Below are some of the most frequently asked questions about carbon fiber that we have heard and answered since Dream's inception in 2003.

For FAQ's related to lightweight mirrors, click here.

1. Q: What is the advantage of using carbon fiber so extensively in your telescopes? 

....A: Dream's telescope structures average 95% carbon fiber and only 5% conventional metals by weight. This is by far the highest of any manufacturer because most use 5-10% carbon fiber and 90-95% aluminum by weight of the structures. There is little to no aluminum in a Dream telescope. Aluminum is readily available, inexpensive as a raw material and inexpensive & quick to machine, which are all reasons for its widespread use. But it has the highest Coefficient of Thermal Expansion (CTE) of any of the common metals and cannot compete with the stiffness of Dream's purpose-built carbon fiber parts. Dream was founded in 2003 to optimize carbon fiber for opto-mechanical structures and to use sandwich core more extensively than ever seen before in an open-market telescope structure, as well as to push lightweight mirrors into the modern era.

....Dream has engineered the carbon fiber for not just high stiffness, essential for today's modern and more demanding optical designs, but also to closely match the CTE of the Dream zeroDELTA lightweight mirrors. One of the most vital areas to use Dream's carbon fiber is in the mirror mounts. A mirror mount needs to maintain the optical surface to within fractions of a wavelength of light. This is a daunting challenge that is nearly impossible to maintain when aluminum mirror mounts, that do not include flexures, are used. Both Dream's extreme stiffness and well-matched CTE offers customers a tremendous advantage due to the extreme mechanical and thermal stability of the telescopes. Making them easily the most user-friendly and maintenance-free telescopes available.

....For the past 15 years a system Dream produces that is sensitive to +/-8µ of focus shift has shown all-sky (when EQ mounted - compound angles, not just the far less demanding altitude-only) mechanical stability that does not go outside this tiny threshold. It does not need focus to partially compensate for tilt errors, since tilt errors cause aberrations that cannot be fully compensated out. This same system has also shown that the telescopes can maintain focus over at least a 20°F ambient temperature change. Dream has nearly 15 years of heritage showning raw, full-frame image data taken throughout the sky on these systems. We've never hidden behind cropped and/or processed images or unstated focus adjustments because Dream's goal has always been performance, not marketing. We've never had to hide anything because the technologies Dream has developed speak for themselves.

2. Q: Are all carbon fiber parts brittle and therefore extremely fragile? 
....A: Dream's carbon fiber parts are not fragile. The videos on this page show just how rugged Dream's carbon fiber and carbon fiber skinned sandwich core parts are. Also keep in mind that Dream's composites are inherently resistent to moisture/humidity, are non-corrosive, have a low CTE, low thermal fatigue and are quite chemically resistent. Because each part is baked in one of our ovens, the parts are also stable at higher temperatures.

3. Q: Do all carbon fiber parts vary in mechanical performance from part to part, making them impossible to use in high-performance opto-mechanical systems, especially when using it to support precision mirrors?
....A: Dream started as an advanced composites company solely for the purpose of making more precise and consistent structures for opto-mechanical systems, which require high-stiffness and low CTE. Opto-mechanical structures also benefit from low mass, which is why Dream has focused so extensively on carbon fiber skinned sandwich core components. Dream isn't a standard "composites shop" that makes fiberglass boats or car bumpers. Such shops lack an understanding of the extremely tight tolerances that are required for opto-mechanical systems. We are the only company in the world that has a deep understanding of optics and a deep understanding of composites. Dream understsands composite materials & methods as they relate to stiffness and CTE requirements for optics. We have tight process controls, like our +/-1°F largest composites oven. Bringing all of this together has allowed us to focus on methods that are highly consistent from part to part. Our tailored composites produce athermal telescopes.

4. Q: Do all carbon fiber parts vary in Coefficient of Thermal Expansion (CTE) from part to part, making them impossible to use in high-performance opto-mechanical systems?
....A: As stated above Dream has always had optical systems in mind for the composties. We are not a general composites company that is trying to shoehorn our composites into precision optical systems. Dream was formed specifically for the opto-mechanical industry. We can produce a part today, then again two years from now with extreme consistency in the part, both mechanical performance (question 2 above) and CTE (question 4 below). They typically coincide with each other (mechanical and thermal), although mechanical properties and thermal properties are two completely different attributes of a part.

5. Q: Do all composite parts have resin-rich areas that will have a different CTE than other areas, therefore causing the part to distort?

....A: From day one Dream has always; paid especially close attention to fiber orientation & resin content, used high-temperature epoxy that is specifically engineered for an optimal match to our mirrors, used vacuum bagging and high temperature baking of the parts to provide the utmost in consistency and part performance. Wildly varying performance is common in wet layup parts, parts that do not use vacuum bagging and by those that have little to no knowledge of the precision required in optics. Overwrapping the parts with weak plastic, similar to Saran Wrap, will have almost no force on the part and therefore are not consolidating the layers of CF. Part thickness can easily be double when full vacuum bagging is not used, leaving more resin in the part. Resin is heavy and has a higher CTE than the carbon fiber. If carbon fiber parts that you have received from others make cracking sounds when loaded, they are inferior parts that not only lack stiffness but can also fail, causing harm to the precision optics. Those are void areas collapsing and are a red flag that the piece is not a high performance part.

....Full vacuum is applying roughly one ton of force on the part per square foot. A part with six square feet of surface area has roughly six tons of force applied across the part. Unlike a press, that inherently will have "hot spots" (higher pressure and subsequent lower pressures), properly executed vacuum bagging will produce the same pressure across the entire part, regardless of it's shape. Although this sounds simple enough like most things in life there is far more to it than just buying a vacuum pump and flipping a switch. It is technically impossible for Dream's advanced composite parts to have wildly different CTE areas because of our extensive experience (2003-), focus on making consistent parts, and proving that performance in the athermal telescopes that we produce that have the best all-sky mechanical and thermal performance of any telescopes in their and surrounding classes.

6. Q: Can carbon fiber parts be fabricated with fiberglass or large pieces of aluminum?

 ....A: Fiberglass has exceedingly low properties when compared to carbon fiber. If the ultimate in performance is the goal, fiberglass is not the raw material of choice. Ferrari, Porsche, Lamborghini, etc., are not making their cars out of fiberglass but out of carbon fiber. The reason some feel the need to use fiberglass is cost. They are trying to pinch pennies anywhere they can. Fiberglass is 20-30x less expensive than carbon fiber.

....Making carbon fiber parts with large pieces of aluminum inside follows along the same logic as above. Since there is no good mechanical or thermal reason to do it, this is probably driven by a company's need to save money. Boeing and other aerospace companies have each spent millions of dollars in R&D specifically related to the bonding of aluminum and other metals to carbon fiber parts. For large surfaces it requires acid-etching of the metal, then bonding immediately after that, since the metal surfaces start to change very rapidly. To do this process properly would cost more than simply throwing out the aluminum and using all carbon fiber... Again, combining larger inexpensive aluminum, which has the highest CTE of any of the common metals, with low CTE carbon fiber is not going to be any engineer's first or even 10th choice. It's a good way to make parts fail some time in the future when they delaminate from each other after numerous thermal and moisture cycles.

7. Q: Others have used aluminum in mirror mounts for a great many decades. Why does Dream focus so much effort on producing athermal mirror mounts?

 ....A: This and many other aspects of what Dream focuses on may seem trivial but remember that a mirror mount is trying to impart the least amount of bending into that precision optic as possible. Optimal performance is obtained only when the optical surface has errors that are fractions of a wavelength of light. A microscope used in middle school science class might operate at 25x magnification. When other students bumped the desk it made the views seem like a 10.0 magnitude Earthquake. At 25x you can see some details inside a 1mm width. In visual spectrum optical systems a diffraction-limited optical surface has peak to valley errors at its surface no larger than about 0.000070mm (70nm). This is 14,286 times smaller than the 25x microscope example. Although a basic telescope is simple to understand, a high-performance telescope has to scrutinize details that are exceedingly small and extradinary compared to everyday life.

....Scale is the reason why details matter in opto-mechanical systems and why ignoring glaring mechanical and thermal issues cannot allow optimal performance. Often such systems are buried in their own errors and therefore cannot see changes attempted because one or more errors in the system are still far larger than the change that was made. This leads to improper conclusions that the system is performing at unrealistic levels.

....At optical scales all optical surfaces bend. Modern engineering analysis and/or modern (real) interferometry easily shows that optical surfaces are not infinitely stiff. Two external (to the mirror) causes of optical surface bending are mechanical and thermal. If a mirror mount lacks stiffness and is also heavy itself, it will bend far more than something with stiffness 10x higher and 2x lower mass. In this mechanical-only view we want the stiffest possible mirror mount because the tolerance for maintaining the optical surface is so incredibly small compared to everyday examples. Being light also benefits system performance because that lower mass will bend structures less, helping to maintain tighter optical alignment tolerances.

....Because the surfaces are so sensitive to bending, that bending can come from inadequate design and a mis-match of the CTE's of the mirror and the mirror mount. Unless flexures are used to take up the CTE differences, an aluminum mirror mount will change shape at a rate roughly 400 to more than one thousand times faster than zero-expansion mirror materials. A mirror mount that much more closely matches the CTE of the mirror is obviously beneficial because as temperature changes, the mirror mount much more closely follows the mirror, causing the least amount of figure distortion in the optical surface. This can eliminate the need for flexures, which can reduce bulk, mass and costs.

"Hello Shane, I can't think of anyone who has delved as deeply into the mechanics of telescopes as you have."

Dream customer

For FAQ's related to lightweight mirrors, click here.

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