D r e a m

Thin Solid Mirrors


Going thinner than the traditional 6:1 aspect ratio solid mirror will make it equalize faster. Unfortunately it comes at the expense of stiffness. The "gain" in thermal time constant is similar to comparing the top speed of two turtles. Neither come close to the performance of the lightweight mirror using thin features. A thinner solid mirror is a Band-Aid on a much larger problem. A problem that Ritchey recognized 100 years ago.
What occurs 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. If you make the glass 1/4 as thick, the stiffness drops by 16x.
Glass is like any other material, when the edge height is decreased, the stiffness will decrease. Whether it is the height of a steel "I" beam, wooden floor joist or glass mirror, everything will have lower stiffness.

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. Even when this mount is created, mounting the mirror properly is an additional complexity, because the mirror's optical surface is so readily bent.
Thin solid mirrors are much more likely to have astigmatism ground and polished into them as well.

In Dream's beginning we tried to use 16.5:1, then 13.2:1 aspect ratio thin solid glass mirrors. 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. Let alone using an inadequately designed (little to no real mechanical engineering and no flexures) mount made from aluminum; ~20ppm different than the mirror material.

The argument that these fundamental, detectable and easy to understand concepts are not happening is akin to ignoring proper optical alignment. 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 and/or the test is inadequate.
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 solid glass mirrors. For them it is another sale of 166 year old tech. For Dream is has always been about performance because we have always made the full, high-performance instruments, not just the mirror.


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