1. Q: What is the advantage
of using carbon fiber so extensively in your telescopes?
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
....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?
Dream's carbon fiber parts
are not fragile. The videos
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?
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
....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?
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
....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
....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.
Shane, I can't think of anyone who has delved as deeply into
the mechanics of telescopes as you have."
FAQ's related to lightweight mirrors, click
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