D r e a m

Thin Solid Mirrors


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 problem.

* 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 (12:1).
* 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 (24:1).
* 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, click here 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 is reduced.
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 distorted.
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.


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