitably stick to your corrector and build up over time and no matter how careful you try to be eventually a clean will be needed. I'm always careful to keep my scope covered when not in use yet it still ended up looking like this:

ChrisH

 

[BGazing]

yes, pollen here is terrible too in may and june, but this was high mountain in july, so much less likely. also, when shined at, pollen is sort of dullish, not clear and reflective like water droplets, i'd say...

[baggywrinkle]

Dew does not fall, though it does appear to.

Dew is a manifestation of water present as a gas in ambient air. The warmer the air the more moisture it can hold, up to 2% in really moist conditions. Other constituents of ambient air are O2 (21%), N2 (78%), CO2, H2 and other trace gases

As your telescope sits outside on a lovely clear night it radiates heat away into the air, this has a cooling effect on the metal and glasswork.

There is a critical point where the boundary between gaseous water vapour and liquid water vapour gets crossed and dew forms on the now cooled OTA. That point is called the dew point. It is also pressure dependant, so variations in atmospheric pressure due to weather fronts also have an effect. That is why on consecutive days of the same temperature it can feel muggy one day and dry the next, the atmospheric pressure has changed.

So though we hanker for clear skies.....they are not always friendly, a clear sky can mean more rapid dew formation that a cloudy one.

A nicer way to contemplate dew point is to look at a nice cool beer or a nice glass of chilled wine...watch the dew form..it has not fallen.

[BGazing]

Thanx for all the suggestions. Have not tried hairdryer, would first have to find one that works on batteries.

Still puzzled by those tiny droplets. They are not pollen from what I see when dried, but I guess I will figure it out at some point. Humidity was 80 plus at the start of the observation, I might be expecting too much...

[Davey-T]

Lots of 12 volt hair driers on E'Bay but beware dodgy ones and they draw a heck of a lot of power, mine draws about 10amps.

Dave

[BGazing]

ups, i use 4.5 amp lipo for my dew heater, so have to look for a less thirsty one...

[baggywrinkle]

On ‎30‎/‎07‎/‎2016 at 12:18, BGazing said:

Thanx for all the suggestions. Have not tried hairdryer, would first have to find one that works on batteries.

Still puzzled by those tiny droplets. They are not pollen from what I see when dried, but I guess I will figure it out at some point. Humidity was 80 plus at the start of the observation, I might be expecting too much...

You could find it is fine particles of both dust and pollen that has allowed moisture to nucleate on the corrector plate.  Not normally visible to the naked eye but small enough to assist dew formation.