Question: black surfaces at lower temp than surrounds - why?

[mental4astro]

I noticed a very curious thing on the weekend. On two consecutive nights, the temperature had fallen to just above freezing. Dew wetting everything. But the secondary mirror of my 17.5" dob was frosted over! The secondary's holder is all metal, painted black & hollow, & filled with a dew heating strap that was NOT ON. I just cannot understand how this secondary mirror could frost over??? :question:

Anyone have an idea as to why, please?

I can only think that the metal casing is acting like some sort of heat sink where the atmosphere is pulling out heat.

AND, I've also noticed up at Katoomba Airfield, black surfaces actually having dew form on them, regardless of the material be it metal, vinyl padded seat, whatever, just black, yet no other surface had dew form on it, including metal ones, even bare metal.

Why is it so??? :shrug:

Mental.

[RB]

Black body radiation.

:einstein: :D

[Shiraz]

yep, painted surfaces radiate heat very much more effectively than bare metal, so they send much of their thermal energy to (very cold) outer space and get cooler than the air in the process - hence dew.

I found that the CF tube on my Newtonian was radiatively cooling below ambient and dewing up, so coated it with aluminised tape and the problem went away. Maybe do the same with your secondary - even a bit of alfoil taped over the back of the holder should help. The advantage of passive techniques is that you are less likely to get air turbulence and resolution loss due to thermal gradients - which will happen if you use heaters.

rough or dark material of any kind, most paints, even glass (eg lenses and corrector plates) are all pretty good thermal radiators and will cool below ambient and dew up if they are pointing at the sky. Clean bare metal is the best material for resisting dew formation.

[AstralTraveller]

Yes, read up on black body radiation. Black objects are (surprise surprise) the best black body radiators. I have a pair of 15x80 binos which used to dew up also instantly. I made dew caps out of heavy Al foil and that helped. I then covered them in foil and that basically solved the problem. Looks dawky though :shrug:.

[multiweb]

Couple of socks should work. :thumbsup: and it won't attract the aliens.

[Don Pensack]

I don't know how deep into the tube or UTA your secondary is, but expanding the length of the tube or UTA above the secondary helps (by reducing the amount of sky into which it can radiate heat.
It won't cut into your true field of view if you add an "SCT dewshield" to the top of the scope so the secondary is, say, about a half meter inside the opening of the scope.

[avandonk]

The simple cause is what is called supercooling. The colour of the object is immaterial.

When any object is under a clear sky it will radiate heat to the much cooler sky. It will then cool below ambient air temperature. If then this new lower temperature is below the dew point your optics are in trouble.


The trick with dew control is to NOT heat your optics. The solution is to heat the optics by radiation. This means very gentle heat to balance the radiative heat loss to the sky.

Dew straps around the tube and or dew shield will radiate heat to balance the heat loss.

A rough number is about 3C of your dew shield above ambient. This number depends on relative humidity.


My RH200 and optic train is held at constant temperature above ambient + or - 0.1 C by an advanced PID controller. This also means my focus just does not change!


Bert

[Allan_L]

I found this interesting.
And it explains some effect of the colour of materials.

http://www.windowoutdoors.com/WindowOutdoors/Dew%20Frost%20Condensation%20and%20 Radiation.htm

[Barrykgerdes]

Obviously the science I learned at school is no longer taught.

White (shiny) surfaces reflect heat and black (matt) surfaces absorb heat from the heat source (sun).

The converse is the case when the source is removed. white (shiny) surface keeps the heat in and the black (matt) radiates the heat quickly.

We proved all this by experiments that covered radiation, conduction and convection in science.

Barry

[Shiraz]

just remembered this very useful thread that would be worth a read.
http://www.iceinspace.com.au/forum/showthread.php?t=83503

re the colour of materials, almost all paints (and most other materials except metal) are pretty black at thermal IR wavelengths, so colour is not important for radiation efficiency - it does make a difference to absorption of sunlight though. The differences in apparent temperatures of a variety of materials under cold sky is likely to be due to thermal conductivity rather than colour - they transfer heat to the surface at different rates.

[avandonk]

It is a simple fact that a hot body will radiate and lose energy to a cold body. Their colour is immaterial.

The Sun is a black body radiator. It just happens to have a surface temperature of about 5000C.

We are now talking about infra red long wavelength radiation. The colour of the emitter is still immaterial.


Bert

[rogerco]

The colour and texture of the surface will effect the the way a material will radiate or absorb heat energy but the thermal capacity (specific heat) of the material will determine how hot or cold it gets for a given loss or gain of energy.

I think new theories of quantum mechanics have changed some of the simpler ideas we had back in the sixties though.

[Eden]

Bert nailed it:



This is what I do on cold Melbourne nights when dew is evident. The temperature of the heat strap is low enough to not radiate heat across the optical path but high enough to keep the dew at bay. It's not even warm to the touch, but does the job. :thumbsup:

[sheeny]

This come down in part to spectroscopy.;)

There is an "infrared window" in the atmospheric spectrum.
http://en.wikipedia.org/wiki/Infrared_window

At night, one of the mechanisms of heat loss (transfer) is via radiation between all the parts of your scope and outer space vis IR radiation in the frequencies in the atmospheric window. Outer space is about 3K and you scope about 300K, so there's a net loss due to radiation.

During the day, the incoming radiation from the sun overwhelms this effect.

It's not entirely accurate to say that the material or colour has no effect. Certainly the colour in the visual spectrum does not have an effect on this mechanism, but the IR colour or emissivity spectrum of the material (or coating on the surface of the material to be more accurate) does. A material with high emissivity in the IR band loses more heat via this mechanism than one that has a low IR emissivity.

If you were to place samples of different materials and coatings out at night and measure their temperatures, you will find differences due to this effect.

This is why a colourbond or galvanised iron roof will produce dew or frost (and hence water into your tank) when a tile roof will not.

Al.

[AstralTraveller]

Not saying you're wrong but I imagine the greater thermal mass of the tiles is also a factor. They are also probably heated more during the day.

[dannat]

here are some good exp.
1.place 2 cups of water into a freezer, same shelf -one room temp water the other with lukewarm water..see which freezes fastest
2. heat up some water in the microwave (for a few minutes) -you may get some bubbles forming, try throwing a teaspoon in quickly {be careful -may splash/burn)

[sheeny]

Getting technical now...:lol:

I don't know which way it would go. There is a lot of thermal mass in a tile compared to corrugated iron, but I would expect the iron roof to actually be hotter than the tile.

However, to back up my theory, I had a tile roof in town and never noticed the sound of dew running in the down pipes. I now have an iron roof running into 2 small rainwater tanks which we use for drinking water only. Our neighbour has a single tank just over twice the capacity of our two tanks and he has a tiled roof. He also only uses the tank water for drinking. During the drought, he had to buy water, where we didn't. We had long periods with no rain at all, but every morning I could hear the dew running into the tanks.:thumbsup:

Al.

[AstralTraveller]

Al, I'm agreeing with you that a tin roof will get colder. I'm just wondering whether emissivity is the complete explanation. Now that you've jogged my memory I used to hear water dripping down our neighbor's down pipes when there was no rain and that was a tin roof.

[sheeny]

There's a lot of variables in that scenario with the dew and roof, David, I agree. I'm not suggesting it is proof, but it is what got me thinking about the what the difference was. Some roof tiles can also be slightly porous, which simply stops the dew of running off before its evaporated.

Going way back, there was an excellent thread here on IIS that involved myself and Bird in particular discussing cooling of scopes and different materials when exposed to the sky. I started off challenging some of Bird's ideas using my engineering experience, but he eventually won me over... I was assuming atmospheric temperature was the governing factor not knowing about the "infrared window". Now I know!;) Its good to learn things...

Al.

[AstralTraveller]

And the neighbour's marriage was pretty frosty too.