Black or White?

[Suzy_A]

Hi Everyone!

I'm making a new telescope - 10" F4. I had everything except a tube.

Yesterday, I finally found a place that not only stocked 300 mm PVC tube, but would also sell me a metre and a half - the first few places I phoned wanted to sell me 500 metres or something like that.

Anyway, as the walls of the PVC is 7 mm thick, the tube weighs about 10 kg and so I'm planning on using it to make a fibreglass mould and then using that to make a carbon fibre tube that will weigh about 3 kg or so.

Now....

my two choices are to use white gelcoat on the outside of the tube OR to leave it black. What do people think?

The way I see it:

White - well - you can see it - you don't bump into the telescope (as much) in the dark as a black one. Secondly, after a late night/early morning session, if you leave the telescope out and the sun comes up it ain't going to fry as much. On the down side is the reflection problem when you look through the eyepiece and the other eye is open, but a bit of black around the focusser should help there. Thirdly, If it is white gelcoat on the outside, scratches don't show up as much and are easily fixed.

Black (carbon fibre) - look sexy! On the down side, the telescope is hard to see in the dark, it will get very hot if you leave it out during the day, and scratches are much harder to cover up.

What do people think?

[ving (David)]

Ooo!! Ooo!!! keep it black!!!
black carbon fibre looks way cool!!!

you could give it a clear coat instead of white....

[gbeal]

Black, and don't leave it outside during the day!!
Not sure of the science, but it is my understanding black dissapates the heat better, and allows better cooling.
I have a PhD (Post Hole Digger) but it isn't the correct qualification for this question. Someone else way brighter will pipe up though, you watch.

[norm]

I prefer black, but white is probably more practical. As suggested about masking the focuser area black, that could work too and is a good idea.

The answer is never black and white , its such a grey area !!! Speaking of which , grey could be a compromise, albeit a tad boring.

[rmcpb (Rob)]

A vote for white. Black tubes ar bumped soooo often it is a real pain.

Cheers

[Suzy_A]

Hi Folks,

Originally Posted by gbeal

Black, and don't leave it outside during the day!!
Not sure of the science, but it is my understanding black dissipates the heat better, and allows better cooling.

On the rare occasion, I may leave it out during the day for a short period of time...

Anyway, how cool something remains depends on the absorbtivity, the emissivity and the reflectivity of an object, along with the incident radiation.

Light falling on an object is absorbed (or transmitted) and reflected. A black surface such as carbon fibre/epoxy would absorb about 90% of the incident light and reflect about 10% while a white (titanium dioxide/epoxy) surface would absorb about 5% and reflect about 95%.

The absorbed light is degraded to heat and then re-radiated at a lower wavelength - in the IR. Some of the heat is also lost by convection - the air passing by carrying of the heat.

The amount of re-radiated energy is dependent upon the emissivity of the surface - the higher the emissivity, the more heat is reradiated and the cooling the object will be.

A black surface in the sun (~1000 W/m-2) will absorb about 900 W/m-2 and need to get rid of this by re-radiation and convection. The equilibrium point -where the heat gain equal the heat loss - will be 50 - 100 C or even more.

A white surface in the sun will only absorb about 50 W/m-2 and so only need to reradiate or lose this by convection.

In general, a ‘black’ surface will reradiate better than a ‘white’ surface. However a black surface will also absorb much more than a white surface. All things being equal, a black surface will become much hotter than a white surface. However all things and not equal and a titanium dioxide surface actually has an extremely high emissivity in the IR bands and so will stay very cool. About the only thing that beats it is a Zinc Sulphide surface.

[ballaratdragons (Ken)]

Go silver. Thats sort of half way between! Black is too hard to see (looks cool though) and white looks scientifically sterile!

[ving (David)]

what if you leave it black and while its in the sun you have a white cover over it?

[Robert_T]

As per Ving's suggestion, I have a black scope that I occasionally leave out during the day, just throw a silver-backed picnic blanket over it with the silver up and she stays nice and cool.


hmmm Suzy, you obviously know a lot about the science behind this stuff

cheers,

[ving (David)]

Originally Posted by Robert_T

As per Ving's suggestion,

gee i am brilliant!!

[gbeal]

What I want to know, apart from how come Ving is so bright, is why leave it out during the day at all???

[Suzy_A]

Originally Posted by gbeal

What I want to know, apart from how come Ving is so bright, is why leave it out during the day at all???

I wouldn't normally leave it out - but there might be a few occasions - with a solar filter looking at the sun, it's on display somewhere, I've been up till 6 am and can't be bothered .....

I have also on a few occasions done daytime viewing, just for the fun of it - its quite easy to see Mercury, Venus, Mars, Jupiter, Saturn, as well as any star brighter than about 1 or 2 mag. Ever seen a binary - alpha Centauri is good - during the day?

I once also took photos of a day-time occultation of Jupiter by the moon.

[gbeal]

Originally Posted by Suzy_A

I wouldn't normally leave it out - but there might be a few occasions - with a solar filter looking at the sun, it's on display somewhere, I've been up till 6 am and can't be bothered .....

I have also on a few occasions done daytime viewing, just for the fun of it - its quite easy to see Mercury, Venus, Mars, Jupiter, Saturn, as well as any star brighter than about 1 or 2 mag. Ever seen a binary - alpha Centauri is good - during the day?

I once also took photos of a day-time occultation of Jupiter by the moon.

Then on those few occasions you have to weigh up in your own mind whether the advantages outweigh the disadvantages. My money is still with black. But let me hasten to add that I have a few "white" scopes, seems the colour you buy them in. The only homebrew I have is indeed black, well nearly.

[fringe_dweller]

geez another brainiac on the forum - you just raised the IIS IQ level 10 more points! stop it! seriously you sound like you know what your on about and have serious passion for astronomy - good to have you on board!
I have heard that black is the best colour
my mate does his tubes powder coated black - he knows the science behind it like you - buit to see it in the dark he puts some astronomy related 'glow in the dark' sticker near the base (the ones that absorb light/heat? during day and glow at night) - i got an image of the world as mine works nicely -
also white scopes reflects the star light, thus ruining your night vision

[ballaratdragons (Ken)]

Not much help are we

[h0ughy (David)]

go black, if you can't see your scope put some very dull red led all over the ends, gee if it is that dark the stars should be lighting up the scope anyway

[ving (David)]

Ooo! glow in the dark!!!
purdy!

gary, i am brite cause i's was bawn thet wey!

[janoskiss (Steve H)]

Bird should be able to tell you what kind of paint to use to keep your OTA dew free. Cover it in Al foil maybe???

[RAJAH235]

on this topic, i will try to relate the facts as stated by beren, I think, re; colours @ night. Anything other than white is bad for collecting dew. Something about ir reflectivity or ? . There's a thread in here somewhere.
ptn, 2 x 10'' dobs, no more than 8 ft apart, mine = white, his = red. Results = mine dry, his was dripping.
Now I totally agree with what's been said re; daytime, but do you normally view then?
fwiw, it's a personal choice. All depends if you want a dry tube or a wet 1 in the car.
Been said, but they are easier to see @ night. L.
ps. that's 1500 for me.Yaa!

[davidpretorius]

Originally Posted by RAJAH235

that's 1500 for me.Yaa!

congrats rajah.

go black i reckon, nothing scientific, it just beats boring grey like mine!