CF vs. Alu

[multiweb]

I've recently acquired a small CF triplet 90/600. I compared a few things with my ED80, similar aperture, similar FL. Besides the razor sharp optics, obviously, I was surprised to see that the scope didn't dew up on first light like the ED80 does. I have to run a dew heater on the ED80 lens and the lens cell and the alu tube do get very cold at night. In contrast the CF tube of the new refractor didn't feel cold to the touch even at 2:00am. As a result, I suspect heat/cold transfer or lack of, the glass was 'nice and warm' and dew free all night. I have also noticed the same with my C11. When I swapped the alu body to a CF tube I reused the old alu as a rigid dew shield. In turn the CF body still feels warm to the touch when the alu 'dewshield' is freezing. Again as a result the corrector plate doesn't seem to get as cold as it used to and although I still run a dew heater I don't have to run it hard. So it seems besides the obvious advantages of minimising focus shift with CF it does also have some interesting properties with heat retention (for lack of a better term) under the open night sky? What do you make of it? Any similar experiences or am I imagining things? :question:

[TrevorW]

Main mirror on my CF GSO RC never dews up

[Moon]

Same here - perhaps because the primary mirror is big enough to retain enough heat to stay a few degrees above ambient.

Dew does form on the secondary and on the outside of the CF tube - to me this indicates the CF is dropping below ambient - i.e. bad.
I think polished AL tube probably has better thermal properties.

James

[Don Pensack]

That which makes carbon fiber beneficial also makes it a detriment.
It will prevent (in the same way an insulator would) the mirrors in the scope from cooling down.
Comparing a carbon-tubed C-11 with and aluminum one, the aluminum one lost the thermal problems with the star images after 3 hours. The carbon-tubed version still displayed thermal image issues at dawn (!).
So I think it matters whether the instrument will be used visually or photographically (aluminum being preferable for visual use).
But, if active cooling is used on the cassegrain, then the tube material isn't really an issue.
Don

[bird]

Hi Marc, the overcooling issue with the Al tube ED80 is from the paint (I assume it's painted? maybe white?) White paint radiates very strongly and will cool quite a ways below ambient.

You might try an experiment, and strip off the paint from your C11 dewshield so it's left as shiny unpainted Al and then re-test it's thermal properties :-) It should track ambient temp very closely then.

The CF tube, as Don says, is a good insulator, so it holds the heat in and keeps the optics warm. It probably takes longer to cool down as a result..

cheers, Bird

[multiweb]

Thanks for the explanations guys. I have fans mounted on the C11 to cool it down. It's stored indoors in my office and I have aircon running so I'd say by the time I take it out it's pretty close to the temp I get at night.

Anthony, my ED80 is painted dark grey, same as the C11. Interesting about the white paint. Who would have thought. I always wondered why your newt tube was bare - now I know. :)

Wondering if covering the tube with some kind of insulation would help with dew prevention. I recall Monte had his FSQ106 wrapped into some thermal fabric embedded with nichrome wire to keep both ends of the tube at the same temp and avoid contraction.

I think Greg Bradley mentioned something about wrapping the tube in rags too to minimise heat loss.

PS: So just I'm clear in my rambling, my problem is not with cooling down the optics, quite the opposite. I need to keep the optics just above the dew point. At the moment all my stuff is going cold too quick. I have no problems with tube current. I took care of that.

[asimov]

I do the same Mark; keep my CF C11 in an airconditioned room. Being in the tropics, I have to watch out though. Take the temp too low & it's instant condensation, just like taking a glass bottle out of the fridge.

[OICURMT]

Doesn't this violate the Law of Thermodynamics... :confused2: