Black vs White ota and mount

[Stu]

50 minutes ago, Skyline said:

Are you serious ! Does it really make a difference in the dark what colour your telescope is, the fluctuations of temperature effects everything.

 

Skyline, please be respectful of other members on the forum, as you would expect from them.

Steve's question is very valid, and he is just trying to understand the physics behind it which is not simple.

Steve, I found this on the web which may or may not help. It comes from the owner of Istar.

Quote......And now about the actual color. We will produce both white AND black, depending on customers preferences. Myself, I still prefer black. Not only for its looks but also for its mostly unnoticed physical properties. The fact is that white outside coat cost us substantially more money and we will have to charge a fee between 55 and 95 dollars depending on style / size of scope. Over 4 years ago when I first started building scopes, I was standing face to face to decision which color to choose. After talking to Zdenek, our master optician, I decided for black. Not only because I did like this color better, but also because there were some important facts playing role in this. You can read more about this subject below. Now, after we produced first white scopes, I may actuall like the white the same as black, they both look absolutely "killer" with our silver, green, red or blue transparent jelly-opal trim. So if you dont mind paying a little bit extra for white, you get the most beatiful scope on the market (to my own humble opinion)... :-)

WHY BLACK? (Copied from an article on internet)
We make our designs including colors of our scopes based on research and laws of physics. In visible light, black surfaces absorb and re-radiate heat much more efficiently than white or reflective surfaces. Light from the sun (with a 5000 K blackbody peak at a wavelength of 0.5 microns) provides the standard reference against which we define colors. However, our common sense definitions of black and white may fail us at other wavelengths appropriate to different blackbody temperatures. Normal telescopes have temperatures around 260-300 K and thus have the peak of their blackbody radiated energy at wavelengths around 10 microns --- in the so-called ``thermal infrared’’. Essentially all painted surfaces appear ``black’’ (high emissivity) at a wavelength of 10 microns, so any painted telescope tube will radiate effectively to its surroundings regardless of the color of the paint. Typically, only metallic surfaces have low emissivity in the thermal infrared. The nature of white is to reflect energy, and black absorbs energy. More energy (light and heat) is reflected from a shinny white surface than from a dull black one. However, because paint is not a perfect reflector, Infrared Radiation (I.R.) will leak through and slowly heat the telescope even if it is housed in an observatory or covered with some material in the outdoors under the Sun. At night, when the air is cooler, we want to remove the heat as fast as possible, so, if the outside of the tube is painted with I.R. reflecting white paint, where does this radiation go?
Yes, inside through the black painted inside of the tube! It may be a good idea first to paint the inside the tube (IR reflecting) titanium white then apply a light coat of flat black over the white paint. The white paint would reflect I.R. to the outside of the tube and of course the black reduces light scattering inside. Remember though, paint is an insulator and should be applied with the least amount of coats as possible on the outside of the tube. One might find it useful to black anodize a small aluminum telescope tube. A tube made this way is black through and though. Given the need to channel heat from within the tube to the outside air a bare aluminum lightly coated with black paint may be the best solution......Unquote

[Sporadic Dobstronomer]

Radiation from your telescope will be far infrared light rather than visible light so you can't tell just by looking at it how bright or dark it is for this purpose.

[Skyline]

Apologies  to the OP. No offense was meant to be intended

[Sporadic Dobstronomer]

My last entry crossed in the post with Stu's...

[Sporadic Dobstronomer]

15 minutes ago, Stu said:

...WHY BLACK? (Copied from an article on internet)

...It may be a good idea first to paint the inside the tube (IR reflecting) titanium white then apply a light coat of flat black over the white paint. The white paint would reflect I.R. to the outside of the tube and of course the black reduces light scattering inside...

I don't think that will work.  I think that heat will simply conduct through the tiny thickness of the white coat so the effect will be almost as though the white coat were not there.

As I see it, if you paint your silver teapot black, all that matters is the black and not the silver underneath.

[SteveBz]

7 minutes ago, Sporadic Dobstronomer said:

I don't think that will work.  I think that heat will simply conduct through the tiny thickness of the white coat so the effect will be almost as though the white coat were not there.

As I see it, if you paint your silver teapot black, all that matters is the black and not the silver underneath.

That's how I see it too.  It's only the surface that matters.

[Sporadic Dobstronomer]

I have been experimenting with using space blankets over the telescope to control over-cooling (those very lightweight silver blankets).

It is too eary to say for certain, but it does look promising.

[SteveBz]

28 minutes ago, Stu said:

Skyline, please be respectful of other members on the forum, as you would expect from them.

Steve's question is very valid, and he is just trying to understand the physics behind it which is not simple.

Steve, I found this on the web which may or may not help. It comes from the owner of Istar.

Quote......And now about the actual color. We will produce both white AND black, depending on customers preferences. Myself, I still prefer black. Not only for its looks but also for its mostly unnoticed physical properties. The fact is that white outside coat cost us substantially more money and we will have to charge a fee between 55 and 95 dollars depending on style / size of scope. Over 4 years ago when I first started building scopes, I was standing face to face to decision which color to choose. After talking to Zdenek, our master optician, I decided for black. Not only because I did like this color better, but also because there were some important facts playing role in this. You can read more about this subject below. Now, after we produced first white scopes, I may actuall like the white the same as black, they both look absolutely "killer" with our silver, green, red or blue transparent jelly-opal trim. So if you dont mind paying a little bit extra for white, you get the most beatiful scope on the market (to my own humble opinion)... :-)

WHY BLACK? (Copied from an article on internet)
We make our designs including colors of our scopes based on research and laws of physics. In visible light, black surfaces absorb and re-radiate heat much more efficiently than white or reflective surfaces. Light from the sun (with a 5000 K blackbody peak at a wavelength of 0.5 microns) provides the standard reference against which we define colors. However, our common sense definitions of black and white may fail us at other wavelengths appropriate to different blackbody temperatures. Normal telescopes have temperatures around 260-300 K and thus have the peak of their blackbody radiated energy at wavelengths around 10 microns --- in the so-called ``thermal infrared’’. Essentially all painted surfaces appear ``black’’ (high emissivity) at a wavelength of 10 microns, so any painted telescope tube will radiate effectively to its surroundings regardless of the color of the paint. Typically, only metallic surfaces have low emissivity in the thermal infrared. The nature of white is to reflect energy, and black absorbs energy. More energy (light and heat) is reflected from a shinny white surface than from a dull black one. However, because paint is not a perfect reflector, Infrared Radiation (I.R.) will leak through and slowly heat the telescope even if it is housed in an observatory or covered with some material in the outdoors under the Sun. At night, when the air is cooler, we want to remove the heat as fast as possible, so, if the outside of the tube is painted with I.R. reflecting white paint, where does this radiation go?
Yes, inside through the black painted inside of the tube! It may be a good idea first to paint the inside the tube (IR reflecting) titanium white then apply a light coat of flat black over the white paint. The white paint would reflect I.R. to the outside of the tube and of course the black reduces light scattering inside. Remember though, paint is an insulator and should be applied with the least amount of coats as possible on the outside of the tube. One might find it useful to black anodize a small aluminum telescope tube. A tube made this way is black through and though. Given the need to channel heat from within the tube to the outside air a bare aluminum lightly coated with black paint may be the best solution......Unquote

Hi Stu,

What a great and complete answer.

So it raises several points:

1) How the paint is constructed.  If the paint is oil-based and the colour is black made from RGB etc, then the absorption spectrum outside the visible is unknown.  However if it is powder-coated with, for instance carbon black, then it would be true black, maybe the same is true of the titanium white.  So it seems that our black shiny telescopes are made from cheap oil-based paint with similar radiative properties to shiny white paint.

2) Reflectivity.   Maybe if, like Quantas planes, our scopes were polished Aluminium on the outside with no paint job they would behave differently.

I probably need to go outside and have a look at things which are not dewed up.

Be back later.

 

[Stu]

50 minutes ago, Sporadic Dobstronomer said:

I have been experimenting with using space blankets over the telescope to control over-cooling (those very lightweight silver blankets).

It is too eary to say for certain, but it does look promising.

I definitely think there is something in this. It is a balance between cooling and over cooling i.e. Get the scope to ambient then wrap it to prevent over cooling and to avoid tube currents and dewing.

[Marci]

Many years ago, when designing the Thermochill line of water cooling radiators I tested various finishes to answer this question, all be it for a different purpose. A standard radiator of brass & copper construction would be electrostatically coated in flat black electrically-conductive paint. 

Stacked 226w tecs were used to create a controllable heatload (up to 1kW), and radiators were shrouded to maintain a controlled fixed airflow in a controlled ambient environment.

Keeping all other factors equal the various finishes were compared against the reference and the c/w efficiency measure to 3dp (thousandths of a degree per watt).

Micron thickness of coating was as controllably constant as possible.

Unpainted, unpainted & polished, electrostatic painted, powder coat (flat, gloss, & flake), and electroplated (chromed) were all run thru the mill for 48hrs each. (Methodology can be found online - look for Bill Adams Thermochill analysis)

Essentially, the only detectable performance difference was between coated vs uncoated, and that difference came down to thousandths of a degree per watt of input heatload.

General consensus was basically if anyone cared about theoretical best thermal efficiency, leave uncoated.

With the heatloads present in an astronomical context being so minuscule, the differences would extend to 4dp per watt or smaller... they exist, as physics determines they should, but have an impact that is almost completely insignificant in _real_ measurable terms.

Take from that what you will...!

[Stu]

3 minutes ago, Marci said:

Many years ago, when designing the Thermochill line of water cooling radiators I tested various finishes to answer this question, all be it for a different purpose. A standard radiator of brass & copper construction would be electrostatically coated in flat black electrically-conductive paint. 

Stacked 226w tecs were used to create a controllable heatload (up to 1kW), and radiators were shrouded to maintain a controlled fixed airflow in a controlled ambient environment.

Keeping all other factors equal the various finishes were compared against the reference and the c/w efficiency measure to 3dp (thousandths of a degree per watt).

Micron thickness of coating was as controllably constant as possible.

Unpainted, unpainted & polished, electrostatic painted, powder coat (flat, gloss, & flake), and electroplated (chromed) were all run thru the mill for 48hrs each. (Methodology can be found online - look for Bill Adams Thermochill analysis)

Essentially, the only detectable performance difference was between coated vs uncoated, and that difference came down to thousandths of a degree per watt of input heatload.

General consensus was basically if anyone cared about theoretical best thermal efficiency, leave uncoated.

With the heatloads present in an astronomical context being so minuscule, the differences would extend to 4dp per watt or smaller... they exist, as physics determines they should, but have an impact that is almost completely insignificant in _real_ measurable terms.

Take from that what you will...!

In as far as I can follow that, am I right in saying that the basic summary is 'it doesn't matter'?

[Peter Drew]

My 6" F10 solar refractor is constructed with externally unpainted oversize aluminium tubing. Difficult to say what benefit this has without another one of different format. 

[Marci]

Pretty much... aye!

[Stu]

2 minutes ago, Peter Drew said:

My 6" F10 solar refractor is constructed with externally unpainted oversize aluminium tubing. Difficult to say what benefit this has without another one of different format. 

Perhaps build a binoviewer version with one channel painted and the other unpainted Peter?  

[Peter Drew]

1 minute ago, Stu said:

Perhaps build a binoviewer version with one channel painted and the other unpainted Peter?  

[Marci]

We actually made a radiator designed to be as perfectly thermally efficient as possible given the restraints of PC Watercooling to satisfy those who were seeking 'perfection' - in the end it didn't look hardcore enough and the performance differences so immeasurable that it was a commercial flop... although ESA ordered 2, and Intel took a few. A company using them to cool lasers ended up being the biggest customer, but only because they wanted to be the theoretical best for PR purposes vs the competition, who were using one of our 'regular' designs.

[Marci]

Ps: proper 'solution' is forced airflow to OUTSIDE surface of the OTA to create turbulence at the boundary layer between air and external surface, not dew heaters, however, this is impractical.

 

[timwetherell]

Interesting thread! I'd presumed that some scopes were white so you didn't trip over them in the dark  But dusting off my now retired physics hat, emissivity (the extent to which a material absorbs or emits radiation) often varies with wavelength so it's not possible to be sure what any given paint looks like in 20µm Infrared based on it's appearance to our eyes in the visible spectrum. The other thing to consider is that once a film of dew has inevitably formed, it's the emissivity of water that becomes important because that's the new surface!

Our homework is to paint a metal plate half black and half white then leave it in the garden at night and observe what happens. Nature will give you the correct answer, she's good like that!

[timwetherell]

3 hours ago, Stu said:

I found this on the web which may or may not help. It comes from the owner of Istar......

Just saw this, yep Stu, he's exactly right  

[SteveBz]

2 hours ago, Marci said:

Ps: proper 'solution' is forced airflow to OUTSIDE surface of the OTA to create turbulence at the boundary layer between air and external surface, not dew heaters, however, this is impractical.

Yes, I was beginning to think the same thing.  Maybe a large fan on a stand, the sort that scans your office in the UK's subtropical climate.

It would be a useful test even if not very portable.

[SteveBz]

1 hour ago, timwetherell said:

Our homework is to paint a metal plate half black and half white then leave it in the garden at night and observe what happens. Nature will give you the correct answer, she's good like that!

... with Celestron paint!

[SteveBz]

For me the only bit that really matters is my camera.  I don't want all those buttons streaming with water as I don't imagine they are intended for scuba use.  I don't want to heat them because really I want them cooled.

I'm beginning to think that a small, non-metal, non-black cover like a silver knitted tea cosy is the answer.

Maybe I could buy a piece of neoprene and make a camera cosy out of it.  That's one option.

The other option is Marci's fan, which I think is my old office fan in a fixed position.  In fact both might be good, but no heaters.

[John]

I used to have one of the black Istars - a 6" F/12. Great scope in many ways but the tube walls were thick and the tube pretty heavy (even for a 6" F/12). Cool down time was long. I can't recall any dewing problems though.

[Ruud]

At 0 °C, the black body radiation peaks around 10500 nanometres. Visible light has wavelengths from about 400 to 700 nanometres.

White paint is highly reflective across the visible range, but it could well be very non-reflective around 10500 nm and therefore be a good black body radiator in the temperature range around 0 °C.

I guess most paints are black when it comes to cooling against the night sky.

[SteveBz]

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