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Equipment Discussions >> Observatories

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CMacD
super member


Reged: 01/23/12

Loc: Ontario, Canada
Re: Metal roof color? new [Re: rimcrazy]
      #6242868 - 12/09/13 03:09 PM

Quote:

Quote:

Quote:

Science says to go with a TiO2 pigment white paint.



Any idea why the big boys buck the science and have big shiny domes?




Paint is expensive, heavy and a maintenance item. I would guess the detractors outweigh the benefit. Most modern telescopes are being built with highly ventilated domes like the DCT. They can rapidly cool down with simply moving the air around the telescope so any thermal dissipation gains/losses due to the color of the dome becomes moot.




I would think that air conditioning such a large structure on a permanent basis would be much more costly than paint, so other factors must be the primary concern here. I have heard that unpainted aluminum can track ambient better than just about anything else and even though it gets plenty hot during the daytime, it is the night time properties that rule the roost. The local seeing at night cannot be controlled as much as the heat inside the dome during the daytime.


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HunterofPhotons
Pooh-Bah


Reged: 04/26/08

Loc: Rhode Island, USA
Re: Metal roof color? new [Re: CMacD]
      #6242917 - 12/09/13 03:33 PM

Quote:

.... I have heard that unpainted aluminum can track ambient better than just about anything else and even though it gets plenty hot during the daytime, it is the night time properties that rule the roost...




I bet the cedar shingles on my observatory roof could beat that. <g>
They don't get hot, don't have much thermal mass, and are naturally insulating.

dan k.


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rimcrazy
sage


Reged: 03/03/12

Loc: Overgaard, AZ
Re: Metal roof color? new [Re: CMacD]
      #6243285 - 12/09/13 06:22 PM

Quote:

Quote:

Quote:

Quote:

Science says to go with a TiO2 pigment white paint.



Any idea why the big boys buck the science and have big shiny domes?




Paint is expensive, heavy and a maintenance item. I would guess the detractors outweigh the benefit. Most modern telescopes are being built with highly ventilated domes like the DCT. They can rapidly cool down with simply moving the air around the telescope so any thermal dissipation gains/losses due to the color of the dome becomes moot.




I would think that air conditioning such a large structure on a permanent basis would be much more costly than paint, so other factors must be the primary concern here. I have heard that unpainted aluminum can track ambient better than just about anything else and even though it gets plenty hot during the daytime, it is the night time properties that rule the roost. The local seeing at night cannot be controlled as much as the heat inside the dome during the daytime.




The only AC on the DCT is downstairs in the operators room. Unless it's raining or snowing, at night the shutters all around the dome are open for natural ventilation. Can't get much cheaper than that. It's all painted grey inside and left the natural metal color on the outside. I honestly don't know if it is steel or aluminum. My guess is galvanized steel.


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CMacD
super member


Reged: 01/23/12

Loc: Ontario, Canada
Re: Metal roof color? new [Re: rimcrazy]
      #6243678 - 12/09/13 09:30 PM

Quote:


The only AC on the DCT is downstairs in the operators room. Unless it's raining or snowing, at night the shutters all around the dome are open for natural ventilation. Can't get much cheaper than that. It's all painted grey inside and left the natural metal color on the outside. I honestly don't know if it is steel or aluminum. My guess is galvanized steel.




Exactly - natural ventilation. The choice of metal roofing is made for night time purposes only so you are not heating or cooling anything when the stars are out. Everything should be as close to equilibrium as possible. You don't want a structure that is gaining or loosing any heat at a different rate than your optics or the surrounding land or else air currents will mess with your seeing. Now during the summer when the sun is beating down on your observatory you don't want daytime heating to increase the temperature of thousands of pounds of glass in your primary mirror that will take countless hours to release. If you were to open the shutters with 90 degrees of temperature outside that would heat the primary big time. I have never been to the DCT but I cannot see how leaving the shutters open during the day does not heat the primary mirror to unacceptable levels even if it feels like a cool breeze. Maybe I am missing something?


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rimcrazy
sage


Reged: 03/03/12

Loc: Overgaard, AZ
Re: Metal roof color? new [Re: CMacD]
      #6243879 - 12/09/13 11:38 PM

Quote:

Quote:


The only AC on the DCT is downstairs in the operators room. Unless it's raining or snowing, at night the shutters all around the dome are open for natural ventilation. Can't get much cheaper than that. It's all painted grey inside and left the natural metal color on the outside. I honestly don't know if it is steel or aluminum. My guess is galvanized steel.




Exactly - natural ventilation. The choice of metal roofing is made for night time purposes only so you are not heating or cooling anything when the stars are out. Everything should be as close to equilibrium as possible. You don't want a structure that is gaining or loosing any heat at a different rate than your optics or the surrounding land or else air currents will mess with your seeing. Now during the summer when the sun is beating down on your observatory you don't want daytime heating to increase the temperature of thousands of pounds of glass in your primary mirror that will take countless hours to release. If you were to open the shutters with 90 degrees of temperature outside that would heat the primary big time. I have never been to the DCT but I cannot see how leaving the shutters open during the day does not heat the primary mirror to unacceptable levels even if it feels like a cool breeze. Maybe I am missing something?




I've been to the DCT numerous times. What I can say is that their is no AC. They vent naturally. I can also say that I know for a fact they spent considerable time and energy designing the facility for correct thermal management. The prototype design they used for both the mirror and the building was the Wynn telescope on Kitt Peak. What ever they do it works and it works very well and the outside of the dome, as you can see from the picture, is not painted but natural metal.


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dawziecat
Carpal Tunnel
*****

Reged: 10/20/10

Loc: Rural Nova Scotia
Re: Metal roof color? new [Re: rimcrazy]
      #6244090 - 12/10/13 04:37 AM

I recall reading, in the distant past, that at least one major observatory cooled the floor to expected nighttime temperatures.

Anyone hear of this?


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CMacD
super member


Reged: 01/23/12

Loc: Ontario, Canada
Re: Metal roof color? new [Re: dawziecat]
      #6244426 - 12/10/13 10:22 AM

Quote:

I recall reading, in the distant past, that at least one major observatory cooled the floor to expected nighttime temperatures.

Anyone hear of this?




Terry, I decided to stop my speculation about the DCT and search for the specifics. The DCT actively cools the primary mirror with twelve non-contacting actively cooled pie-shaped plates. This system will provide approx. 140W of cooling power and will help to keep the mirror surface temperature to typically within 0.5C from ambient. Facility provided instrument glycol is used as a coolant. I couldn't find any information about the use of air conditioners within the scope housing area but I would like to know how they keep the mirror from sweating during the daytime regardless of whether they open the shutters or not and are cooling the mirror to night time temps. Perhaps the air is dry enough on the mountain and the difference is not great enough to cause it to drip? Dunno. But there you go - they are doing more than "natural ventilation" to keep that sucker cool.


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Spectral Joe
super member


Reged: 02/28/11

Loc: Livermore CA
Re: Metal roof color? new [Re: CMacD]
      #6244718 - 12/10/13 12:54 PM

If you happen to be at Kitt Peak you have an opportunity to do an interesting experiment. Go over to the McMath-Pierce solar telescope during the daytime and feel both the upper (sun-exposed) and lower (shaded and facing the ground) surfaces of the angled structure. The upper side will be cooler than the lower, the sky is cool in the infrared and the ground warm. It's not a guess or a theory, I and many others have tried it and it just makes sense. Yes, the structure is actively cooled, but you can still feel the difference. Ever noticed that the upper parts of the Apollo lunar lander have silver insulation and the lower parts are gold? Same reason. I suspect that professional observatories make a tradeoff decision based on cooling rates, effectiveness of active climate control, etc. Painting a large dome is a pain, and expensive. A thin sheet metal structure comes to equilibrium rather quickly, so maybe none of this matters once the sun goes down. I do virtually all of my observing in the full sunlight these days, so it matters more to me than it would to nocturnal observers. I have visited and worked at a number of professional observing structures, and white is the dominant color, followed closely by bare metal.

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Mittag56
sage


Reged: 07/08/09

Loc: West Amboy, N.Y.
Re: Metal roof color? new [Re: dawziecat]
      #6247247 - 12/11/13 03:03 PM

I went with a dark color to fit in with other buildings...now wish i had gon with a lighter color....alot of heat can build up in the sun....this spring will see a few changes for insulation and air ducting to try to keep it cooler....

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John Fitzgerald
In Focus
*****

Reged: 01/04/04

Loc: AR
Re: Metal roof color? new [Re: pstarr]
      #6248412 - 12/12/13 07:29 AM

Arizona is AZ, not AR. Regardless of the heat on the outside, the inside of my observatory covered with bright (silver) galvanized steel does not get hot. It is also well ventilated.

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hottr6
Carpal Tunnel
*****

Reged: 06/28/09

Loc: 7,500', Magdalena Mtns, NM
Re: Metal roof color? new [Re: dawziecat]
      #6254597 - 12/15/13 02:31 PM

The OP is in rural Nova Scotia.

Unless you want your shed to appear like a sore thumb in every satellite photo and google map, I'd suggest a dark green.

I live in NM and went with beige to match the surrounding desert grasses - keeps my workshop cool as well.


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Raginar
Post Laureate
*****

Reged: 10/19/10

Loc: Rapid CIty, SD
Re: Metal roof color? new [Re: hottr6]
      #6254671 - 12/15/13 03:24 PM

That would be neat

I had to check. My shed camps in pretty well

Edited by Raginar (12/15/13 03:37 PM)


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pstarr
Carpal Tunnel
*****

Reged: 09/17/04

Loc: NE Ohio
Re: Metal roof color? [Re: hottr6]
      #6254756 - 12/15/13 04:21 PM

Any color is good as long as it's white.

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