CF Tube vs Aluminum Tube

[syyuen168]

The following thread talks about the reason why large scope do not use carbon fiber tube. There must be some reasons why the industry folks do not use large CF tubes. Sure, everybody wants light weight but may not be practical in terms of inherent problems like, thermal expansion between glass and CF, ductility, experience to handle large CF tube and so on.

http://www.cloudynights.com/ubbthreads/ ... ll/fpart/1

[Subaru]

The following thread talks about the reason why large scope do not use carbon fiber tube. There must be some reasons why the industry folks do not use large CF tubes. Sure, everybody wants light weight but may not be practical in terms of inherent problems like, thermal expansion between glass and CF, ductility, experience to handle large CF tube and so on.

http://www.cloudynights.com/ubbthreads/ ... ll/fpart/1

It is just some talkings, without measurement (focus shift compensation of metallic tube and lens). My experience of imaging under freezing temperature is, re-focus was indeed needed for most of the telecopes and tele-lens during temperature change (let's say 10 degrees), unless the telescope is made and tested with temperature compensation by design.

Nevertheless, I think both Aluminium, glass fibre and CF tube are all suitable materials for telescope tube.

[willis]

The following thread talks about the reason why large scope do not use carbon fiber tube. There must be some reasons why the industry folks do not use large CF tubes. Sure, everybody wants light weight but may not be practical in terms of inherent problems like, thermal expansion between glass and CF, ductility, experience to handle large CF tube and so on.

http://www.cloudynights.com/ubbthreads/ ... ll/fpart/1

The guy, PJ Anway, in the link u gave, has answered the question.

"One reason given by Roland Christen on another site was the thermal characteristics. 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. "

Roland Christen is the Boss of Astrophysics. Carbon fibre's expansivity cannot compensate the expansivity of the lens. But alumnum does compansate better. Although the tensile stress of carbon fibre is high, but it is not very stiff. That's why no pure carbon fibre is used, but in a form of composite material. Don't think carbon fibre can reduce the weight of the telescope a lot. It is becasue there are still other parts like tube ring, mounts, dovetail, focuser, the OTA tube is not a significant part in terms of weight. This is a common misconception that carbon fibre telescope is much lighter than that of aluminum version. In fact the weight of carbon fibre version and the weight of alumnium version are simliar. See the attached links:

Ordinary C8 (12.5 lb)
http://www.telescopes.com/telescopes/op ... 20c8%20ota

Carbon C8 (12.5 lb)
http://www.telescopes.com/telescopes/ca ... atible.cfm

Another not exact example (Both are using LOMO 80/480 lens, but different telescope makers)
APM/LOMO 80/480 (6.05 lb)
http://www.apm-telescopes.de/super-apo- ... 72637.html

A&M/TMB 80/480 (2.6 kg = 5.72 lb)
http://www.opticstar.com/Run/Astronomy/ ... _10_1_2_30

In addition, if the aluminum is painted in white, the cool down time is lesser. In HK the temp. difference between day & night is small, usually less than 10 degress, the cool down time may be about 1 hr. The material of the tube doesn't affect much. However, in some locations the temp. differenece is large, the cool down time may be as long as several hours. The material of the tube becomes important. Carbon fibre is light & expensive. It is for for some application, say racing car or racket etc, but it is a gimmick in telescope making. White painted aluminum is a cheap yet good telescope material.

PS* As far as I know there is another material made by Germany, but I fogot the name, which is light and the expansivity can compensate the lens expansivity better than aluminum...

[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...

[willis]

The reason why carbon fiber is not a good material to build telescope tube compared with aluminum or other material is not weight, but expansivity. I guess most amateur know that carbon fiber is light weight and tough, perhaps luxurious. This gives an image of carbon fiber is the Holy Grail for all applications. As one can see that most leading telescope makers, say Takahashi, APM, Astrophysics, TEC etc, they seldom or don’t use carbon fiber as tube material. Even for big reflector, if you check those national grade observatory or research grade reflectors in universities, it is hardly find carbon fiber tube reflectors. I think most of the opticians in those leading telescope makers are experience in building telescopes. They all know the reasons behind. I don’t think they haven’t done any engineering or calculation about the carbon fiber tube. As a layman like me, just look at their products, it is hardly from carbon fiber tube. When you talk about a really big telescope, the most significant cost comes from the lens or mirrors not the tube. Try to think about refractors of bigger than 10” or reflectors bigger than 16”, the tube is cheap when compared with the lens or mirror. In fact there is few telescope makers use carbon fiber tube. But the trade-off is that they limit their glass choices. As carbon fiber has nearly zero expansivity, in order to lower the cool down time, one needs to use so-called zero expansion glass. (Zero expansion doesn’t really mean zero, but close to zero) There are only few choices, say Zerodur or quartz. (Note that fluorite, ED, float glass and pyrex are not zero expansion glass.) Or may be u can dig out some more in the Internet. But these zero expansion glasses do not have good dispersion properties to make good refractor lens. For reflector, the problem is relatively mirror. The famous reflector mirror maker, Zambuto, she uses Corning #7740 Pyrex, which is not zero expansion glass. http://zambutomirrors.com/mirrors.html Anyway, this is the one I find which use carbon fiber tube. Note they use Zerodur.
http://www.atscope.com.au/rcos.html
Anyway, one can insists to use carbon fiber to build telescope tube and at the same time use good glass to make the lens. However, the cool down time will be…
So aluminum is cheap yet gives good cool down time material to make telescope tube.

PS* The light weight & match the expansivity material produced by Germany I mentioned in the previous reply is called Kruppax.

[syyuen168]

Besides the thermal expansion issue, the ductility is also another major issue. Aluminum is much more ductile than carbon fibre thus, post fabrications become much flexible.

[Eddy So]

Besides the thermal expansion issue, the ductility is also another major issue. Aluminum is much more ductile than carbon fibre thus, post fabrications become much flexible.

袁兄：請問用那一個牌號的鋁合金會比較適宜做鏡筒？

[willis]

Besides the thermal expansion issue, the ductility is also another major issue. Aluminum is much more ductile than carbon fibre thus, post fabrications become much flexible.

I think we should consider stiffness instead of ductility.
This is the definitiono of stiffness from http://www.answers.com/topic/stiffness-3:
The ratio of the force applied to a structure (or a structural element) to the corresponding displacement.

This is the definition of dutility from http://www.answers.com/topic/ductility
(materials) The ability of a material to be plastically deformed by elongation, without fracture.

We don't expect to make the telescope tube to have plastic deform. But we expect the telescope tube does not deform under heavy loading, say a heavy guide scope and DSLR are mount on it. That is how hard is the tube, stiffness. In layman term is "hardness". Pure carbon fiber is not very stiff, but with high tensile stress. As I mentioned in the previous reply they are in form of composite material. If I remember correctly, the carbon fiber is embeded in resin in order to make it stiff.

In addition, if cabon fiber is used in the reflector, a zero expansion glass should be used in order to produce good thermal response reflector. (i.e. zero expansion mirror + zero expansion tube with precise structure design) The resultant product would have good thermal properties. But I guess the price would be sky high. Here I assume zero expansion glass has good homogeneity, which I am not sure about it.

[鄧登凳]

The reason why carbon fiber is not a good material to build telescope tube compared with aluminum or other material is not weight, but expansivity. I guess most amateur know that carbon fiber is light weight and tough, perhaps luxurious. This gives an image of carbon fiber is the Holy Grail for all applications. As one can see that most leading telescope makers, say Takahashi, APM, Astrophysics, TEC etc, they seldom or don’t use carbon fiber as tube material. Even for big reflector, if you check those national grade observatory or research grade reflectors in universities, it is hardly find carbon fiber tube reflectors. I think most of the opticians in those leading telescope makers are experience in building telescopes. They all know the reasons behind. I don’t think they haven’t done any engineering or calculation about the carbon fiber tube. As a layman like me, just look at their products, it is hardly from carbon fiber tube. When you talk about a really big telescope, the most significant cost comes from the lens or mirrors not the tube. Try to think about refractors of bigger than 10” or reflectors bigger than 16”, the tube is cheap when compared with the lens or mirror. In fact there is few telescope makers use carbon fiber tube. But the trade-off is that they limit their glass choices. As carbon fiber has nearly zero expansivity, in order to lower the cool down time, one needs to use so-called zero expansion glass. (Zero expansion doesn’t really mean zero, but close to zero) There are only few choices, say Zerodur or quartz. (Note that fluorite, ED, float glass and pyrex are not zero expansion glass.) Or may be u can dig out some more in the Internet. But these zero expansion glasses do not have good dispersion properties to make good refractor lens. For reflector, the problem is relatively mirror. The famous reflector mirror maker, Zambuto, she uses Corning #7740 Pyrex, which is not zero expansion glass. http://zambutomirrors.com/mirrors.html Anyway, this is the one I find which use carbon fiber tube. Note they use Zerodur.
http://www.atscope.com.au/rcos.html
Anyway, one can insists to use carbon fiber to build telescope tube and at the same time use good glass to make the lens. However, the cool down time will be…
So aluminum is cheap yet gives good cool down time material to make telescope tube.

PS* The light weight & match the expansivity material produced by Germany I mentioned in the previous reply is called Kruppax.

With due respect, I cannot concur with your arguement.

To begin, the properties of carbon fibre reinforced polymers (composite) may vary depending on the type of carbon wire, the type of woven carbon fabrics, the polymer used and the thermal processing. In general, it is true to say

The most important class of properties of carbon-carbon composites is their thermal properties. C-C composites have very low thermal expansion coefficients, making them dimensionally stable at a wide range of temperatures, and they have high thermal conductivity.

With such property, carbon fibre goes better with glass than aluminium. Let’s for argument sake consider a carbon fibre composite with zero coefficient of thermal expansion, 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. The difference in expansion is what will cause problems in telescopes. One of the problems is pinched optics because of more contraction of the tube as compared with the lens when the telescope cools down.

Pressure from mounting elements usually causes some form of astigmatism, due to a typically radially symmetric distribution of the points of support and/or retaining. Typical pinching pressure, for instance, induces trefoil - a three-winged form of astigmatism, quickly revealing itself in the appearance of diffraction pattern. Mounting pressure can result from thermal expansion of optical elements and/or mechanical structure, which is one more reason why optical elements should be left slightly loose within mechanical structures holding them.

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

As mentioned in the above quote from the Materials Science and Engineering Department of the Michigan Technological University, carbon-fibre composites have high thermal conductivity (quite comparable to metals), there cannot be cooling down problems in any sense more serious than an aluminum telescope.

In short, thermal expansion coefficients and thermal conductivities are NOT reasons to prefer aluminum over carbon-fibre composites in making OTAs.

[willis]

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. An engineering approach is a quick way to find the answer. That is do experiemnt. I think the boss of Astrophysics, Roland Christen, did a lot of experiments and come to the conclusion. Otherwise he would not make such conslusion. See the link in CN. Remember this is not my conclusion, though I agree with it. Similar observations found in telescope makers. That's why a Germany company invented Kruppas. If ur discusion is correct, how to explain Roland Christen's experiment observation. The point is how to overcome the focal shift. May be u can sent an e-mail to Roland Christen and ask for more experimental detail.

Lastly, if
"In short, thermal expansion coefficients and thermal conductivities are NOT reasons to prefer aluminum over carbon-fibre composites in making OTAs."

is correct, then

"In short, thermal expansion coefficients and thermal conductivities are NOT reasons to prefer carbon-fibre composites over aluminum in making OTAs."

is also true.

[willis]

Pressure from mounting elements usually causes some form of astigmatism, due to a typically radially symmetric distribution of the points of support and/or retaining. Typical pinching pressure, for instance, induces trefoil - a three-winged form of astigmatism, quickly revealing itself in the appearance of diffraction pattern. Mounting pressure can result from thermal expansion of optical elements and/or mechanical structure, which is one more reason why optical elements should be left slightly loose within mechanical structures holding them.

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

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...

[Chanlunlun]

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

[syyuen168]

Eddy, off my head, I would say 7075 is a suitable alloy for the tube. Reasons are : hard and readily available.

Besides the thermal expansion issue, the ductility is also another major issue. Aluminum is much more ductile than carbon fibre thus, post fabrications become much flexible.

袁兄：請問用那一個牌號的鋁合金會比較適宜做鏡筒？

[syyuen168]

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.

Besides the thermal expansion issue, the ductility is also another major issue. Aluminum is much more ductile than carbon fibre thus, post fabrications become much flexible.

I think we should consider stiffness instead of ductility.
This is the definitiono of stiffness from http://www.answers.com/topic/stiffness-3:
The ratio of the force applied to a structure (or a structural element) to the corresponding displacement.

This is the definition of dutility from http://www.answers.com/topic/ductility
(materials) The ability of a material to be plastically deformed by elongation, without fracture.

We don't expect to make the telescope tube to have plastic deform. But we expect the telescope tube does not deform under heavy loading, say a heavy guide scope and DSLR are mount on it. That is how hard is the tube, stiffness. In layman term is "hardness". Pure carbon fiber is not very stiff, but with high tensile stress. As I mentioned in the previous reply they are in form of composite material. If I remember correctly, the carbon fiber is embeded in resin in order to make it stiff.

In addition, if cabon fiber is used in the reflector, a zero expansion glass should be used in order to produce good thermal response reflector. (i.e. zero expansion mirror + zero expansion tube with precise structure design) The resultant product would have good thermal properties. But I guess the price would be sky high. Here I assume zero expansion glass has good homogeneity, which I am not sure about it.

[syyuen168]

The reason why carbon fiber is not a good material to build telescope tube compared with aluminum or other material is not weight, but expansivity. I guess most amateur know that carbon fiber is light weight and tough, perhaps luxurious. This gives an image of carbon fiber is the Holy Grail for all applications. As one can see that most leading telescope makers, say Takahashi, APM, Astrophysics, TEC etc, they seldom or don’t use carbon fiber as tube material. Even for big reflector, if you check those national grade observatory or research grade reflectors in universities, it is hardly find carbon fiber tube reflectors. I think most of the opticians in those leading telescope makers are experience in building telescopes. They all know the reasons behind. I don’t think they haven’t done any engineering or calculation about the carbon fiber tube. As a layman like me, just look at their products, it is hardly from carbon fiber tube. When you talk about a really big telescope, the most significant cost comes from the lens or mirrors not the tube. Try to think about refractors of bigger than 10” or reflectors bigger than 16”, the tube is cheap when compared with the lens or mirror. In fact there is few telescope makers use carbon fiber tube. But the trade-off is that they limit their glass choices. As carbon fiber has nearly zero expansivity, in order to lower the cool down time, one needs to use so-called zero expansion glass. (Zero expansion doesn’t really mean zero, but close to zero) There are only few choices, say Zerodur or quartz. (Note that fluorite, ED, float glass and pyrex are not zero expansion glass.) Or may be u can dig out some more in the Internet. But these zero expansion glasses do not have good dispersion properties to make good refractor lens. For reflector, the problem is relatively mirror. The famous reflector mirror maker, Zambuto, she uses Corning #7740 Pyrex, which is not zero expansion glass. http://zambutomirrors.com/mirrors.html Anyway, this is the one I find which use carbon fiber tube. Note they use Zerodur.
http://www.atscope.com.au/rcos.html
Anyway, one can insists to use carbon fiber to build telescope tube and at the same time use good glass to make the lens. However, the cool down time will be…
So aluminum is cheap yet gives good cool down time material to make telescope tube.

PS* The light weight & match the expansivity material produced by Germany I mentioned in the previous reply is called Kruppax.

With due respect, I cannot concur with your arguement.

To begin, the properties of carbon fibre reinforced polymers (composite) may vary depending on the type of carbon wire, the type of woven carbon fabrics, the polymer used and the thermal processing. In general, it is true to say

The most important class of properties of carbon-carbon composites is their thermal properties. C-C composites have very low thermal expansion coefficients, making them dimensionally stable at a wide range of temperatures, and they have high thermal conductivity.

With such property, carbon fibre goes better with glass than aluminium. Let’s for argument sake consider a carbon fibre composite with zero coefficient of thermal expansion, 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. The difference in expansion is what will cause problems in telescopes. One of the problems is pinched optics because of more contraction of the tube as compared with the lens when the telescope cools down.

Pressure from mounting elements usually causes some form of astigmatism, due to a typically radially symmetric distribution of the points of support and/or retaining. Typical pinching pressure, for instance, induces trefoil - a three-winged form of astigmatism, quickly revealing itself in the appearance of diffraction pattern. Mounting pressure can result from thermal expansion of optical elements and/or mechanical structure, which is one more reason why optical elements should be left slightly loose within mechanical structures holding them.

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

As mentioned in the above quote from the Materials Science and Engineering Department of the Michigan Technological University, carbon-fibre composites have high thermal conductivity (quite comparable to metals), there cannot be cooling down problems in any sense more serious than an aluminum telescope.

In short, thermal expansion coefficients and thermal conductivities are NOT reasons to prefer aluminum over carbon-fibre composites in making OTAs.

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 ?