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]

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

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 :lol: , its such a grey area !!!:lol: Speaking of which , grey could be a compromise, albeit a tad boring.

[rmcpb]

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

Cheers

[Suzy_A]

Hi Folks,



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]

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

[ving]

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:confuse3:

cheers,

[ving]

gee i am brilliant!! :D

[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]

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]

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! :einstein: 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 :earth: works nicely -
also white scopes reflects the star light, thus ruining your night vision :P

[ballaratdragons]

Not much help are we :lol:

[h0ughy]

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]

Ooo! glow in the dark!!! :D
purdy!

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

[janoskiss]

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. :D L.
ps. that's 1500 for me.Yaa!

[davidpretorius]

congrats rajah.

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

[Photon]

Hey Dave,
Interesting Rajah's comments. Maybe if we go "White" we may not have to use the hairdryer again!! Boy that last meet was a 'wet' one.
IanG

[Suzy_A]

No, but mildly amusing...

[ving]

glad to be of sevice m'am ;)
:P
made up your mind yet?

[Suzy_A]

Maybe chequered?

Or hot pink?

Actually, still undecided. On Monday I'll go to the place that sells fibreglass, carbon fibre, polyester and epoxy resin, gelcoat etc and ask them about the practical aspects of making the tube.

The main problem that I see is actually constructing the tube.

The mould is no problem - a two piece mould - ( and ) - if you know what I mean, with the smooth side on the inside of the mould. Basically just sand, polish and prepare the PVC tube outside and then make a two-piece mould around it using fibreglass.

Then make the actual tube - and this is the hard part.

Either I can join the two parts of the mould together () and try and use a single piece of carbon fibre wrapped around the inside of it. I can see that this may be difficult and a bit messy, but will give a one-piece seamless tube. If I first use a layer of clear resin or gelcoat, then the carbon fibre, then the epoxy or polyester resin, then it will look nice and shiny and black. Or I could use white gelcoat, then carbon, then resin, for a seamless white tube.

The other way would be to make the two halves and then join them later. This would be much easier to do but if it is only clear resin and the carbon, then there will be two obvious joins. Using white gelcoat, carbon and resin would mean that the join would be invisible.

[iceman]

You should talk to forum member "Bird". He got his own tube made out of aluminium, and there was a thread a while back about it and how it is much better in resisting dew.

It looks great too, nice and shiny.

[iceman]

Here's one of the threads about it. some interesting reading.

http://www.iceinspace.com.au/forum/showthread.php?t=5256&highlight=dew+aluminium

[stinky]

As for colour and bumping the tube - on sailing boats it's an old trick to paint deck fittings black so that they stand out at night (obviously the deck could not be black!). Visibility in low light is a question of contrast - black to any other colour in low light is as extreme as it gets.

[ving]

thats brilliant sco.... i mean guest!
have your tube black and the area surrounding the telescope a brighter colour so it stands..... hang on, do you have an ovservatory?

[RAJAH235]

Do you really need to go to all that trouble to mould a tube? Why not use 'Sonotube' like Meade do. Cardboard concrete/pillar moulding type tube. Lots of sizes. :D L.
ps. If you paint it white, you'll have less prob with dew as well. :P
pps. Good thermal Q's 2.

[fringe_dweller]

:rofl:

pssst ...over here ... paint it black..dont tell rajah ;-))

[Suzy_A]

Sonotube is unavailable in Perth.

Large diameter cardboard tubing use to be available in Perth but not anymore. I once bought some large diameter plain tubing (not coated) and fibreglassed it and it was perfect! But the place that made it was bought by a multi-national and then closed down.

The largest size of 'formatube' now available in WA is 250 mm diameter. A few places offered to get some 300 mm for me, but the cost of shipping was unreasonable - it works out only a bit more to buy the PVC tubing, make a fibreglass mould and then a carbon fibre tube.

Not only that, I think that using carbon fibre/epoxy only, I can make a tube that weighs only 2 - 3 kg, as against about 7 for sono/carbon/epoxy, or 11 for PVC. The other option is 1 mm aluminium as that would only weigh about 3 - 4 kg.

[stinky]

..don't paint the walls black - it's not cool anymore.

But seriously if there is ANY light at all black stands out.

[ving]

i have heard that mint green is the new black... is this true?

[acropolite]

Black for me, or you could be radical and opt for a cow pattern like Ving's chair.

[mickoking]

WHITE WHITE WHITE WHITE WHITE


Real telescopes are White ( I wish mine were) Black is so passe ;) However having some black around the focusser is a good idea.


Telescopes should be white
Motorcycles should not be white.......:P

[ving]

mine is silver... thats almost white.

by the way, silver is the new white :P

[33South]

I've heard of white gold but never white silver. :P

[mickoking]

Mine is silver too, I don't feel in my heart that silver is the new white :lol:

[AstroJunk]

Sometimes you know instinctively what is right!

[astro_south]

.....or you could compromise like I did :D

[ving]

just about every car in the past sold was white.... now just about every car sold is silver. hence silver being the new white :)

[Suzy_A]

Hi Folks,

well, this morning I went and bought 2 m2 of carbon fibre and resin etc. I also got some white gelcoat - so I can do either...