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. 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. Neither come close to the performance of Dream's
zeroDELTA lightweight mirrors. 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. Please note
that both are 6:1 aspect ratios 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 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 better test is needed. The precision of easy and cheap
tests can be substantially worse than believed.
The mirror is absolutely bending.
Everything from freeware FEA to full-blown custom modeling and
FEM/FEA to actual telescope tests using a camera, all easily
shows that overly thin solid mirrors are bending by far larger
amounts and they are highly sensitive to over-constrained conditions.
The weaker the mirror, the easier it is to over-constrain.
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 easy to
distort. This was using Dream's in-house fabricated carbon fiber
mirror mounts, which are close to 1ppm/°C within the CTE
of borosilicate glass. Using an inadequately
designed (little to no real mechanical engineering and
no flexures) mount made
from aluminum, which is ~20ppm different than the mirror material,
is even worse. Dream's results are based on both testing of the
full telescopes under the stars and engineering analysis. Bending
was readily evident in both the physical and the virtual worlds.
The vast majority of optical
test reports are not accounting for real-world mirror
mounts, or real-world 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 mirror properly is an additional
challenge, because the mirror's optical surface is so readily
Thin solid mirrors are much more
likely to have astigmatism ground and polished into them. The
largest red flag statement a consumer can ever hear is that a
mirror has no errors, has zero astigmatism, zero RMS surface
roughness, etc. Without exception this cannot live in the real
world and statements like this persist because consumers did
not push for proper testing. This leaves the consumer to live
on faith instead of facts.
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
and/or telescope structure stiffness. If you can't see a difference,
then you need to ask why not. The answer is usually that the
instrument is swimming in thermal problems, the test is inadequate
or is improperly
conducted. If you
aren't exposed to better performance, the status quo seems fine
because it hasn't been compared to something better. Intelligent
consumers know that 166 year-old technology is not modern in
any way. Mirror-seeing is as old as mirrors.
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.
Traditional opticians are married to solid glass mirrors. For
them it is another sale of 166 year old technology. Going thinner
is simply a shiny bow on old technology. For Dream is has always
been about improved performance, not traditional performance,
because we have always made the full, high-performance instruments,
not just the mirror, and we have never let others stand in the
way of performance gains, logic and sound engineering.