CF Tube vs Aluminum Tube

[willis]

Willis, all metals are ductile, ductilty in term of fabrication : dwelling, bending and cutting. All metals deform in a some sense under loading like a building structure and a bridge.

Dear SY. I know it. This is physics or material science. But when we talk about dutility, it means plastic deform. plastic defrom is not reversible. If we apply a small force to bend a metal bar, it will deform a little bit and restore the orginal shape afterward. This is elastic deform. However, if we apply a large enough force to bend a metal bar, it will deform plastically and will not restore to it's original shape (拗咗). This is plastic deform. In the case of telescope, the loading, say DSLR or guiding etc, will not make the telescope tube to undergoe plastic deform. So the stiffness is more relevant to the discussion. Stiffness is simply the resistance to deform. A stiff tube can avoid misalignment of the focus better.

[anguslau]

I can understand and do agree that CF may not have much advantage in refractor scopes. It may be better suited for reflectors. The reduction in weight may not be significant (or even meaningful) for small scopes such as the C8, but will become more and more prominent for larger scopes, I believe...

Dear Mr. Lau,

May be you are right. But experiences tell that big telescopes, especially reflectors, are being more unlikely to use tubes. They tend to use struss.

Don't mistaken me, I am talking about telescopes with apertures bigger than 20 inches. C11 and C14 are not classified as big.

Best regards
Chan Yuk Lun

Dear mirror god, thanks for your comments!

Looking at the scopes available commercially, it seems that ~16" is the breaking point between closed and open OTAs. I guess >~16", an open OTA begins to make more sense because of weight, ease of fabrication and other characteristics...

For a truss design, do you think carbon truss or aluminum truss tubes make more sense? Will the weight reduction be significant? What about the thermal perspective?

I have been exploring the feasibility of making a 16" astrograph (~f/4). I notice that Royce makes conical mirrors which claims significant weight reduction plus other advantages. Would you recommend going for such designs?

[syyuen168]

willis - It is the yield point of the material. I think the weight of DSLR + Guiding will never approach the yield point of the material. This belongs to the topic of visco-elastic properties of materials - A very interesting topic to be further discussed.

Willis, all metals are ductile, ductilty in term of fabrication : dwelling, bending and cutting. All metals deform in a some sense under loading like a building structure and a bridge.

Dear SY. I know it. This is physics or material science. But when we talk about dutility, it means plastic deform. plastic defrom is not reversible. If we apply a small force to bend a metal bar, it will deform a little bit and restore the orginal shape afterward. This is elastic deform. However, if we apply a large enough force to bend a metal bar, it will deform plastically and will not restore to it's original shape (拗咗). This is plastic deform. In the case of telescope, the loading, say DSLR or guiding etc, will not make the telescope tube to undergoe plastic deform. So the stiffness is more relevant to the discussion. Stiffness is simply the resistance to deform. A stiff tube can avoid misalignment of the focus better.

[willis]

willis - It is the yield point of the material. I think the weight of DSLR + Guiding will never approach the yield point of the material. This belongs to the topic of visco-elastic properties of materials - A very interesting topic to be further discussed.

Willis, all metals are ductile, ductilty in term of fabrication : dwelling, bending and cutting. All metals deform in a some sense under loading like a building structure and a bridge.

Dear SY. I know it. This is physics or material science. But when we talk about dutility, it means plastic deform. plastic defrom is not reversible. If we apply a small force to bend a metal bar, it will deform a little bit and restore the orginal shape afterward. This is elastic deform. However, if we apply a large enough force to bend a metal bar, it will deform plastically and will not restore to it's original shape (拗咗). This is plastic deform. In the case of telescope, the loading, say DSLR or guiding etc, will not make the telescope tube to undergoe plastic deform. So the stiffness is more relevant to the discussion. Stiffness is simply the resistance to deform. A stiff tube can avoid misalignment of the focus better.

Yes! That's the yield point. Beyond which, there is plastic deform. Regarding carbon fiber tube, the cool down time between aluminum tube & carbon fiber tube will not be a great difference as the temperature difference in HK is small. In locations with great temperature difference, the white painted aluminum tube can have shorter cool down time compared with carbon fiber. Anyway, one cannot deny the fact that carbon fiber is luxurious. An exaggerated analogy is that a mechanical Rolex cannot beat quartz Casio in terms of accuracy. But everyone knows Rolex is the symbol of luxury, class and status.

[syyuen168]

Willis, some consumers wants to buy rolex because it is a rolex. Some consumers like to buy casio becasue it is a watch. Two different consumer behaviours here and there is no right or wrong here.

[willis]

Willis, some consumers wants to buy rolex because it is a rolex. Some consumers like to buy casio becasue it is a watch. Two different consumer behaviours here and there is no right or wrong here.

Couldn't agree any more!!! [爽] [爽] [爽] Certainly u will buy Rolex. I am a pragmatic guy. So I’ll buy Casio.

[鄧登凳]

Are u mixing up the problem?

Pressure from mounting elements usually causes some form of astigmatism, due to a typically radially symmetric distribution of the points of

We am not taking about the mounting elements (the lens or mirror holder)? We are talking about the tube...

No, I have not mixed up the problem. For most refractor telescopes, the objective lenses are mounted with three screws from the optical tube radially pointing towards the edges of the lenses. When the tube contract more than lenses in cold weather, the pressure on the lenses from the screws increases a lot causing pinched optics. That is why the link I quoted suggests to keep those screw loose at room temperature (but may cause other problems if they are too loose).

[鄧登凳]

Mr. Tang, I can not concur to this statement. Carbon Fibre composite is a polymeric composite and is organic in nature. How can an organic polymer be comparable in thermal conductivity than metal ? Maybe it is a sepcial type of reinforement polymeric composite. Can you elaborate more on your statement to fill me up with facts and figures ?

Unfortunately, Mr Anyone is out of town, otherwise he probably can confirm that diamond (a non-metallic substance) has a higher thermal conductivity than metals. Diamond has thermal conductivity more than twice that of silver and gold and nearly five times that of aluminun. That is how people test diamond!! So don't assume metals are the only type of materials with good thermal conductivity.

As I mentioned in my previous post, carbon fibre composites vary in compositions and properties, including their thermal conductivity. However, I can give you at least an example of one type with thermal conductivity much better than copper and aluminum.

Thornel® Continuous Carbon Fibers are continuous strand fibers designed for typical thermal management applications. These materials are used in applications where high performance and maximum weight reduction must be combined. Thanks to ultra-high longitudinal thermal conductivity, Thornel fibers demonstrate 2 to 3 times the performance of copper, and 4 to 5 times the performance of aluminum.

Moreover, it is not correct to say that top astrographic telescopes do not use carbon fibre. Orion Optics is one of the top manufacturer of astrographs in UK. Including one used by the famous Peter Shah (see: http://www.hkastroforum.net/viewtopic.p ... 8B&start=0). Their 8" to 16" astrograph newtonians are all made with carbon fibre. See: http://www.opticstar.com/Run/Astronomy/ ... 10_1_5_100

[willis]

No, I have not mixed up the problem. For most refractor telescopes, the objective lenses are mounted with three screws from the optical tube radially pointing towards the edges of the lenses. When the tube contract more than lenses in cold weather, the pressure on the lenses from the screws increases a lot causing pinched optics. That is why the link I quoted suggests to keep those screw loose at room temperature (but may cause other problems if they are too loose).

I don’t know what u mean by “most refractor”. I am afraid that is not the case. As far as I know my Sky-watcher Equinox 80ED & 120ED, Takahashi FS60C, Lunt LS35T’s lens are not mounted by what u said “three screws from the optical tube radially pointing towards the edges of the lenses”. My friend who once owned Sky90 and now APM said that there is no such construction. A few weeks ago, Xiaoer & I took off the lens of both Equinox 80ED & Maxvison 80ED for preparing the solar filter adaptor. There is no such “three screws from the optical tube radially pointing towards the edges of the lenses” in both telescopes. May be Sky-watcher, Takahashi, Lunt, Maxvision & APM are minority. [疑惑] As u said most refractor are “mounted with three screws from the optical tube radially pointing towards the edges of the lenses”, can u name some models?

As I mentioned in my previous reply that:
“One cannot simply look at the expansivity of the carbon fiber & aluminum and then make conclusion. Otherwise, finding macthing material to make telescope is simple. Even a form 5 student can do it. The mismatch causes focal shift. The tube material selected should be able to compensate the focal shift. In addition, carbon fiber are wove in carbon fiber composite, the expansivity is different in different direction. That is anisotropic. Therefore the geometry plays an important role. Clearly the problem cannot be easly solved theoretically, anisotropic problem. Even simple geometry, tensor is needed.”

The definition of expansivity is fractional increase in length per degree. That's why the unit is per degree Celsius.

Let consider a very simple geometry, a wire, virtually an 1D object. Suppose we have two aluminum wires of 10cm and 20cm respectively. They are undergoing same amount of temperature increase. If the expansion of the 10cm aluminum wire is 10um, what is the expansion of the 20cm aluminum wire? Obviously it is 20um. So in this simple case, the expansion depends on the original length of the wire. This discussion is also true for temperature decrease, but contraction instead of expansion. If we are considering a lens, an extended object, the situation is more complication. Since there is no material has infinity thermal conductivity and the lens is an extended object, expansion takes time. The expansion of the lens diameter would not be equal to the expansion of the thickness of the lens in general. This will result in changing curvature duration cool down. That’s why we need to refocus during cool down. There is no simple relationship between lens contraction to the shrinking of focal length. The ultimate task is to find a tube material which can compensate well with the shrinking of focal length. If the focal length shrink the same % as the contraction of the lens/glass, everything would be simple. BUT it is not the case. That’s why telescope makers, including Astrophysics’ Roland Christen, need to do experiments. They are not looking at the data book and then find out which materials are right choices.

[syyuen168]

My dear Mr. Tang,

I am not talking selling or buying pure diamonds here because I have no money to buy from sir anyone. I am talking about a composite material. Below are some basic organic chemistry concepts :-

I never assume that only metals have good thermal conductivity. You mix up carbon fibre and carbon fibre polymeric composites. Carbon fibre polymeric composites are NOT pure carbon fibre. It consists of mainly epoxy (A thermoset plastics) or other thermo plastics, carbon flbres and maybe other additives to further enhance the functional properties. Now I agree carbon in pure form (diamond is a form of carbon) has much better thermal conductivity than most metals. Epoxy is a thermoset polymer that has a low thermal conductivity of 0.59 W / (m-K) (in pure form). Now, if the main resin (we call that carrier resin) in the composite is EXOXY (maybe more than 70% or even more). How can you expect the thermal conductivity of the composite as good as the carbon (Diamond) in pure form? If you can pull out some facts and figures or research papers to support the conductivity of carbon fibre composite is better than aluminum or copper, let me know at once.

Why do I know all this? Because I study hard in my Form 2 chemistry in the old days and got a good grade in HKCE

Mr. Tang, I can not concur to this statement. Carbon Fibre composite is a polymeric composite and is organic in nature. How can an organic polymer be comparable in thermal conductivity than metal ? Maybe it is a sepcial type of reinforement polymeric composite. Can you elaborate more on your statement to fill me up with facts and figures ?

Unfortunately, Mr Anyone is out of town, otherwise he probably can confirm that diamond (a non-metallic substance) has a higher thermal conductivity than metals. Diamond has thermal conductivity more than twice that of silver and gold and nearly five times that of aluminun. That is how people test diamond!! So don't assume metals are the only type of materials with good thermal conductivity.

As I mentioned in my previous post, carbon fibre composites vary in compositions and properties, including their thermal conductivity. However, I can give you at least an example of one type with thermal conductivity much better than copper and aluminum.

Thornel® Continuous Carbon Fibers are continuous strand fibers designed for typical thermal management applications. These materials are used in applications where high performance and maximum weight reduction must be combined. Thanks to ultra-high longitudinal thermal conductivity, Thornel fibers demonstrate 2 to 3 times the performance of copper, and 4 to 5 times the performance of aluminum.

Moreover, it is not correct to say that top astrographic telescopes do not use carbon fibre. Orion Optics is one of the top manufacturer of astrographs in UK. Including one used by the famous Peter Shah (see: http://www.hkastroforum.net/viewtopic.p ... 8B&start=0). Their 8" to 16" astrograph newtonians are all made with carbon fibre. See: http://www.opticstar.com/Run/Astronomy/ ... 10_1_5_100

[鄧登凳]

My dear Mr. Tang,

I am not talking selling or buying pure diamonds here because I have no money to buy from sir anyone. I am talking about a composite material. Below are some basic organic chemistry concepts :-

I never assume that only metals have good thermal conductivity. You mix up carbon fibre and carbon fibre polymeric composites. Carbon fibre polymeric composites are NOT pure carbon fibre. It consists of mainly epoxy (A thermoset plastics) or other thermo plastics, carbon flbres and maybe other additives to further enhance the functional properties. Now I agree carbon in pure form (diamond is a form of carbon) has much better thermal conductivity than most metals. Epoxy is a thermoset polymer that has a low thermal conductivity of 0.59 W / (m-K) (in pure form). Now, if the main resin (we call that carrier resin) in the composite is EXOXY (maybe more than 70% or even more). How can you expect the thermal conductivity of the composite as good as the carbon (Diamond) in pure form? If you can pull out some facts and figures or research papers to support the conductivity of carbon fibre composite is better than aluminum or copper, let me know at once.

Why do I know all this? Because I study hard in my Form 2 chemistry in the old days and got a good grade in HKCE

I am sure you can afford many good diamonds form Sir Anyone. However, the reason I quoted the case of diamond is that carbon fibre in its pure form is a relative of diamond. Actually pure carbon fibre has thermal conductivity two or three times that of diamonds so it is 10 to 15 times that of aluminium.

Of course composites containing carbon fibre cannot reach that 10X or 15X of aluminum. However, composites have never lost all the properties of one of its major components. The link that I provided contains the following information:

Composites made from ThermalGraph® discontinuous fibers offer several advantages over most other materials including thermoplastic and thermoset resins.
Higher thermal conductivity than possible with other particulate fillers such as metals or ceramics

How high can the thermal conductivity of these composite (not pure fibre but the actual plastic) depends on a lot of factors. Just give a paper which is published in 2002: http://www.springerlink.com/content/mve ... lltext.pdf. As shown by the following chart extracted from the paper, the composites containing up to 45% carbon fibre have thermal conductivity of 60 to 80 W/mK, which is comparable to some types of steel. Noting that currently there are carbon fibre composites with 70% carbon fibre, the composites can conduct heat better than uncoated steel. So it cannot be far off from coated aluminum.

[xiaoer]

I don’t know what u mean by “most refractor”. I am afraid that is not the case. As far as I know my Sky-watcher Equinox 80ED & 120ED, Takahashi FS60C, Lunt LS35T’s lens are not mounted by what u said “three screws from the optical tube radially pointing towards the edges of the lenses”. My friend who once owned Sky90 and now APM said that there is no such construction. A few weeks ago, Xiaoer & I took off the lens of both Equinox 80ED & Maxvison 80ED for preparing the solar filter adaptor. There is no such “three screws from the optical tube radially pointing towards the edges of the lenses” in both telescopes. May be Sky-watcher, Takahashi, Lunt, Maxvision & APM are minority. [疑惑] As u said most refractor are “mounted with three screws from the optical tube radially pointing towards the edges of the lenses”, can u name some models?

哈哈，係呀，上次偶一不慎拆左maxvision 的主鏡出黎，幾片玻璃完全冇screw上著的，玻璃之間只有一金屬圈圈隔開(air spacing)。

幾年前讀過的材料科學好多都俾返老師，又懶得查書，科學就無謂在這裡亂講了，只是市面上的折射鏡甚少用CF composite,我估點都唔係價錢同技術o既問題掛，因為呢D在貴價鏡市場都唔係問題！乜野材料做鏡筒最好，好似好多因素要考慮，我估最尾點都要by trial and error搵出黎。淨係焦距點隨溫度改變我估都未必係線性關係。

[willis]

哈哈，係呀，上次偶一不慎拆左maxvision 的主鏡出黎，幾片玻璃完全冇screw上著的，玻璃之間只有一金屬圈圈隔開(air spacing)。

幾年前讀過的材料科學好多都俾返老師，又懶得查書，科學就無謂在這裡亂講了，只是市面上的折射鏡甚少用CF composite,我估點都唔係價錢同技術o既問題掛，因為呢D在貴價鏡市場都唔係問題！乜野材料做鏡筒最好，好似好多因素要考慮，我估最尾點都要by trial and error搵出黎。淨係焦距點隨溫度改變我估都未必係線性關係。

Ur Maxvision is a triplet. It has 3 pieces of lens. Yes! "by trial and error" means repeated experiments. As u pointed out that the relationship is may not linear, 未必係線性關係, do experiments is an easier way to find the suitable materials.

[鄧登凳]

I don’t know what u mean by “most refractor”. I am afraid that is not the case. As far as I know my Sky-watcher Equinox 80ED & 120ED, Takahashi FS60C, Lunt LS35T’s lens are not mounted by what u said “three screws from the optical tube radially pointing towards the edges of the lenses”. My friend who once owned Sky90 and now APM said that there is no such construction. A few weeks ago, Xiaoer & I took off the lens of both Equinox 80ED & Maxvison 80ED for preparing the solar filter adaptor. There is no such “three screws from the optical tube radially pointing towards the edges of the lenses” in both telescopes. May be Sky-watcher, Takahashi, Lunt, Maxvision & APM are minority. [疑惑] As u said most refractor are “mounted with three screws from the optical tube radially pointing towards the edges of the lenses”, can u name some models?

哈哈，係呀，上次偶一不慎拆左maxvision 的主鏡出黎，幾片玻璃完全冇screw上著的，玻璃之間只有一金屬圈圈隔開(air spacing)。

幾年前讀過的材料科學好多都俾返老師，又懶得查書，科學就無謂在這裡亂講了，只是市面上的折射鏡甚少用CF composite,我估點都唔係價錢同技術o既問題掛，因為呢D在貴價鏡市場都唔係問題！乜野材料做鏡筒最好，好似好多因素要考慮，我估最尾點都要by trial and error搵出黎。淨係焦距點隨溫度改變我估都未必係線性關係。

我沒有那麼多可以任我折開的高級折射鏡

用碳纖做鏡筒的折射鏡, 也有不少是實驗成功的 , 包括Stellarvue Raptor series (70 ED doublet 至 130 ED triplet), A&M (見http://www.opticstar.com/download/Astro ... atalog.pdf), William Optics, 還有 TMB 的TMB-80CF (見http://www.astronomics.com/main/product ... id/TMB80CF)

三口螺絲設計的例子, 網上的圖來自Explore Scientific 127 triplet ED:
, 顯示用"夾"鏡的螺調光軸。

網上資料顯示Skywatcher 80ED是沒有這個設計的, 但120ED似乎是有的。

附圖而TMB似乎是有同樣的設計, 圖片來自TMB 92 signature series的手冊http://www.astronomics.com/main/documen ... manual.pdf[/url]

[willis]

Pls note that the topic of this thread is “CF tube vs Aluminum Tube”. U drag around and keep on taking about mounting of lens”… Anyway, I am sorry that u may mix up again.

1. From the TMB photo u showed, the screws are used for fixing and collimating the lens cell only. (Lens cell is used for carrying the lens) One can clearly see that the direction of the screws are pointing towards the end of the telescope, not the one u mentioned “three screws from the optical tube radially pointing towards the edges of the lenses”.

2. What do u mean by "碳纖做鏡筒的折射鏡, 也有不少是實驗成功的"? Do u mean as we have carbon fiber tube products, so carbon fiber tube telescope are successful? If this logic is correct, I guess aluminum tube is more sucessful becasue we have lot more aluminum tube products.

3. From the discussion of this thread, the expansivity of different material tubes, cool down etc, now u drag to mounting of the lens. If the mounting of the lens is more important than the tube material on the contribution of cool down time, those telescope makers should do more research on the lens cell or mounting technology instead of the tube material. I really do not get ur point. Anway, I don't need to get ur point. But u can e-mail Roland Christen as he said:

"an ED/Fluorite refractor lens moves inward a significant amount with dropping temperatures. This is partly compensated in an aluminum tube which also contracts with falling temperatures. However, as already stated, a carbon fiber tube will not change length with falling temperatures. The resulting shift in focus is not enough to bother visual observing, but not good for astrophotography where focusing is more critical. "

Give him more suggestions. u can contact him via Astrophysics homepage.

4. In ur previous reply u said that "I cannot concur with your arguement." by listing the expansivities and saying:

"Pyrex glass has a thermal expansion coefficient of 4.9 X 10^-6 /℃; aluminum 22.2 X 10^-6 /℃, so the difference between Pyrex glass and carbon fibre composite is 4.9, and that between pyrex glass and aluminum is 17.3."

and

"Aluminum optical tubes, with three time the expansion coefficient difference than carbon fibre OTA are much more prone to this problem. "

Do u insist on this now? If the explaination/theory doesn't fit the fact, change the explaination/theory to fit the fact. OR change the fact to fit the explaination/theory. u think about it. I pointed out already that one cannot simply look at the expansivities and then conclusion. If that is the case, this is a F5 level question. An experimental approach is easier way and ineeded telescope makers do experiments.

5. What I learn from this forum is that Equinox is not class 1 telescope and it is China made. And I only got one class 1 telescope Tak FS60C. So I don't have many prestigious telescope as u said "我沒有那麼多可以任我折開的高級折射鏡". But I learn a lot from other Tak & APM owners. Whenever there is chance, I go & see. So I gain experience.

6. You are very hard working. In order to show yourself is correct, “For most refractor telescopes, the objective lenses are mounted with three screws from the optical tube radially pointing towards the edges of the lenses.”. u found a few photos to prove this. However, it turn out to be…. Are u insist on saying “most refractor…”?

Finally, although I am not a scientist, I am serious about science. I quoted Roland Christen's finding and I agree with him because it sounds scientific. If u find his finding is wrong, the best way is to contact him. If ur suggestion is right and constructive, I think he will be more than happy to know it and change the design afterward. This is all I can say about the topic. I guess forum members who has read this thread had the conclusion already...