[ATM] Carbon Fiber for Spider Vanes

["Dream - Telescopes & Accessories, Inc." <shane@dreamscopes.com>]

Peter,

A business colleague sent me your posting regarding carbon fiber.

I am a composite's company that specializes in carbon fiber products for 
the astronomy and opto-mechanical industries.  The biggest issue with 
composite's, among a great many, is resin content.  The strength is in the 
fibers, not in the resin.  In order to maximize properties, low resin 
contents and also extremely low void content are vital.  Resin weighs much 
more than the fibers too so either not using as much resin to begin with or 
somehow getting some resin back out of the composite lay up is of utmost 
importance.  Vacuum bagging and using traditional vacuum bagging materials, 
like breather, is a very good way to reduce the resin content in the part.

There are a tremendous amount of variables in composites that can affect 
their strength and stiffness.  In astronomy we are much more concerned with 
stiffness than strength, in our opto-mechanical systems.  We're not 
building bridges but simply holding (relatively light) optics as stiff as 
possible, especially for imaging applications.

Parts with high resin contents, voids (areas that weren't wet out properly) 
and general inconsistencies of the wet out all make for a very poor 
part.  Such a part would be heavy, brittle and generally will have low 
properties (strength and stiffness).  If however the part is made in a 
fairly meticulous manner concentrating on fundamentals of lay up, the part 
(carbon fiber) can have greater stiffness than steel yet weigh 
less.  Remember that steel is stronger than aluminum so carbon fiber can be 
far superior to aluminum.

Again, there are so many variables in not only the lay up portion of making 
a composite but also materials (dry goods, resins, prepregs, etc.) used 
that flatly stating "carbon fiber" is "five times stronger than steel" is 
simple ignorance.  If the part is engineered with certain types of carbon 
fiber & resins and it is laid up and processed correctly, it can have 
numbers exceeding the typically heard "fives times..."  BUT, parts not 
designed or made properly will not meet that statements claims.

Here's an example that further illustrates my point.  A given part can be 
made by two different companies or even two different people within the 
same company.  Both parts are produced using the exact same carbon fiber 
dry goods, exact same resin, same equipment, same room, under the same 
temperature, humidity, etc..  One part is made in a sloppy manner and the 
other very meticulously.  The latter will exhibit not only lighter weight 
but higher properties.  It will also be extremely consistent.  The sloppy 
part is very heavy, very brittle (since many resins are brittle and this 
sloppy part is more resin than fiber).  Testing the sloppy part in one area 
will yield wildly different results than another test sample from somewhere 
else on the part.

So the answer to your question is yes, it can be extremely beneficial to 
have spider vanes, or any other part of the telescope made from carbon 
fiber.  BUT, it won't be if the person making the part isn't meticulous and 
pushing their techniques and QA on every single part.  Just the simple use 
of carbon fiber alone does not automatically give the product the ultimate 
properties it can attain.  There are many, many variables along the way 
that greatly affect performance.

You can see a few things that my company has produced using carbon fiber 
below.  Just click on the home icon at the bottom of the page to get to 
photos of other carbon fiber products.
http://www.dreamscopes.com/pages/projects-04/oven/26.htm

The above mirror cell uses truss style members that will be almost 
identical to spider vanes I will be producing in the coming months.

The (general) properties of carbon fiber is extremely beneficial for 
astronomy applications.  Carbon fiber does not like to hold heat.  My 
painter took a strand of carbon fiber from my shop and put a small butane 
torch to it.  He waited until it glowed orange/red hot, expecting it to 
burn up or at least fall into two pieces.  It did not.  He then took the 
torch away and waited 15 seconds before trying to touch the strand with his 
bare fingers at the point where the flame had been.  He kept doing this 
heat up cycle until he was touching the strand approximately three seconds 
after the flame was off of it.  It cooled that fast.  Somewhat similar to 
the ceramic tiles on the Space Shuttle.

Carbon fiber is, in general, the stiffest of the composite dry 
goods.  Stiffer than E-glass (common fiberglass), S-2 glass (a stronger 
version of E-glass), Kevlar, etc..  It is also very light.  The only other 
common material that is lighter is Kevlar.

Carbon fiber also has a very low CTE (coefficient of thermal 
expansion).  In an opto mechanical system dissimilar CTE's in the structure 
and glass will cause, among other things, focus shifts.  Carbon fiber can 
be highly engineered and thus gives the designer the ability to match the 
carbon fiber's CTE to the optics materials used.

You can see that the material properties are very appealing: stiffer, 
stronger, lighter and doesn't hold heat.  These properties are so appealing 
that two companies, that I've spoken with, have been in R&D to produce 
carbon fiber mirrors.  The US company has been in R&D for more than 10 
years and is working on the ULTRA project.  The other company is in the 
UK.  Besides ULTRA, the US company is under contract to produce a complete 
1m telescope (mount, OTA, optics, etc..) that will weigh 80 lbs or less.

Structural components are very feasible to produce, albeit expensive 
compared to metals and especially wooden products.  Producing an optical 
surface on composites is another story.  My company has done some 
preliminary R&D on this but other than seeing a reflection, additional work 
would be needed to get the optical surface even remotely close my own 
optic's standards.

I hope this helps answer your question...

Sincerely,
Shane Santi
Dream - Telescopes & Accessories, Inc.
http://www.dreamscopes.com
610 - 365 - 2833 

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