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Observatory dish??

Community Veteran
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Registered: 08-10-2010

Observatory dish??

As a bus driver, I go past the Mullard Radio Astronomy Observatory on a daily basis, and in my idle musings I asked myself..'Why are observatory dishes painted white?'
Surely a radio observatory, listening  to radio waves would not be affected by colour? I could understand an optical observatory  being affected by colour.
Any ideas anyone?? Huh
7 REPLIES
shalom2010
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Registered: 28-12-2012

Re: Observatory dish??

I think they are white to reflect the sun, so keeping them cool. But that's only a guess! (looks like this link confirms it http://www.physicsforums.com/showthread.php?t=698125)
Community Veteran
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Registered: 30-08-2007

Re: Observatory dish??

The Jodrell Bank dish was painted grey/silver (and apparently rust!) for a long time, but is now white overall.
Experience; is something you gain, just after you needed it most.

When faced with two choices, simply toss a coin. It works not because it settles the question for you. But because in that brief moment while the coin is in the air. You suddenly know what you are hoping for.
Community Veteran
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Registered: 11-08-2007

Re: Observatory dish??

I would hazard a guess, that white is used to make the surface easier to clean, by making it easier to see where "you've missed a bit"

[quote=shalom2010]I think they are white to reflect the sun, so keeping them cool
I highly doubt that white is used to reflect the sun to keep the dish cool !
If that was true, by the very nature of a parabolic dish focusing incoming radiation at the receiving element located in the centre, then by your theory all that reflected heat would be concentrated at the sensitive electronics in the receiver, which is obviously undesirable unless you want to build a solar powered barbecue.
itsme
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Re: Observatory dish??

Quote from: purleigh
I highly doubt that white is used to reflect the sun to keep the dish cool !

If the dish get hot it would expand and therefore change the performance of the dish.
Community Veteran
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Re: Observatory dish??

Quote from: purleigh

I highly doubt that white is used to reflect the sun to keep the dish cool !

As I understand it.... colours do make a significant difference to the heat dissipation on any surface...
White is the most effective in keeping temperatures low from radiated heat.... Black is the most efficient in transferring heat to a surface..
Cricket players and bowls players and tennis players know this well...... and consider that the elements on a solar system are encased in a black background to transfer as much heat as possible from the sun`s rays.
Community Veteran
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Registered: 23-09-2010

Re: Observatory dish??

I expect the truth is that they had run out of grey paint and the local shop had a half price offer on white.
alanb
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Registered: 24-05-2007

Re: Observatory dish??

White paint is probably cheaper in large quantities.
Many many years ago I worked for a specialist paint company that manufactured bespoke paints for all sorts of weird applications. More than half of all the paint made by the company was white, and there were more different shades of white than any other colour. Most of the coloured paints also started off as white in the manufacturing process and then were changed to the desired colour by adding tinters later in the process.
The colour of paint used on older radio dishes probably did not matter, but some of the newer receivers operating at very short wavelengths cannot be painted because the paint affects performance. These are given passivation coatings such as chromate conversion or anodisation to prevent the metal corroding.