Cooling Fan and Dew Formation

[Bart]

I might be imagining it or just unlucky, but recently on at least 2 occasions, when I left the cooling fan on running on the Dob, the primary ended up dewing up after a while, say 1.5-2 hours. I dont remember leaving it pointed to the sky in between coffees/beers/breaks/map reading etc. Neither do I remember the primary dewing up ever when the fan wasn't running, and I have had some looooooong sessions.

Can anyone tell me if this is predicatable or any other suggestions. In my naievity, I thought leaving the fan on might avoid dew, but thats not what I found..

Any thoughts would be welcome, thanks.

PS unless I'm doing something wrong, the fan will, in future, be turned off once i start observing.

Bart

[iwatkins]

Very likely that on very still evenings, leaving the fan on will draw cooler, damper air from ground level up into the tube. This well mixed air would then meet the cold mirror and condense.

Once you have your mirror down to ambient, I would certainly turn the fan off. If nothing else, the fan will introduce air currents in the tube.

Cheers

Ian

[Bart]

you're probably right, Ian, thanks

Barry

[jimmyjamjoejoe]

Dew forms when the mirror becomes cooler than the ambient air. It will eventually happen due to radiation over time, and yes, cooling with a fan will accelerate the effect.

The trick is to cool to ambient (or very near) with the fan, then turn it off, and use the conventional dew shield to slow down the onset.

[danielbb]

Im a noob so am likely wrong and im happy to be corrected.

So here is my question..

How can cooling it effectively with the ambient air temperature make it cooler than the ambient air? Is it not that the mirror is getting too cold and needs a better ambient air flow to bring it back up to temp?

ie rear cooling not being sufficient to keep the front of the mirror temperature high enough?

I like this small front facing fan idea

Telescope Optics Topics[/url

........

[jimmyjamjoejoe]

Because it isn't just the air that is cooling it, it is also radiating infrared energy. Both methods together shed more heat than can be replaced by blowing ambient air onto it.

[danielbb]

Increase air flow?

[cs1cjc]

My impression is that leaving the fan on inhibits dew, but this is on a solid tube.