Why white

[TrevorW]

Is their any particular reason that a lot of mounts and scopes only come in white

[mental4astro (Alexander)]

Yes. White is the colour that is least problematic when it comes to dew condensing on it, and has less technical consequences to the instrument and mount.

Black is the very worst colour possible. At night it causes the item to drop its temperature below ambient, and as a result it will not only attract more dew, but it will dew up first, and if the ambient temperature is close to zero, the item that is painted black will have the dew turn to frost.

Not only this temperature characteristic of black, it also causes much greater shrinkage of the material. Particularly composite materials.

You will never see a professional instrument painted black for the last reason alone. The complications that uneven shrinkage causes makes it impossible for engineers to calculate the shrinkage of the various materials so they can best control the instrument and accuracy of tracking. How? A big scope is designed with flex in mind, so as the scope slews across the sky, the primary mirror's cell moves the same amount as the secondary cage no matter where in the sky it is pointed to. If the materials that are used in a truss instrument start shrinking at non predictable rates, the alignment of the optics goes out the window. White is the colour you will find the vast majority of professional scopes. This colour provides more predictable shrinkage characteristics, and as mentioned earlier, is least prone to dewing up.

I have paid close attention to the temperature and dew characteristics of items painted black over many years. At the dark sky site I use, dew is an infrequent occurrence, and often when it does form it is light. However, I have noticed that items that are painted black will always attract what little moisture is in the air while every other non-black painted item stays dry. And without fail, when dew does form, those items painted black will be sodden long before anything else is wet, and often have formed frost while all other items are only wet.

It is only amateur instruments that have their instruments painted black or other dark colours, and entirely only because of visual appeal, and without any thought given to possible consequences. And because of the shrinkage and dew issues with black and other dark colours, amateurs go bloody nuts trying to come up with ways to combat heavy dew.

It is becoming increasingly popular to insulate Maksutov and SCT's. This insulation is primarily to prevent a heat plume from developing inside the OTA and the scope can be used at high magnification straight away. Scopes that are painted black show a much more intense heat plume than white coloured ones of the same design and size. As soon as these scopes are insulated, and the key here is the outside of the insulating wrap IS NOT BLACK, and not only does this prevent the heat plume form forming, but dew is also much less of problem for those scopes that were painted black or other dark colour.

Alex.

[TrevorW]

OK thanks for the details info but why all these dark carbon fiber tubes so popular then does CF perform differently

[sunslayr (David)]

Carbon fibre can be made lighter and stiffer than steel or aluminium and it also doesn't expand as much. Although I have heard that it's not really an advantage because glass expands and contracts more than the carbon fibre and it can put stress on the optics unless its well designed to take that into account. It would be nice to replace the steel tube on a 10" newt to make it a little easier to handle.

[mental4astro (Alexander)]

Because the black CF is seen as been "sexy", and that is how it is marketed with its black aspect, plain and simple. It is somehow implied that because the tube is made from CF that it is somehow "better", but no one seems to ask how or why, and mostly this comes out of not knowing any better - and being "stronger" is how stronger?

Putting less strain on optics? It is ONLY the tube that is CF. The primary's cell is the same alloy one. ALL cells are designed to be passive. CF or metal makes no difference here. None.

Which would sell more, a black CF tube with the snazzy CF woven mat look, or a plain white CF tube? The plain white CF tube would get passed over only because of the look, regardless of the thermal advantages it presents. If it really was anything else, then white CF tube would be also offered. That's human psychology...

A lot of money is spent in the marketing departments of big corporations, and this includes working out what colours to use for their scopes. This is entirely a visual exercise. Nothing else matters because people on the whole just don't know any better. With amateur instruments, the prime driver is selling units.

Professional instruments are not designed or built by astronomers. Rather these instruments are designed and built by teams of engineers, technicians and opticians whose job is to know all of these factors and many, many more, and to present the keys of the finished product at the end. With amateur instruments, the prime driver is selling units.

[TrevorW]

Thanks you for that because is it worth $450 more to get the CF 8" GSO RC or the steel tube white one, if you compare the weights as advertised there is very little difference between the two ??

[sunslayr (David)]

Not sure how you would even go about making a white CF tube, you would need to dye the resin so much that it would probably ruin the tube, what with carbon being black and all.

[TrevorW]

You'd just paint it like most CF bikes

[sunslayr (David)]

Originally Posted by TrevorW

Thanks you for that because is it worth $450 more to get the CF 8" GSO RC or the steel tube white one, if you compare the weights as advertised there is very little difference between the two ??

From memory the steel one is a little bit better balanced.

[mental4astro (Alexander)]

It comes down to what is more important for you - the way it looks or the way it performs overall.

GSO makes no claims regarding the performance, pros or cons between the two. They only provide a weight difference. If the small weight difference is boarderline for your mount, then the truth is your mount is not strong enough or you are hanging too much off it.

Most people who could afford the more expensive CF would go with the CF and it entirely because of the way it looks and an implied "better", but without understanding all the aspects involved. To say CF is somehow stronger, but without reason for why it is or why this is somehow better than a steel tube is not a reason, and this does not take into account the thermal issues black brings with it.

[mental4astro (Alexander)]

David, you bring up an interesting point. But I need to ask, how is a steel tube better balanced? What makes one scope better balanced than another? What criteria are you using for this? I am not preferencing CF or steel here in any way. My question is purely a mechanical one. If one is more heavy towards the primary cell than the other, how is this a pro or con?

[sunslayr (David)]

I'll be honest I've only read reviews on RCs I've never had the pleasure of owning them. RC's in particular seem to be quite back heavy and the extra weight of the tube pushed out from the primary seems to help counter this. Otherwise you would probably need a weight on the front or maybe a longer dovetail to set it further in the mount.

[JA]

Originally Posted by TrevorW

Is their any particular reason that a lot of mounts and scopes only come in white

Radiant heat transfer is reduced with white and it is easier to see in low light are a couple of possibilities.

Best
JA

[mental4astro (Alexander)]

David, you are right in saying you just need to move the OTA if the balance point is towards the primary cell if one OTA is lighter than the other.

There are many OTA's that have the COG (centre of gravity) heavily set towards the primary cell. The Takahashi Mewlon is one such instrument. And this is not a problem, so why would it be a problem for a CF instrument? (this is a rhetorical question ) Why would this be a problem between a steel tube vs CF too? Now if this is just a comparison note from someone, then that's how it should be taken and not as meaning one is "better balanced" than the other because this is nonsensical.

Just remember, I am not advocating the use of black CF. My comments above are wholly about mechanical aspects.

Alex.

[sunslayr (David)]

I'm not sure I really understand what your trying to say? I guess it would be more accurate to say 'easier to balance' rather than 'better balanced' but then we are just getting into semantics.

[mental4astro (Alexander)]

Yep

[TrevorW]

Its interesting in that GSO apparently are no longer supplying the 10" CF tube RC not sure about the 10" steel tube one but they are doing a 12" RC at around $2500AUD compared to nearly $5000 for a truss CF RC which comes in at 23.7KG compared to the steel tube one of 25kg, now considering optics are probably no different where does the extra $2500 come into the picture ???

[Top_oz (Tim)]

I can't find any literature that indicates white is any better or worse than black paint. All non-specialty paints seem to be around the same emissivity and are 'black' in the IR spectrum (black in this context should not be associated with visible colour).

There are some high and low emissivity coatings out there but I'd suspect this is not used on amateur gear. My guess is that colour choice is down to aesthetics (marketing).

If emissivity is really a concern, the lowest emitters are polished metallic surfaces and metallised films. Not too far behind are aluminised paints.

[JA]

Originally Posted by Top_oz

I can't find any literature that indicates white is any better or worse than black paint. All non-specialty paints seem to be around the same emissivity and are 'black' in the IR spectrum (black in this context should not be associated with visible colour).

There are some high and low emissivity coatings out there but I'd suspect this is not used on amateur gear. My guess is that colour choice is down to aesthetics (marketing).

If emissivity is really a concern, the lowest emitters are polished metallic surfaces and metallised films. Not too far behind are aluminised paints.

Emissivity and Albedo are two different properties of a material or surface. Both important, along with many other factors, in radiant heat transfer.

Emissivity is the proportion of usually long wave / Infra Red thermal energy emitted by an object compared with a thermally "Black Body" at the same temperature and is used in the Stefan Boltzman Law to account for the heat transfer performance of thermally "grey" bodies. As a second order complication the emissivity is also a function of wavelength, angle, shape, etc...

Albedo on the other hand relates to the reflectiveness of a surface and is the proportion of incident solar radiation reflected from a surface. Albedo is sometimes more generally referred to as reflectivity. Albedo is defined as between Zero (0= Not reflective at all) to 1 (= Perfect reflector). As an example snow typically has an albedo of 0.9, whereas asphalt is around 0.1 and white and black paints/surfaces follow similar widely different values.

Emissivity and Albedo are 2 important parameters in radiant heat transfer. One you can think of as emission from the surface, the other reflection from the surface. Whilst the emissivity of white and black (say painted) surfaces may not be that different, their albedo or reflectivity in the solar radiation wavelength range(~250-2500nm) is very different and will result in more solar radiation being reflected from the white surface object. If you have your scope outside in the sun, either solar imaging or setting up in preparation for nightime imaging, there's no need for it to bake in the sun so much if you go for a lighter/white colour. Note that albedo / reflectivity is effected by dirt and dust on surfaces.

Best
JA

[Top_oz (Tim)]

My comments relate to night time and emmisivity only. Obviously daytime has other things to consider.