CF Tube vs Aluminum Tube

[syyuen168]

My dear Mr. Tang,

The conductivity of aluminum in pure form is about 240 W (m-K) and 120 -200 W (m-K) in its alloys. As indicates in your graph, the conductivity of modified carbon fibre is only 50 – 80 W (m-K) which is still far behind the conductivity of aluminum. Can you produce a carbon fibre composite better than aluminum in thermal conductivity? I assume so but at what cost? It can be done by various means. Of my head, nano aluminum particles can be embedded by using a coupling agent into the structure of the carrier resin is a way. The other way maybe a kind of “core and shell” structure which can turn the structure more conductive.

Now, we are all living in a real world. The cost of doing this maybe too high, not practical, and/or the technology may not be mature enough for the industry to invest in it. So, come back to square one, why not using aluminum, which is cheap, readily available and a super ductile metal of all?

For the question CF tube in Orion website, how successful do you know their products in the market? A company pushing a technology into the market does not mean it is great and superb. My dear friend, what we need is to listen what the market says not what the individual company bluffs about their products. If Orion is successful in using carbon fibre tube, why do APM, TV and Takahashi not follow and copy quick for their large tubes? Why do these companies still using aluminum tubes? These companies have been around for long and the way they produce their products reflects lot of successful as well as bad experiences behind the doors. Again, we are all living in a real world. We do not trust entirely what the internet says and we do not trust entirely what the producers say. What we need to trust is our sense to judge what the mind says.

Still not enough. No problem. Come back again for more technical as well as commercial discussions.

[sn-10]

"We do not trust entirely what the internet says and we do not trust entirely what the producers say."



Most knowledges can be found on the internet, but it is not your knowledge until you have really digested it. Being a searcher is easier than being a real scholar.

[willis]

Now, we are all living in a real world. The cost of doing this maybe too high, not practical, and/or the technology may not be mature enough for the industry to invest in it. So, come back to square one, why not using aluminum, which is cheap, readily available and a super ductile metal of all?

Dear SY, I agree with u that we are living in a real word & the cost really matters. One info. from boss of APM,Markus Ludes. that Titanium is also a good choice of material in terms of thermal propertise and mechanical propertise.

The weight reduction with titanium len cell on 8" Apo is 1 kg only, increased price cost was 3 years ago about US $ 3,000, the 228 mm apo and bigger using already Titanium Cells , 180 mm and 203 mm modells using already special alloy material.

So the price is outrageous for just the Titanium lens cell only!!! Therefore we don't even have Titanium tube in the market. The price will be scary!!! That's why APM used Kruppax which is lighter, better thermal propertise and cheaper.

Regards,
Willis

[鄧登凳]

Dear Willis,

Sorry that I got the wrong photo for the TMB lens cell, the following should be the one (from Grandeye website), the radial screws can be easily seen.



I may be wrong in using the word "most" as that some but not most use screws to control the position of the lens as well as the radial pressure exerted by the telescope tube or lens cell on the lens, other design simply use non-adjustable clips or even glue. However, not using screws does not remove the problem of excessive pressure on the lens when the tube/cell contract significantly more than lens.

This quote has been provided on the first page of this thread.

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.

Dear Mr Yuen,

I think that we have come to consensus that carbon-fibre composites can be made to be good conductors of heat. Actually, there is a commercial availble carbon-fibre reinforced polymer that can conduct electricity, which by this very nature I speculate (no data available) it to be reasonable conductor of heat as well, although it is currently very expensive.

Please note that Orion Optics, a UK based company and respectable astrograph newtonians maker, is different from the USA based Orion telescope. It is not a mass producer but its optics are highly valued and used by some top astrophotographers.

The fact that some long established telescopes makers are not following suit is morely likely a result of resistant to change. Just consider the fact that Takahashi is still insisting on the production of wooden tripods for its equatorial mounts and that few equatorial mount makers venture into the use of carbon-fibre composites for their tipods and new alloys (more durable and less weight) for their gears.

Regards to both,
Tang

[willis]

Dear Mr. Tang,
There is no doubt some refractor’s lens are mounted by the 3 crews method u mentioned. I agree that there is some pressure exerted on the lens by the screws. However, the most important factor that contributes to the focal shift is the mismatch of expansion of the lens and the tube. Otherwise telescope makers should put most of the effort on lens mounting or the lens cell design. From the thread provided by SY from the very beginning has already showed the idea of Roland Christen, the Boss of Astrophysics, that this is due to the mismatch of the expansion of lens & tube. He pointed out that aluminum tube does the job better than carbon fiber tube. In this forum, all the members are educated & I believed we all got sufficient science knowledge to do judgment. There is lots of information in the Internet. Some are facts, some are opinions, some are theories, some are lies and some are garbage etc. One needs to be open-minded and critical enough to sort out the answer. Sometimes, we never know it is true or not. But if we are open-minded, careful, critical & knowledgeable enough, we may at least find out the relatively reliable answers. I have presented what I believe is that from cool down consideration, aluminum tube is better than carbon fiber tube.

Today, I found an amazing coincidence! In HK, we are hot in discussing the carbon fiber tube and aluminum tube. In Cloudy Nights, they are hot in discussing the same topic! Look the day of the thread (The staring day is 26/1/2010)!
http://www.cloudynights.com/ubbthreads/ ... art/1/vc/1
One of the participant is Markus Ludes, the boss of APM. If u read the thread, we can find this:

Hello All

I want to speak some general words about Carbonfiber tubes used on Refractors , not on this product just in general. I hope its not against TOS.

These days if a manufactur install a big focuser and offers a flattner he has imaging in mind, right ?

Lenses from any manufactur around the world are doing all the same thing , the focuse is shifting when temperature changes.

Now a manufactur who thinks a bit and use his brain a bit when he build a telescope , he want to compensate such focuse shift. To compensate the focuse shift of a lens , you want to use a tube material which expands or schrinks in opposite direction then your lens does, to get as close as possible to 0 shift of the system. Depends on the lens design either aluminum tubes , or like our Krüpax50 tubes doing a dam good job.

Carbon fiber is known and used either as tubes or Rods to keep Sital and Zerodur Mirrortelescopes at zero shift. Sital and Zerodur do not have a focuse shift , so such mirror scopes need a tube material with zero expansion , like Carbonfiber

In a Lens Telescope Carbonfiber is the worsest material you can use , it delivers to the imagers to biggets possible focuse shift.

Therefore I am asking myself who came up with the Idea to build Refractors with Carbon Fiber and why ?

Weight ? forget the weight issue on the normal size refractors. the main tube and dew shield , made out of thin alu or our material does not really weight more or much more then Carbonfiber.

we have compared weights of carbonfiber optical tubes aganst alu made tubes and either they have been equal , sometimes the alu tube was even less heavy.

Would be nice to find out what manufacturs , who building carbonfiber refractors thinking

--------------------
clear skies

Markus Ludes
anfrage@apm-telescopes.com

This means Markus Ludes, got the same conclusion as Roland Christen that aluminum tube balance better than carbon fiber tube in terms of cool down time.
In the thread, there are some carbon fiber tube owners said there is no cool down problem. However, there are some carbon fiber tube owner said the cool down in carbon fiber tube is bad. Here is an example of a carbon fiber C8 (Again u can find this in the thread):

Markus,

I agree with everything you said. Additionally, cooldown
is an issue. I remember the C8 I owned with a carbon fiber
tube. It took forever to cooldown, especially if it had
been out in the hot sun before use. I find aluminum works
quite well on the AP180EDT for imaging. I've been able to
keep good focus over a 2 to 3 hour series of exposures
without having to refocus, as long as the temperature drop
is not extreme. I look at carbon fiber as more of a
marketing issue than anything useful. Others may feel
differently.

Tom

So which one is better? Aluminum tube or carbon fiber tube? We have both side!!! In Markus Ludes’ second reply, he pointed out that this is significant when one does astrophotography, especially long time exposure! Markus Ludes & Roland Christen arrived the same conclusion. Markus is a German & Roland is American. Markus is from APM & Roland is from Astrophysics. This means two independent leading telescope makers arrived the same conclusion! In this situation, I guess aluminum tube gives better cool down time than carbon fiber tube is highly possible true. Here is Markus Ludes’ 2nd reply:

Hi Tom

just this time , my new optical designer, Massimo Riccardi , me and LZOS are in the stage of designing the tubes for our new 4-element Super Apo Petzval Astrographs and Massimo give extrem greatr input for building the cell and the tube combination to decrease the focuse shift as much as possible.

Its not a big issue for visuel observers and those who image with a webcam or a digital camera for a few minutes, bu its a big issue for those who doing long time exporsure imaging and we have to take care on them.

--------------------
clear skies

Markus Ludes
anfrage@apm-telescopes.com
www.apm-telescopes.de
20 year anniversary of APM 1990-2010

I trust Roland Christen & Markus Ludes more than other amateurs, since they are technical people of high reputation in the field. If they say something wrong or irresponsible in public, they just risk their own reputation and their companies will be… So after gathering the information from Internet & analysis, I would say I opt for aluminum tube instead of carbon fiber tube.
Regards,
Willis

[鄧登凳]

Dear Willis,

Thank you for passing on the information.

I think that we talked about different things. I suggested that the significantly larger expansion coefficient of aluminum than glass would likely cause stress on the optics (both mirrors and lenses) and you now limit the discussion on the focus shift issue for astrophotographic refractors (dropping the talk about pyrex mirror etc).

Mr syyuen168 started this thread to discuss "why the industry folks do not use large CF tubes".

My response was that thermal expansion and conductivity should not be the issue. I gave, initially, an example of a top UK based astrographic newtonian maker "Orion Optics" as an example of large tubes 8" to 16" using carbon fibre composites.

To give a further example of large scopes (as Mirror God Mr Chan argues that nothing less than 20" can be large), Orion Telescopes & Binoculars (of USA) produces their Monster Dobsonians 36" to 50". These scopes qualified as large ones use carbon fibre struss tubes.

At least, Markus Ludes agrees with making large newtonian with carbon fibre material in saying "so such mirror scopes need a tube material with zero expansion , like Carbonfiber".

So up to this point, I have not seen any valid argument against making large mirror based telescopes using carbon fibre.

Let's us for the moment put aside the issue whether commercial refractors qualify as large scopes. I think that the so called focus shift issue contains a lot of talk and no hard data.

Very early on in this thread, Mr Subaru indicated that focus shift is present in most telescopes and tele-lenses. As far as I know, tele-lenses are made with aluminum. So that does not eliminate focus shift. Mr Subaru is one of the top astrophotographers in Hong Kong and I trust his experience.

Astrophysics seemingly is claiming that aluminum will do the trick. Does APM agree on that? No, at least Mr Fong (APM HK dealer) does not think so (see http://hk.myblog.yahoo.com/galaxyscient ... le?mid=238 "APM的APO折射鏡，100mm口徑或以上的鏡筒，均採用Kruppax 50物料製造，其特性是頗能抵消物鏡因為溫度變化而造成的焦距差，但鋁製鏡筒則在不同溫下受到一定因素影響").

As I mentioned, Stallarvue Raptors are refractors with carbon-fibre tubes. I cannot find any review report compaining that these scopes have bad focus shift problems.

I remain skeptical about this focus shift issue until hard data or measurements can be shown.

Best regards,
Tang

[BossKwan]

.....So many viewpoints in this thread!
I am just back after a few days off for vacation and would like to be a follower.....挿下咀..

It is a fact that for smaller refractors (under 4"), the difference in weight is very small between these two materials and similar the case in schmidt telescopes around 8 to 11 inches which are already known by most fans.

Then, why more and more manufactures produce scopes using carbonfibre and put on the markets?
I am sure that the main reason is neither because of the considertion of weight nor the good advangtage of the material's low thermal conductivity, at least, most of their purposes are mostly because "carbonfibre" is a real and good selling point for their new products: Nowadays, the tastes of people are always in changing, some people like different, the traditional white color (or other colors) aluminiun telescope tube might bore some people, an OTA with CF tube really features diferent color and different looks, like with a more elegent and new-fashioned cloth that could more please some people, and besides, it really has other advantages as some of you described before.... This is already rather a business policy more than its practical advantages.

Why some well known manufacturers still insist using aluninium tubes for their refractors?
Surprisingly, most of you seem to have skipped disscusing a problem of "machining processing". All old brand manufacturers know that clearly, aluninium is very good material for easy machining processing over others, particularly some parts or procedures need to be finished by using CNC machines, on the other hand, CF materials would cause many production problems from processing, difficulties come out more particularly in producing bigger refractors. I am sure that using which materials can more easily guarantee prefect finishing, they know all the answers very clearly.......
Therefore, we can see, on the current markets, most CF OTAs are only available in smaller apertures.......