Using a solid mirror
that is thinner than the traditional 6:1 aspect ratio will
make the solid mirror equalize faster, compared to a 6:1 solid
mirror of the same material. Unfortunately it comes at the expense
of stiffness and the "gain" in thermal time constant is similar to comparing the difference in top speed
of two turtles, relative to a Formula 1 car. A thinner solid
mirror is a Band-Aid "solution" on a much, much larger
* What happens to the stiffness
of the solid mirror when you make it half as thick? The stiffness of the mirror becomes
4x lower than it was before; 12" diameter by 2" thick
(6:1) is 4x stiffer than a 12" diameter by 1" thick
* What happens to the stiffness
of the solid mirror when you make it 1/4 as thick? The stiffness of the mirror becomes
16x lower than it was before; 12" diameter by 2" thick
(6:1) is 16x stiffer than a 12" diameter by 0.5" thick
* How do the 12" examples
above compare to a 6" diameter solid mirror that is 1"
thick? A 12"
diameter by 2" thick (6:1) is 4x lower in stiffness than
the 6" diameter by 1" thick (6:1) mirror. Both are
6:1 aspect ratio but the 12" is 4x lower in stiffness. When
we compare the 12" by 0.5" thick (24:1) to the 6"
diameter by 1" thick mirror we find the 12" is 64x
lower in stiffness. To see charts and additional information,
for a short paper on aspect ratio & stiffness.
Glass is like any other material,
when the height is decreased, the stiffness will decrease. Whether
it is a steel "I" beam, wooden floor joist or glass
mirror, everything will have lower stiffness when the height
If bending of the optical surface
can't be detected in situations where the thin, solid mirror
has substantially lower stiffness, is far easier to bend and
engineering analysis shows it is bending in large amounts, concluding
that it is not bending is wishful
thinking. Common sense
says a higher resolution (better) test is needed.
The "precision" of
easy and cheap tests can be substantially worse than believed.
Even data from a $450,000 interferometer still has to abide by
the laws of mechanics. The interferometer doesn't know how to
properly test the large optic but the operator needs to. An ignorant
(lack of expertise) operator may subtract out more than just
noise, thus leading to data with higher uncertainty.
One thing to keep in mind regarding
optical testing. It is often done with the optical axis horizontal.
This places the face of the mirror in shear. For a thin solid
mirror it has high stiffness in shear, relative to axial stiffness.
Axial being the thin solid mirror zenith-pointing (optical axis
vertical). So even in situations where the optical metrology
equipment used in horizontal testing is high enough in resolution,
the data will not show how weak the mirror is.
At fractions of a wavelength
of light, all mirrors bend. Everything from freeware FEA to full-blown
custom modeling and FEM/FEA to actual telescope tests using a
camera, all easily show that overly thin solid mirrors are bending
by large amounts and they are even more sensitive to over-constrained
conditions. The lower the stiffness of the mirror, the easier
it is to over-constrain and warp the optical figure due to the
mirror mount. For all optics larger than 1-2" in diameter,
the mirror mount is an integral part of the optical surface,
not separate from it. With proper optical metrology and
a desire to learn, this is easily discovered.
Dream has tried 16.5:1 and 13.2:1
aspect ratio thin solid glass mirrors in the 16" diameter
range. Even the 13.2:1 was abandoned because it was so readily
distorted by the mirror mount. This was using Dream's in-house
fabricated carbon fiber mirror mount, which was essentially athermal
to the mirror. Using an inadequately
designed (little to no real mechanical engineering and
no flexures) mount made
from aluminum, which is eight to hundreds of times higher in
CTE than the mirror material chosen, is even worse. Dream's results
are based on both testing of the full telescopes and engineering
analysis. Bending was readily evident in both the physical and
the virtual worlds. Believing in, something for nothing, is wishful
lives in the details, not in baseless and easy to discount claims.
The vast majority of optical
test reports are not accounting for real-world mirror
mounts, or fina-use mirror angles. In order to achieve the same
performance as the 6:1 mirror requires a mirror mount of far
greater complexity, especially as the mirror diameter gets larger
and larger. Even when a more complex mount is created, physically
mounting the low-stiffness mirror properly is an additional challenge,
because the mirror's optical surface is so readily distorted.
Thin solid mirrors are much more
likely to have astigmatism ground and polished into them. The
largest red flag statement a buyer can ever hear is that a mirror
has no errors. Without exception this cannot live in the real
world and statements like this persist because buyers have not
pushed for proper testing or done their own research to more
deeply understand optical metrology. This leaves the buyer living
on faith instead of facts and creates a market
where claims abound.
The argument that these fundamental,
detectable and easy to understand realities of a physical object
are not happening is akin to ignoring proper optical alignment,
structure stiffness and critical focus. If a difference can't
be seen, the test
undoubtedly does not have the accuracy that was assumed. Simultaneously
the instrument is often swimming in thermal and mechanical errors.
If we aren't exposed to better performance, the status quo seems
fine because it hasn't been compared to something better. Intelligent
buyers know there will always be a better mousetrap.
It doesn't take a world-class
site to detect these types of problems, and consequently anyone
can see improvements when these issues are properly addressed.
This tends to turn site seeing on its head because often it was
the instrument itself with the largest thermal and/or mechanical
errors. Traditional opticians are married to solid glass mirrors.
For them it is another sale of 167 year-old technology. Going
thinner is simply a shiny bow on an old technology, which slightly
improves the thermal aspect(s) while providing you with an unwanted
side effect; loss of stiffness.
For Dream it has always been
about improved performance, not traditional performance, because
we have always made the full, high-performance instruments, not
just a mirror blank or a portion of the structure. Dream has
never let others stand in the way of performance gains, logic
and sound engineering, which is why it has become such a vertically
integrated company over the past 15 years.