University Lowbrow Astronomers

Observatory Paint.

by Tom Ryan
Printed in Reflections: November, 2001;
Title Changed: November, 2003.

Paint.  In Clifford D. Simak’s excellent short story, “The Big Front Yard”, we encounter advanced alien traders whose alienness is evident by the fact that they have never invented paint.  In our own world, paint is so ubiquitous that we rarely think of it, other than to notice that our houses could use another coat, or to wonder what the car companies were thinking when they produced that shade.  A deep thinker about paint might delve into it’s moisture permeability, or it’s psychological effect on the viewer, but since most astronomical equipment is made from rust free materials, and most astronomers don’t pay any attention to things that don’t last at least a few thousand years, astronomers end up looking at paint from a completely different perspective.

Roger Tanner, in Tucson, Arizona, made this clear recently when he and I were discussing the color to paint his observatory roof.  I said that I remembered reading in ATM II or III that stellar observatory domes were painted white, and solar observatories were painted silver, but I couldn’t remember why.  He replied:

“On the white roof, Here is what I have learned from the people designing observatories around here and in my radiometry class.

Aluminum absorbs only about 5% of incident visible radiation, reducing the large heat input.  Typical white paint absorbs about 8-10% so it will absorb more radiation.  When these objects heat up they radiate in the 10 micron range and the white paint has a emissivity of about 90% ( it is effectively black at 10 microns) while aluminum is about 2%!.  While this means a reflectivity of 98%, great coating for an IR telescope, only bettered by gold, it isn’t going to radiate away much heat.  What happens is the aluminum gets hotter since it can’t radiate heat away.  More heat would get carried away with convection cooling with aluminum, this is what limits the temp.  Aluminum roofs around here are much hotter than white roofs, but not as bad a black asphalt roof.  That is also why a chromed metal door handle on a black car can get much hotter then the paint on a windless day.  Temperature is controlled on a windless day by IR emissivity / Vis absorption ratio.

Problem is at night, the white domes radiate away their heat to a 3 degree Kelvin sky through a 180 degree Kelvin atmosphere; they actually cool below the air temp.  Then the air next to the dome cool off and rolls off the roof into the slit and causes seeing problems.  The aluminum dome would be good because it would not cool off at all and stay near air temp.

On the WIYN telescope, they went with a different solution.  They used aluminized mylar on an insulating foam-on-steel structure.  The aluminized mylar is mylar side out, this gives it the low absorptivity of aluminum during the day in the solar peak at 0.5 microns, and the mylar is transparent at this wavelength.  The mylar is mostly opaque at 10 microns (with an emissivity of like 0.4) and so the roof can radiate well and it carries the heat away from the aluminum.  They choose the thickness of the mylar to get this emissivity.  Also the mylar is thin enough not to isolate the aluminum from being cooled.  Then at night the emissivity is not so high that it cools and has cold air rolling off the roof.  The very low thermal mass of the aluminized mylar means the building cools off very rapidly at night.  I think that my be why the Desert Storm Shield works to keep a scope cool during the day also.  If you just wrap the scope in aluminum, it can get pretty hot unless there is a good breeze to keep the convection going.  They said the only down side is the mylar will require replacing in 5-8 years.  This is not that bad as a white dome needs repainting about this often.  Aluminum domes don’t need anything.  Maybe solar observatories are cooled from the inside with convection?  The McMath solar telescope on Kitt peak is painted white.

That is why I choose white, even though it cost $700 over just galvanized steel.  Galvanized steel is more absorbent than aluminum and more emissive, but it still gets hotter than white paint.  Since my observatory will be in another building, I won’t have problems with the cold air coming off the roof.  I have painted the roof of my present observatory with a local paint called KoolKote.  It is loaded with barium sulfate or something like that to give it the lowest visible absorptivity; around 6% when new.  It is also very black in the IR, like 94% emissivity.  The door handles to the observatory are often too hot to touch during the day, I just pull the door open with the key.  But the roof was just as cold or hot as the air.  The sides of the building are painted a very light tan, almost white.  They are usually warmer than the roof even though the roof is getting the sun at a more normal angle.”

Roger’s insight into the use of paint is, like most of his thoughts, both informative and practical.  Did you notice that Ford is having a much tougher time of it since he left them to work on Mars landers?  Well, their loss is our gain.

“Paint.  It’s not just for looking at any more.”


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