Black vs White ota and mount

[SteveBz]

Hi Guys,

I have a black ota and mount from Celestron plus my dslrs are also black (clearly).  At night it's a constant battle against dew with heaters, controllers and shields.  Given that the accepted explanation is about heat radiating from the setup to a clear black sky (3 degrees above absolute zero) would a white setup be better?

What do you think?

Anyone got both?

Regards

Steve.

[Marci]

No. Makes no noticeable difference. I have both, & they all get as wet as each other.

[SteveBz]

15 minutes ago, Marci said:

No. Makes no noticeable difference. I have both, & they all get as wet as each other.

Hmm... So maybe we haven't got the explanation quite right. White and black do radiate differently and so they should cool differently.

Thanks.

Steve.

[Moonshane]

Black absorbs heat more readily than white but never thought about radiation.  I would think materials are more important for that.

[nightfisher]

I dont see colour of OTA making any difference in the dark, it would make a difference in sunny weather as i found out with a black Maksutov doing Solar

[SteveBz]

15 minutes ago, nightfisher said:

I dont see colour of OTA making any difference in the dark, it would make a difference in sunny weather as i found out with a black Maksutov doing Solar

It makes a difference when radiating heat from the ota to the even colder sky. Well I say it makes a difference, but I'm really saying it should make a difference. 

Google black-body radiation.

What happened doing solar with your black Mak?

[nightfisher]

Putting a black OTA in the sun for Solar imaging means the tube will get more thermal currents compared to a white tube

[SteveBz]

37 minutes ago, Moonshane said:

Black absorbs heat more readily than white but never thought about radiation.  I would think materials are more important for that.

Materials are important for conducting heat around things from hot to cold, but colour is important for radiating heat away.

[SteveBz]

4 minutes ago, nightfisher said:

Putting a black OTA in the sun for Solar imaging means the tube will get more thermal currents compared to a white tube

Did it make it hard to image?

[Moonshane]

I may have misunderstood black body radiation but don't think it has anything to do with colour. The way I read it, black, white and grey bodies' colour name are illustrative and just indicate rates of absorption/radiation?

 

[Chris]

Black absorbs more energy from the Sun compared two white which is good at reflecting energy. I don't think you need to worry about it at night, only when doing solar

[SteveBz]

29 minutes ago, Moonshane said:

I may have misunderstood black body radiation but don't think it has anything to do with colour. The way I read it, black, white and grey bodies' colour name are illustrative and just indicate rates of absorption/radiation?

 

If something absorbs heat, like a black object, it also radiates in order to keep it in thermal equilibrium otherwise a black sphere in space would just keep heating up with no way of losing heat. If it doesn't absorb instead it reflects, like a white object, it also doesn't radiate and for the same reason.

[John]

I've owned white, black and gold and other coloured optical tubes. The colour of the tube did not seem to make any difference to dewing or the cooling speed of the scopes, some of which were the same aperture and focal ratio.

Tube material can make a difference. My TMB 5.1 apo refractor uses a tube made from a material known as Kruppax 50 and seems totally immune from dewing, whatever the conditions. It seems to cool at about the same rate as a conventional aluminum or steel tube.

[SteveBz]

6 minutes ago, John said:

I've owned white, black and gold and other coloured optical tubes. The colour of the tube did not seem to make any difference to dewing or the cooling speed of the scopes, some of which were the same aperture and focal ratio.

Tube material can make a difference. My TMB 5.1 apo refractor uses a tube made from a material known as Kruppax 50 and seems totally immune from dewing, whatever the conditions. It seems to cool at about the same rate as a conventional aluminum or steel tube.

Ok, that's very interesting, so it seems that the model we are being given of the scope dropping past the dew point because it radiates heat is not correct. What other models are there?

[mikeDnight]

Theoretically the black surfaces should lose heat more rapidly at night, but would absorb heat more rapidly in sunlight. The mounts and tubes are small and thin and I doubt black or white will make any noticeable difference in reality. 

Mike

[Moonshane]

49 minutes ago, SteveBz said:

Ok, that's very interesting, so it seems that the model we are being given of the scope dropping past the dew point because it radiates heat is not correct. What other models are there?

I can't agree with this. Surely other variables affect the behaviour ?

The fact is that scopes dew up when It is a moist night and the scope is colder than  ambient. As the scope clearly gets colder than ambient (see above) it must radiate heat faster than the environment.  There might be a slight difference between colours and materials but as everything ends up equally wet it Is not a big difference and seems essentially moot?

[John]

I would have thought tube colour was a minor factor compared with tube thickness, tube material, tube volume, whether a closed or open optical system, mass of the glass, etc, etc

 

[SteveBz]

1 hour ago, John said:

Tube material can make a difference. My TMB 5.1 apo refractor uses a tube made from a material known as Kruppax 50 and seems totally immune from dewing, whatever the conditions. It seems to cool at about the same rate as a conventional aluminum or steel tube.

Hi John, What is Kruppax 50, the one that is immune from dewing? Steve.

 

[Chris]

A black scope would only radiate more heat than a white one at night if it's been in direct sunlight during the day to accumulate the heat energy to radiate back out again, wouldn't it if we're talking infra red photon energy not ambient temp? So it could be a factor if you have your scope stored in front of a sunny window for example, but as said above there are a number of other factors which outweigh the albedo effect.

 

[SteveBz]

29 minutes ago, Moonshane said:

I can't agree with this. Surely other variables affect the behaviour ?

The fact is that scopes dew up when It is a moist night and the scope is colder than  ambient. As the scope clearly gets colder than ambient (see above) it must radiate heat faster than the environment.  There might be a slight difference between colours and materials but as everything ends up equally wet it Is not a big difference and seems essentially moot?

Well maybe it's just like a mist coming down.  Scopes pointing downwards or under trees don't get the mist because it lands on the tree or the back of the scope.  Warm scopes just dry off as fast as the mist falls.  Maybe we just don't have any need of the radiation hypothesis.

[John]

4 minutes ago, SteveBz said:

Hi John, What is Kruppax 50, the one that is immune from dewing? Steve.

 

I don't know much about it other than it's a very specialised form of compressed card. The German company APM Telescopes have been using it for their high end refractor tubes for over a decade now.

It is lighter than aluminum or steel - closer to carbon fibre. I found this description on another forum:

" The tubes are made from paper tape wound on mandrels and impregnated with phenolic resin. Similar to fiberglass and CF, but with phenolic instead of epoxy as the binder. They are cheaper to make and much easier to machine. Finishing operations on E-glass and CF require carbide or diamond tooling and machining either produces hazardous waste. Kruppax has none of those downsides."

I think I might have misspelled it - I think Krupax with an umlaut over the u is correct. It is a German product.

[SteveBz]

1 minute ago, John said:

I don't know much about it other than it's a very specialised form of compressed card. The German company APM Telescopes have been using it for their high end refractor tubes for over a decade now.

It is lighter than aluminum or steel - closer to carbon fibre. I found this description on another forum:

" The tubes are made from paper tape wound on mandrels and impregnated with phenolic resin. Similar to fiberglass and CF, but with phenolic instead of epoxy as the binder. They are cheaper to make and much easier to machine. Finishing operations on E-glass and CF require carbide or diamond tooling and machining either produces hazardous waste. Kruppax has none of those downsides."

I think I might have misspelled it - I think Krupax with an umlaut over the u is correct. It is a German product.

Very cool, maybe that is the answer.  Can you get other components from it, like eyepieces and finders?  I guess glass is glass.

[SteveBz]

18 minutes ago, John said:

I would have thought tube colour was a minor factor compared with tube thickness, tube material, tube volume, whether a closed or open optical system, mass of the glass, etc, etc

 

Well, I guess that other physical mechanisms are more relevant than radiation, but it is radiation that is the accepted explanation of dewing up.   Things like wind speed, altitude or air pressure probably play a part but we don't play a fan across our lens, we heat it.  Maybe a fan would be a better solution with the additional advantage that it doesn't create thermals or affect our CCD chips.

[John]

12 minutes ago, SteveBz said:

Very cool, maybe that is the answer.  Can you get other components from it, like eyepieces and finders?  I guess glass is glass.

As far as I know, the only astro related application has been refractor tubes from the APM Telescope company.

Here is a link to a German supplier of the material:

http://www.krueger-und-sohn.de/tubes-made-of-kruepax/150/91/119/

I suspect that a major order would be needed to make a specific alternative astro application viable.

[Skyline]

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